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UNIVERSIDADDEVALLADOLID
ESCUELADEINGENIERÍASINDUSTRIALES
DEPARTAMENTODEINGENIERÍAQUÍMICAY
TECNOLOGÍADELMEDIOAMBIENTE
NITROGENREMOVALINDOMESTICWASTEWATER
AFTERANAEROBICTREATMENT
PresentadaporLaraPelazPérezparaoptaralgradodeDoctorporlaUniversidaddeValladolid
Dirigidapor:
Prof.Dra.MaríaFdz-PolancoÍñiguezdellaTorre
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UNIVERSIDADDEVALLADOLID
ESCUELADEINGENIERÍASINDUSTRIALES
DEPARTAMENTODEINGENIERÍAQUÍMICAY
TECNOLOGÍADELMEDIOAMBIENTE
ELIMINACIÓNDENITRÓGENOENAGUASRESIDUALES
DOMÉSTICASDESPUÉSDETRATAMIENTOANAEROBIO
PresentadaporLaraPelazPérezparaoptaralgradodeDoctorporlaUniversidaddeValladolid
Dirigidapor:
Prof.Dra.MaríaFdz-PolancoÍñiguezdellaTorre
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MemoriaparaoptaralgradodeDoctor,
presentadaporlaIngenieraQuímica
LaraPelazPérez
SiendolatutoraenlaUniversidaddeValladolid
Dra.DªMaríaFdz-PolancoÍñiguezdelaTorre
Valladolid,dede2016
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UNIVERSIDADDEVALLADOLID
ESCUELADEINGENIERIASINDUSTRIALES
Secretaría
Lapresentetesisdoctoralquedaregistradaenelfolionúmero
______delcorrespondientelibroderegistronúmero_____.
Valladolid,dede2016
Fdo.Elencargadoderegistro
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MaríaFdz-PolancoÍñiguezdelaTorreProfesoraTitulardeUniversidad
DepartamentodeIngenieríaQuímicayTecnologíadelMedioAmbienteUniversidaddeValladolid
Certifican:
QuelaingenieraquímicaLARAPELAZPÉREZharealizadobajosudirecciónel trabajo “Nitrogen removal in domestic wastewater after anaerobictreatment”, en el Departamento de Ingeniería Química y Tecnología delMedioAmbientedelaEscueladeIngenieríasIndustrialesdelaUniversidaddeValladolid.Considerandoquedichotrabajoreúnelosrequisitosparaserpresentado como Tesis Doctoral expresa su conformidad con dichapresentación.
Valladolid,dede2016
Fdo.MaríaFdz-PolancoÍñiguezdellaTorre
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Reunido el tribunal que ha de juzgar la tesis doctoral titulada “Nitrogenremoval in domestic wastewater after anaerobic treatment” presentadaporlaingenieraLaraPelazPérezyencumplimientoconloestablecidoenelRealDecreto99/2011de28deenerode2011haacordadoconcederpor___________lacalificaciónde_______________.
Valladolid,a_____de_______de2016
PRESIDENTE SECRETARIO
1erVOCAL 2ºVOCAL 3erVOCAL
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1
Reunido el tribunal que ha juzgado la Tesis Doctoral titulada “Nitrogen
removal in domestic wastewater after anaerobic treatment” presentada
por la Ingeniera Química Lara Pelaz Pérez y en cumplimiento con lo
establecido en el Real Decreto 1393/2007 de 29 Octubre ha acordado
concederporlacalificaciónde_______________.
Valladolid,a_____de_______de2016
PRESIDENTE SECRETARIO
1erVOCAL 2ºVOCAL 3erVOCAL
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3
Amispadres,SilvioyMªRosa
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5
Contents
Abstract................................................................................................................................9
Resumen............................................................................................................................15
AbbreviationsandSymbols...............................................................................................21
Chapter1:Stateof theArt.Nitrogenremoval indomesticwastewaterafteranaerobic
treatment...........................................................................................................................25
AimsandContents.............................................................................................................55
Chapter 2. SBR system for nitrogen removal in domestic wastewater from anaerobic
treatment...........................................................................................................................61
Chapter 3. Denitrification of the AnMBR effluent with alternative electron donors in
domesticwastewatertreatment.......................................................................................83
Chapter 4. Advanced denitrification of anaerobic treatment effluent of domestic
wastewaterbyusingwastedgas.....................................................................................101
Chapter5.Nitrogenremovalindomesticwastewater.Effectofthenitraterecyclingand
theCOD/Nratio...............................................................................................................117
Chapter6.Techno-economicalstudyofadomesticwastewatertreatmentsystem....137
Conclusions......................................................................................................................159
Agradecimientos..............................................................................................................165
Acknowledgements.........................................................................................................165
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Contenido
Abstract................................................................................................................................9
Resumen............................................................................................................................15
Abreviacionesysímbolos..................................................................................................21
Capítulo 1: Estado del arte. Eliminación de nitrógeno en aguas residuales domésticas
despuésdetratamientoanaerobio...................................................................................25
Objetivosycontenidos......................................................................................................55
Capítulo2:SistemaSBRparaeliminacióndenitrógenoenaguasresidualesdomésticas
despuésdetratamientoanaerobio...................................................................................61
Capítulo 3: Desnitrificación del efluente de un AnMBR con dadores de electrones
alternativosenaguasresidualesdomésticas....................................................................83
Capítulo4:Desnitrificaciónavanzadadelefluentedetratamientoanaerobioutilizando
gasusado.........................................................................................................................101
Capítulo 5: Eliminación de nitrógeno en aguas residuales domésticas. Efecto de la
recirculacióndenitratosydelarelaciónDQO/N...........................................................117
Capítulo6:Estudiotecno-económicodeunsistemadetratamientodeaguasresiduales
domésticas.......................................................................................................................137
Conclusiones....................................................................................................................159
Agradecimientos..............................................................................................................165
Acknowledgements.........................................................................................................165
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9
Abstract
Nitrogenremovalindomestic
wastewaterafteranaerobic
treatment
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Abstract
11
Domesticwastewatercontainsexcessivenutrients,harmfulbacteria/virusesandhousehold
chemicals thatmay contaminate the land andwaters and threaten public health. Therefore,
beforethedomesticwastewatersdischarge,itwillbenecessaryatreatmenttopreventdiseases
inpeople,andtoprotectthefaunaandflorapresentinthenaturalreceptorbody.Wastewater
treatmentiscloselyrelatedtothestandardssetfortheeffluentquality.
Anaerobictechnologyfororganicmatterremovalisveryfavorableundertheperspectiveof
sustainable development. However, the anaerobic effluent usually requires a post-treatment
step as a means to adapt the treated effluent to the requirements of the environmental
legislation and protect the receivingwater bodies. Themain role of the post-treatment is to
eliminatethenutrientsandcompletetheremovaloforganicmatter.
For the biological removal of organicmatter and nitrogen, anaerobic, anoxic and aerobic
biological processes should be combined. For this purpose, different treatment systems are
beingdevelopedtomaximizetheadvantagesofbothaeratedandnon-aeratedprocesses.
Theaimof thisPhDThesis is todevelopandevaluatedifferent treatmentprocessesofan
anaerobic reactor effluent fedwith domestic wastewater. For this purpose, different reactor
configurations are developed: SBR and biofilters, with different reaction ways to treat the
effluentofananaerobic reactor.Nitrogenremovalefficiencyandenvironmental sustainability
havebeenconsideredtocomplythedischargestandardsindomesticwastewater.
In Chapter 2, this work presents the performance of a sequencing batch reactor (SBR)
systemusedasnitrogenremovaltreatmentofdomesticwastewaterpreviouslytreatedwithan
anaerobic reactor and as consequence, with a low C/N ratio. The aim of the work was to
determine the feasibility for the removalofnitrogen from thedomesticwastewater.A5 Lof
workingvolumeSBRwasinvestigatedatdifferentcycletimesof12h,8hand6h,at18ºC.The
treatment efficiency of SBR varied with the duration of the cycle time, being optimal the
anoxic/aerobic/anoxic sequence cycle with 6 h of duration. Due to the low organic matter
presentinthedomesticwastewaterafteranaerobictreatment,anadditionalsupplyofexternal
carbonbeforethesecondanoxicstagewasnecessary.Theadditionofmethanolwasakeypoint
in thedenitrificationprocessemployedasamodel for thewastewaterby-pass inwastewater
treatmentplants(WWTP).Theremovalefficienciesobtainedwere:98%forTKN,84%fortotal
nitrogen and 77% for soluble COD. The reactor showed viability, so this process can be
successfullyappliedasapost-treatmentofananaerobicreactortreatingdomesticwastewater,
fortheremovalofnitrogen.
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Abstract
12
InChapter3,theperformanceofafixedfilmbioreactorforpartialandtotaldenitrificationof
the effluent from an anaerobic membrane bioreactor (AnMBR) treating domestic water was
investigated.Wastewaterafteranaerobictreatment,withalowC/Nratio,containsaremaining
COD which is not enough for the conventional heterotrophic denitrification. As the effluent
from the low-temperature anaerobic reactor holds methane and sulfide dissolved and
oversaturated, itwas evaluated the feasibility of using these reduced compounds as electron
donorstoremove80mgNOx
--N/LatdifferentHRTobtainingtheoptimumat2h. Inaddition,
theinfluenceoftheNO2
-/NO3
-ratio(100%/0%;50%/50%;25%/75%and0%/100%)inthefeed
was studied. Satisfactory results were obtained achieving total nitrogen removal in the
denitrifyingeffluent,beingawareofthecasewith100%NO3
-inthefeed,thatwasatthelimitof
theprocess.Methanewas themainelectrondonorused to remove thenitrites andnitrates,
withmorethan70%ofparticipation.
InChapter4,apilotplantofdenitritationwasoperatedformorethanfivemonthstreating
domestic wastewater with high ammonium nitrogen concentration from anaerobic process
underambienttemperatureconditions(18ºC).Theprocessconsistedononebiofilterwith2h
ofHRTfordenitritation.Tostudythefeasibilityofthedenitritationprocess,differentsynthetic
nitrite concentrations were supplied to the anoxic reactor to simulate the effluent of a
nitritation process. The present work investigates an advanced denitritation of wastewater
using theorganicmatterandotheralternativeelectrondonors fromananaerobic treatment:
methane and sulfide. The denitrifying bacteria were able to treat water at an inlet nitrite
concentration of 75 mg NO2
--N/L with removal efficiency of 92,9%. When the inlet nitrite
concentrationwashigheritwasnecessarytorecirculatethegasobtainedintheanoxicreactor
toenhancethenitriteremoval,achieving98,3%ofNO2
-eliminationefficiency.
In Chapter 5, a denitrification/nitrification pilot plantwas designed, built and operated to
treattheeffluentofananaerobicreactor.Theplantwasoperatedtoexaminetheeffectofthe
nitraterecyclingandtheC/Nratioonthenitrogenandtheremainingorganicmatterremoval.
The systemconsistedof a two stages treatmentprocess: anoxic andaerobic. TheHRTof the
systemwas2hfortheanoxicbioreactorand4hfortheaerobicone.Theincreaseinthenitrate
recycling ratiodidnot supposea significant improvement in thenitrogen removaldue to the
insufficientcarbonsource.ThewastewatertobetreatedhadaC/Nratioof1.1showingalack
of organic carbon. The addition of methanol was a key point in the denitrification process
employed as a model for the traditional wastewater by-pass in the WWTP. The maximum
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Abstract
13
nitrogenandorganicmatterremoval(84.7%and96%,respectively)wasachievedwithanitrate
recyclingratioof600%andaC/Nof8.25,adjustedbymethanoladdition.
In Chapter 6, the techno-economical feasibility of themembrane anaerobic treatment of
wastewater eliminating nitrogen has been simulated. The process was simulated using
experimental data analyzing the influence of different electron donors (methane, organic
matter and sulfide) on the nitrogen elimination capacity. Different scenarios have been
assessedchangingtheconcentrationoftheinvolvedcomponentsandevaluatingtheireffecton
the nitrogen elimination capacity as well as the ability to produce biogas in the anaerobic
treatment.Thesescenariosimplyontheonehand,theincrementoftheavailablesolubleCOD
forthenitrogeneliminationstage.TheCODfeedtothereactorwasadjustedatvaluesbetween
15% and 30% assuming different mixing ratios with the influent stream of the anaerobic
reactor.Ontheotherhand,differentflowsofbiogasfromtheanaerobicreactorwerepumped
tothedenitritationreactor.Thegoalwastoachieveanitrogeneliminationcapacitytoreachan
effluentwith10-20mgN/L.Then,themostpromisingscenariowasstudiedindetailanditwas
comparedtothecostsassociatedtotheWWTPwithabiologicalanaerobic treatmentusinga
MBR system. The results indicated that the proposed process is feasible since the fixed and
variablescostsofbothtreatmentplantsaresimilar.
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Resumen
Eliminacióndenitrógeno
enaguasresidualesdomésticas
despuésdetratamientoanaerobio
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Resumen
17
Lasaguasresidualesdomésticascontienenunexcesodenutrientes,bacterias/virusdañinos
ysustanciasquímicasdomésticasquepuedancontaminarlatierrayelaguayponerenpeligro
lasaludpública.Porlotanto,antesdeladescargadeaguasresidualesdomésticas,esnecesario
untratamientoparaprevenirenfermedadesenlaspersonas,asícomoparaprotegerlafaunay
la flora presente en el cuerpo receptor natural. El tratamiento de aguas residuales está
estrechamenterelacionadoconlasnormasestablecidasparalacalidaddelefluente.
Latecnologíaanaerobiapara laeliminacióndemateriaorgánicaesmuyfavorabledesdeel
punto de vista del desarrollo sostenible. Sin embargo, el efluente anaerobio generalmente
requiereunaetapadepost-tratamientoparaadaptarelefluentetratadoa losrequisitosde la
legislación ambiental y proteger los cuerpos de agua receptores. El papel principal del post-
tratamientoeseliminarlosnutrientesycompletarlaeliminacióndelamateriaorgánica.
Para laeliminaciónbiológicade lamateriaorgánicaynitrógeno, losprocesosbiológicosde
tratamientoanaerobio,anóxicoyaerobiodebencombinarse.Paraestepropósito,estánsiendo
desarrollados diferentes sistemas de tratamiento para maximizar las ventajas de ambos
procesosaerobiosynoaerobios.
Elobjetivodeestatesisdoctoralesdesarrollaryevaluardiferentesprocesosdetratamiento
del efluente de un reactor anaerobio alimentado por aguas residuales domésticas. Para este
propósito, se han desarrollado diferentes configuraciones de reactor: SBR y biofiltros, con
diferentescaminosdereacciónparatratarelefluentedeunreactoranaerobio.Paraacatarlas
normas de descarga de aguas residuales domésticas, se han considerado la eficiencia de
eliminacióndenitrógenoylasostenibilidadambiental.
En el capítulo 2, se presenta el rendimiento de un reactor discontinuo secuencial (SBR),
utilizadocomotratamientoparalaeliminacióndenitrógenodelasaguasresidualesdomésticas
previamentetratadasconunreactoranaeróbicoy,comoconsecuencia,conunabajarelación
C/N. El objetivodel trabajo fuedeterminar la factibilidadpara la eliminarnitrógenoenaguas
residuales domésticas.Un reactor SBRde 5 litros de volumende trabajo fue investigado con
ciclosdediferentestiempos:12h,8hy6h,a18ºC.LaeficienciadeltratamientodelSBRvarió
en función de la duración del tiempo de ciclo, siendo óptimo el ciclo con la secuencia
anóxico/aerobio/anóxicocon6horasdeduración.Debidoa lapocaconcentracióndemateria
orgánica presente en el agua residual doméstica después del tratamiento anaerobio, fue
necesario un suministro adicional de carbono externo antes de la segunda etapa anóxica. La
adición demetanol fue un punto clave en el proceso de desnitrificación, empleado comoun
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Resumen
18
modeloparasimularelby-passdepartedelacorrientealreactoranaerobio,enlasplantasde
tratamientodeaguas residuales. Laseficienciasdeeliminaciónobtenidas fueron:98%parael
NKT,el84%denitrógenototalyel77%paralaDQOsoluble.Elreactormostróviabilidad,porlo
queesteprocesopuedeseraplicadoconéxitocomopost-tratamientodeunreactoranaerobio
quetratadeaguasresidualesdomésticas,paralaeliminacióndenitrógeno.
En el capítulo 3, se investigó el rendimiento de un biorreactor de película fija para la
desnitrificaciónparcialy totaldelefluentedeunAnMBRquetrataelaguadoméstica.Elagua
residualdespuésdeltratamientoanaerobio,conbajarelaciónC/N,contienepartedelaDQOno
eliminadapreviamentequenoes suficientepara ladesnitrificaciónheterótrofa convencional.
Comoelefluentedelreactoranaerobiodebajatemperaturacontienemetanoysulfurodisuelto
ysobresaturado,seevaluó laviabilidaddeutilizarestoscompuestos reducidoscomodadores
deelectronesparaeliminar80mgN-NOx
-/Ladiferentestiemposderesidencia,obteniendoel
óptimoen2h.Además,seestudióla influenciadelarelaciónNO2
-/NO3
-(100%/0%;50%/50%;
25%/75% and 0%/100%) en la alimentación. Se obtuvieron resultados satisfactorios
consiguiendo la eliminación total de nitrógeno en el efluente de desnitrificación, siendo
conscientesdelcasocon100%deNO3
-enlaalimentación,queestabaenellímitedelproceso.
Elmetanofueeldadordeelectronesprincipalqueseutilizóparaeliminarlosnitritosynitratos,
conmásde70%departicipación.
En el capítulo 4, una planta piloto de desnitritación operó durante más de cinco meses
tratando de aguas residuales domésticas con alta concentración de nitrógeno amoniacal
procedentedelprocesoanaerobio,encondicionesdetemperaturaambiente(18ºC).Elproceso
consistía en un biofiltro con 2h de tiempo de residencia hidráulico (TRH) para desnitritación.
Para estudiar la viabilidad del proceso desnitritación, se suministraron al reactor anóxico
distintasconcentracionesdenitritosintéticoparasimularelefluentedeunprocesonitritación.
Se investigó la desnitritación avanzada de aguas residuales utilizando materia orgánica y
dadoresdeelectronesalternativosprocedentesdeuntratamientoanaerobio:metanoysulfuro.
Las bacterias desnitrificantes fueron capaces de tratar el agua con una concentración de
entrada de nitrito de 75mg N-NO2
-/L con una eficacia de eliminación del 92.9%. Cuando la
concentraciónalimentacióndenitritofuemásalta,fuenecesariorecircularelgasobtenidoen
el reactoranóxicoparamejorar laeliminacióndenitrito, lograndounaeficaciadeeliminación
deNO2
-de98.3%.
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Resumen
19
Enelcapítulo5,unaplantapilotodedesnitrificación/nitrificaciónfuediseñada,construiday
operada para tratar el efluente de un reactor anaerobio. Se examinó el efecto de la
recirculación de nitrato y la relación C/N en la eliminación de nitrógeno y materia orgánica
residual.Elsistemaconsistíaenunprocesodetratamientodedosetapas:anóxicayaerobia.El
TRHdelsistemafuede2hparaelbiorreactoranóxicoyde4hparaelaerobio.Elaumentodela
recirculación de nitrato no supuso una mejora significativa en la eliminación de nitrógeno
debidoa la insuficienciade fuentedecarbono.Elaguaresiduala tratar teníaunarelaciónde
C/Nde1.1,mostrandofaltadecarbonoorgánico.Laadicióndemetanolfueunpuntoclaveen
el proceso de desnitrificación empleado como un modelo simular el by-pass de parte de la
corriente en el reactor anaerobio. La máxima eliminación de nitrógeno y materia orgánica
(84,7%y96%,respectivamente)selogróconunarelaciónderecirculacióndenitratode600%y
unC/Nde8.25,ajustadoporlaadicióndemetanol.
Enelcapítulo6,sehasimuladolaviabilidadtécnicayeconómicadeltratamientoanaerobio
demembranadeaguasresidualesylaeliminaciónnitrógeno.Elprocesosesimulóusandodatos
experimentalesanalizando la influenciadediferentesdadoresdeelectrones(metano,materia
orgánica y sulfuro) en la capacidad de eliminación de nitrógeno. Se evaluaron diferentes
escenarioscambiando laconcentraciónde loscomponentes implicadosyevaluandosuefecto
sobrelacapacidaddeeliminacióndenitrógeno,asícomolacapacidaddeproducirbiogásenel
tratamiento anaerobio. Estos escenarios implican por una parte, el incremento de la DQO
solubledisponiblepara laetapadeeliminacióndenitrógeno.LaalimentacióndelaDQOenel
reactorseajustóavaloresentre15%y30%,asumiendodiferentesrelacionesdemezclaconla
corrientedealimentacióndelreactoranaeróbico.Porotrolado,sebombearondiferentesflujos
debiogásprocedentesdelreactoranaerobioalreactordedesnitritación.Elobjetivofuelograr
unacapacidaddeeliminacióndenitrógenotalquesepuedaconseguirunefluentecon10-20
mgN/L.Acontinuación,seestudióendetalleelescenariomásprometedorysecomparócon
loscostesasociadosa laEDARconun tratamientobiológicoanaerobiousandounsistemade
membranas.Losresultadosindicaronqueelprocesopropuestoesviableyaqueloscostosfijos
yvariablesdelasdosplantasdetratamientosonsimilares.
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AbbreviationsandSymbols
Page 35
AbbreviationsandSymbols
23
AnMBR Anaerobicmembranebioreactor
AOB Ammoniumoxidizingbacteria
BOD Biologicaloxygendemand(mgO2/L)
BOE OfficialSpanishBulletin
BNR Biologicalnutrientremoval
COD Chemicaloxygendemand(mgO2/L)
DO Dissolvedoxygen(mgO2/L)
FA Freeammonia
FNA Freenitrousacid
GC Gaschromatography
GHG Greenhousegases
HPLC high-liquidperformancechromatography
HRT Hydraulicretentiontime(h)
MBR Membranebioreactor
NLR Nitrogenloadingrate(kgN/m3d)
NOB Nitriteoxidizingbacteria
OM Organicmatter
ORP Oxidation-reductionpotential(mV)
Qin Inletflow
QL Quantificationlimit
SAF Submergedaeratedfilters
sCOD Solublechemicaloxygendemand(mgO2/L)
SBR Sequencingbatchreactors
SND Simultaneousnitrification/denitrification
SRT Solidretentiontime(d)
Page 36
AbbreviationsandSymbols
24
TKN TotalKjeldahlnitrogen(mgN/L)
TN Totalnitrogen
TSS Totalsuspendedsolids(mg/L)
UASB Upflowanaerobicsludgeblanket
VSS Volatilesuspendedsolids(mg/L)
WWTP Wastewatertreatmentplant
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Chapter1:
StateoftheArt.
Nitrogenremovalin
domesticwastewaterafter
anaerobictreatment
Abstract
Awastewater treatment systemhas to remove suspendedmaterial, dissolved
OM, pathogens and dissolved inorganic material. Such treatment systems must
fulfillmanyrequirementstobefeasiblyimplemented,suchassimpledesign,useof
non-sophisticated equipment, high treatment efficiency, and low operating and
capital costs. Conventional nitrification/denitrification and other alternatives are
proposedtoremoveN.AnaerobicprocessesachievehighOMremovalefficiencies
withoutoxygenrequirement.Anaerobicmembranetechnologycanproduceasolid
freeeffluent,andenablesshortHRTandhighSRT.Toobtainaneffluentthatmeets
requirements of the environmental legislation regarding N and protects the
receiving water bodies, anaerobic membrane bioreactors can play an important
role with post-treatment systems based on biofilters and sequencing batch
reactorsamongothers.Muchprogresshasbeenachievedinthelastyearsinterms
of understanding the pollutants elimination from wastewater. However, some
challenges must still be overcome before a sustainable and efficient domestic
wastewatertreatmenttechnologyisachieved.
Keywords: Domestic wastewater • Nitrogen • Organic matter • SBR
•Biofilters.
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1. INTRODUCTION:WaterandWastewater
Theuncontrolleddisposaltotheenvironmentofdomestic,industrialandagriculturalliquid,
solidandgaseouswastesconstitutesoneofthemostseriousthreatstothesustainabilityofthe
human race because ofwater sources, land and air contamination and because its potential
contribution to globalwarming[1]. The amount and type ofwaste produced in households is
influencedbythebehavior,lifestyleandlifestandardofinhabitantsaswellasthetechnicaland
juridicalframeworkthatregulatesthedisposalstandards.Inthecaseofhouseholdwastes,the
compositionofwastewaterandsolidwastes fromhouseholds isaresultof thedistributionof
contributions from various sources within the household[2, 3]
. Moreover, every community
producesairemissions.Inthisintroduction,thetechnologiesreviewisfocusedintheanalysisof
thewastewatergenerationandtreatment.
The liquidwastewater is basically thewater supplied to the community after it has been
used inavarietyofapplications.Fromthestandpointofgenerationsources,wastewatermay
be defined as a combination of the liquidwastes removed from residences, institutions, and
commercialandindustrialestablishments,togetherwithsuchgroundwater,surfacewater,and
stormwaterasmaybepresent[4]. Increasinglyamountsofdomesticandindustrialsewageare
generated due to rapid population growth, expansion of cities and industrial development.
These increasing activities make it of ultimate importance to redouble efforts to maintain a
cleanandsafeenvironment[5,6]
.Inthiscontext,thequalityofwateriscentraltoalloftheroles
thatwater plays in our lives[7].Water is the source of life on earth, and human civilizations
blossomed where there was reliable and clean freshwater. Use of water by humans – for
drinking, washing, and recreation – requires water free of biological, chemical, and physical
contaminations. Plants, animals, and the habitats that support biological diversity also need
clean water to develop themselves.Water of a certain quality is also needed to grow food,
powercitiesandrunindustries[7].
Wastewatersaredischargedintoriversandstreams,whichcouldcausedeteriorationofthe
environment if thewastewater is not correctly adapted to the receiving source. This activity
modifiesthenatureoftheriverandreceivingbodies,whichcanprovokeseveralproblems,such
aseutrophicationandpollution.Asconsequence,bigproblemsforhumanhealthaswellasfor
aquaticfloraandfaunacanbearisen.Forthesereasons,wastewatersmustbetreatedbefore
they are discharged to the receiving water bodies. “Treatment” is defined as the process of
reducingthepollutantsintolessharmfulendproducts,adaptingthewastewatercompositionto
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28
a desired one that does not change the natural composition of the receiving bodies. The
processmaybeaccomplishedbyeitherphysical,chemical,orbiologicalmeans[8,9]
.
1.1.DOMESTICWASTEWATER:definition
Domesticwastewater,alsoknownasmunicipalwastewaterorsanitarywastewaterorsimply
sewage, is the usedwater, which has been discharged from the residential, commercial and
institutionalzonesofacityoratownoracommunityandcollectedthroughseweragesystem.
Sometimes, partially treated liquid wastes form small industries are also collected and
dischargedintothesanitarysewersandthusincludedwithdomesticwastewater[10]
.Domestic
wastewater is the most abundant type of wastewater that falls into the category of low-
strength waste streams, characterized by low organic strength and high particulate organic
mattercontent[11]
.Itiscomposedofhumanbodywastes(faecesandurine)togetherwiththe
waterused for flushing toilets, and thewastewater resulting frompersonalwashing, laundry,
foodpreparationandthecleaningofkitchenutensils[12]
.
1.2.DOMESTICWASTEWATER:constituentsandcomposition
Typical domestic wastewater consists of about 99.9% wt. water and 0.1% wt. pollutants.
About60 to80%of thepollutants are foundasdissolvedmaterial and the rest are foundas
suspendedmatter. The pollutants includemineral and organicmatters, suspended solids, oil
and grease, detergents, nitrogen, phosphorous, sulfur, phenols, and heavy metals among
others.Domesticwastewatersalsocontainlargeamountsofbacteriaandviruses,someofthem
pathogenic[1]. The constituents in domestic wastewater can be divided into nine main
categories,whicharedisplayedinTable1.
Theconcentrationsfoundinwastewaterresultfromacombinationofpollutantloadandthe
amountofwaterinwhichthepollutantis“diluted”.Thecompositionofamunicipalwastewater
variessignificantlyfromonelocationtoanother.Onagivenlocationthecompositionwillvary
withtimeduetovariationsinthedischargedamountsofsubstances.Thecompositionoftypical
domestic wastewater is shown in Table 2 where concentrated wastewater (high) represents
caseswithlowwaterconsumptionand/orinfiltration.Dilutedwastewater(low)representshigh
waterconsumptionand/orinfiltration.
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29
Table1.Categoriesofconstituentsfoundindomesticwastewater[2].
ConstituentsCategories ActualConstituents Risks
Microorganisms Pathogenicbacteria,virusand
wormseggs
Riskwhenbathingandeating
shellfish
Biodegradableorganic
materials
Oxygendepletioninrivers,
lakesandfjords
Fishdeath,odors
Otherorganicmaterials Detergents,pesticides,fat,oil
andgrease,coloring,solvents,
phenols,cyanide
Toxiceffect,aesthetic
inconveniences,bioaccumulation
inthefoodchain
Nutrients Nitrogen,phosphorus,
ammonium
Eutrophication,oxygendepletion,
toxiceffect
Metals Hg,Pb,Cd,Cr,Cy,Ni Toxiceffect,bioaccumulation
Otherinorganic
materials
Acids,forexamplehydrogen
sulfide,bases
Corrosion,toxiceffect
Thermaleffects Hotwater Changinglivingconditionsfor
floraandfauna
Odor(andtaste) Hydrogensulfide Aestheticinconveniences,toxic
effect
Radioactivity Toxiceffect,accumulation
Table2.Typicalcompositionofrawdomesticwastewater(ppm)[2].COD:ChemicalOxygenDemand.BOD:
BiologicalOxygenDemand.VFA:VolatileFattyAcids.NTotal:TotalNitrogen.Ammonia-N:Nitrogenas
ammonia.Ptotal:Totalphosphorus.Ortho-P:phosphorousasphosphate.TSS:TotalSuspendedSolids.
VSS:VolatilesSuspendedSolids.
ParameterHigh
concentration
Medium
concentration
Low
concentration
CODtotal 1200 750 500
CODsoluble 480 300 200
CODsuspended 720 450 300
BOD 560 350 230
VFA(asacetate) 80 30 10
Ntotal 100 60 30
Ammonia-N 75 45 20
Ptotal 25 15 6
Ortho-P 15 10 4
TSS 600 400 250
VSS 480 320 200
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30
Theorganicmatteristhemajorpollutantindomesticwastewater[2].Theamountoforganic
matter in domesticwastes determines the degree of biological treatment required[13]
. Some
studieshavereportedthattheorganicmatterindomesticwastewatersiscomposedmainlyof
proteins, lipids and carbohydrates.[12, 14, 15]
. Sincemostof thenutrients arenormally soluble,
they cannot be removed by settling, filtration, flotation or other means of solid-liquid
separation[2].
2.WASTEWATERTREATMENT
There isan increasingneedtodevelopreliabletechnologiesforthetreatmentofdomestic
wastewatertoprotectbothpublichealthandthoseofthereceivingbodiesorusers.Treatment
generallymeansthepartialreductionorcompleteeliminationoftheimpuritiespresent inthe
wastewater so that their concentration reaches an acceptable level for its final disposal or
properreuse.Definingthelevelofwastewatertreatmentandselectingthetreatmentprocesses
dependsmainlyon theeffluentquality standardsprescribedby the Law.A treatment system
has to remove suspended material, dissolved organic material, pathogens and, sometimes,
dissolved inorganic material. Such treatment systems must fulfill many requirements to be
feasibly implemented, such as simple design, use of non-sophisticated equipment, high
treatmentefficiency,andlowoperatingandcapitalcosts[1,10,16,17]
.
AsoutlinedinFigure1,aconventionaltreatmentplantconsistsofatrainof individualunit
processes set in a series,with the effluent of one process becoming the influent of the next
inline process. The sewage treatment processes can be classified in four groups: preliminary
treatment, primary treatment, secondary treatment and tertiary treatment.Many treatment
processes also generate sludge as by-product and there are several alternatives for sludge
treatment[1,16]
.
Figure1:Processdiagramofaconventionalsewagetreatmentplant.
Biological Reactor
Influent Effluent
Screen Grit
Primary Treatment
Primary Sedimentation
Tank
Secondary Sedimentation
Tank
Filter
Beds
Chlorine Mixing Tank
Preliminary Treatment
Secondary Treatment
Tertiary Treatment
SludgeRecycle
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31
A general treatment scheme includes a preliminary treatment (e.g. screens and grit
chambers)thatremovesmostofthecoarseandheavyinorganic(typicallygarbageandgrit)and
organic solids (coarse food particles). A large fraction of total suspended solids and a fair
proportionof theorganicmatter in suspendedsolids canbe removedbygravity inaprimary
sedimentationtank.Preliminaryandprimarytreatmentsarebasedonphysicalprocesses.The
secondary treatment is based on biological processes. Biological reactors are employed to
remove the biodegradable organics. Tertiary treatment is usually based on physicochemical
processes. Polishing to remove fine particles and disinfection are typically carried out in
filtration and chlorination or UV disinfection reactor respectively[16]
. Wastewater treatment
plants (WWTPs) have been evolved over the time to adapt to the growth of cities, the
environmental changes (including climate change), the economic conditions and, finally, the
requirementsofsocietyundertheinfluenceofbothenvironmentandeconomy[18,19]
.
Suspended solids are the most visible of all impurities in wastewater and may be either
organicorinorganicinnature.Itisthereforenotsurprisingthatthefirstwastewatertreatment
systems,introducedbytheendofthe19thcentury,weredesignedasunitsfortheseparationof
solids from liquidsbymeansofgravity settling:aprocessknownas theprimary treatmentof
wastewater.Whenthefirstefficientandreliabletreatmentunitsenteredintooperation,itsoon
became clear that these could treat wastewaters only partially for a simple reason: a large
fractionoftheorganicmaterialinwastewaterisnotsettleableandthereforeisnotremovedby
primary treatment. With the objective of improving the treatment efficiency of wastewater
treatmentplants,secondarytreatmentwasdevisedintheearlyyearsofthe20thcentury,and
nowformsthebasisofwastewatertreatmentworldwide.Secondarytreatmentischaracterized
bytheuseofbiologicalmethodstoremovetheorganicmaterialpresentinthewastewater[20,
21]. With appropriate analysis and environmental control, almost all wastewaters containing
biodegradable constituents with a BOD/COD ratio of 0.5 or greater can be treated easily by
biological means. In comparison to other methods of wastewater treatment, it also has the
advantagesofloweringtreatmentcostswithnosecondarypollution[22]
.
Both, aerobic and anaerobic, processes can be used as biological treatments to the
wastewater streams. Aerobic processes involve the use of free or dissolved oxygen by
microorganisms(aerobes)intheconversionoforganicwastestobiomassandCO2.Inanaerobic
processes,complexorganicwastesaredegradedintomethane,CO2andH2Othroughfourbasic
steps (hydrolysis, acidogenesis, acetogenesis andmethanogenesis) in the absence of oxygen.
Aerobicbiological processes are commonlyused in the treatmentoforganicwastewaters for
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32
achieving high degree of treatment efficiency. On the other hand, considerable progress has
been achieved in anaerobic biotechnology for waste treatment based on the concept of
resourcerecoveryandutilizationwhilestillachievingtheobjectiveofpollutioncontrol[22]
.
Spain,asamemberoftheEuropeanUnion,isobligedtocomplywiththeCommunityrules.
CouncilDirective91/271/EECof21May1991,establishedtheminimumrequirementsforthe
collection,treatmentanddisposalofdomesticwastewater.ThisDirectivewastransposedinto
Spanish law by Royal Decree Law 11/1995, committed to achieve good ecological status of
waters for 2015 set out in the Water Framework Directive (Directive 2000/60/EC of the
EuropeanParliamentandtheCouncilof23October2000,establishingaCommunityframework
actioninthefieldofwaterpolicy)[23]
.
Initially,thegoalofWWTPswastosimplyreleasethewaterofthedrainsfromthepollutants
beforedischarging itback to theenvironment.Asa result, theWWTPsweredesignedon the
principleoftheactivatedsludgeprocess.Aerationofmunicipalsewageresultedinanincreased
removalrateoforganicmaterial,whileatthesametimetheformationofmacroscopicflocswas
observed, which could be separated from the liquid phase by settling, forming a biological
sludge.Theadditionofthissludgetoanewbatchofwastewatertremendouslyacceleratethe
removal rate of the organicmaterial. The sludge bacteria, togetherwith some protozoa and
othermicrobes, are collectively referred to as activated sludge. The concept of treatment is
verysimple.Thebacteriaremovesmallorganiccarbonmoleculesby‘eating’them.Asaresult,
the bacteria grow, and the wastewater is cleansed. The activated sludge process is energy
consuminganddoesnottakeintoaccountthepotentialofenergyandnutrientrecovery[18,20,
21].Conventionalactivatedsludgerequireshighelectricalpowerconsumptionforpumpingand
aeration. The excess of sludge generated in this system is a secondary solid waste, and its
disposal isamajorenvironmentalconcern[19, 24]
.Furthermore, this technology is inefficient in
eliminatingcontaminants,resultingthusintheirdisseminationintotheenvironment.Advanced
effluent treatment has also severe limitations depending on the type of treatment and
compoundtoberemoved.Allofthemcanonlyremovecertaincompoundscompletely.Some
compoundsareremovedonlypartiallyandothersarenotremovedatall[25]
.
The technological achievements in the fields of monitoring and controlling the design of
stable and efficient processes (both physicochemical and biological) together with the
development of suitable benchmarking and economic tools have begun to change the
philosophy of WWTPs from treatment to valorization facilities. This means that sewage
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33
treatment should be incorporated into a more holistic management scheme, which aims at
reducing thepollutants aswell asenhancingnutrient,water andenergy recycling inorder to
maintaintheenvironment’sintegrityinaneconomicfeasiblebutalsoefficientway[18]
.
The presence ofmineral compounds in the effluent, especially the nutrients nitrogen and
phosphorus, could cause a serious disruption of the ecological equilibrium in the receiving
water[20]
.Someoftheproblemsofexcessivenutrientsinwaterbodiesincludereducedoxygen
concentrationinwater,whichcanleadtofishdeath,eutrophication,andover-fertilization[26]
.
Eutrophicationreduceswaterquality,alterstheecologicalstructureandfunctionoffreshwater,
and poses many potential hazards to human and animal health[27]
. The increasing public
concern for environmental protection has led to stricter nutrient discharge standards in
domesticwastewater[28]
.Asconsequence, toprotect thewaterquality in thereceivingwater
bodies, most of the efforts have been focused on the development of new technologies in
which, inadditiontotheremovalofsuspendedsolidsandorganicmaterial,alsothenutrients
nitrogen and phosphoruswere eliminated[20, 28]
. A variety of physicochemical, chemical, and
biological methods have been used to remove nutrients from wastewater[26]
. However,
biologicalnitrogenremovalispreferredoverphysicochemicalprocessesbecauseitiscapableof
removingfixednitrogenouscompoundstoharmlessdinitrogengas(N2)inamoreeffectiveand
economicalway[27]
.
Inthenextthreesections,thebiologicalprocessofnitrogeneliminationisreviewed.Section
2.1 in this chapter reviews themechanismsofnitrogenelimination fromwastewater. Section
2.2 reviews the Sequential Batch Reactor Technology and finally, section 2.3 reviews the
developed technologies of anaerobic biological treatments for nitrogen and organic matter
removalfromwastewaterasasecondarytreatmentinaWWTP.
2.1.NITROGENREMOVALFROMWASTEWATER:Biologicalmechanisms
Nitrogenisessentialforlife,asitisthefourthmostabundantelementinthebiosphere.The
N cycle in the biosphere is governed by various catabolic processes, anabolic processes and
ammonification.TheseprocesseshavebeenengineeredovertheyearsandappliedinWWTPs
toimplementbiologicalNremovaltoproduceeffluentswithalowerenvironmentalimpact[28]
.
2.1.1.ConventionalNitrification/Denitrification.
Inthe1950s,additionaltotheorganicmaterialremoval,nitrificationwasintroducedinthe
activated sludge process[20]
. Conventional N removal comprises two completely different
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34
microbialprocesses:nitrificationanddenitrification[29]
.
Nitrificationisatwo-stepbiologicaloxidationofammonium,usingoxygen[20]
.Thefirststep,
nitritation(Eq.[1]),istheoxidationofammoniumtonitrite(NO2
-).TheNH4
+servesasNsource
forthesynthesisofnewbiomassandasthesoleenergysourceforthegrowthofammonium-
oxidizingbacteria(AOB).
Nitritation:NH4
++2HCO3
-+1.5O2→NO2
-+2CO2+3H2O[1]
Thesecondstep,nitratation(Eq.[2]),istheoxidationofnitritetoproducenitrate(NO3
-)by
nitrite-oxidizingbacteria[30]
.
Nitratation:NO2
-+1.5O2→NO3
-[2]
ThecompleteoxidationofNH4
+toNO3
-byAOBandNOBisoverallcallednitrification
[20,28].
Bothfunctionalgroupsofnitrifiersareaerobicandchemolithoautotrophic[29]
.
Denitrificationisthereductionofnitrate(Eq.[3])tonitrogengas. It isasequentialprocess
that consists of the following reduction steps: NO3
-to NO2
-, nitric oxide (NO), nitrous oxide
(N2O),andN2.Biologicaldenitrification iscarriedoutentirelybyheterotrophicbacteria,which
requiresabiodegradableorganiccarbonsourceasanelectrondonortocompletethereduction
process[28]
.Denitrificationonlydevelops in an anoxic environment,which is characterizedby
thepresenceofnitrateornitriteandtheabsenceofdissolvedoxygen.
Heterotrophicdenitrificationovernitrate:
NO3
-+1.08CH3OH+0.24H2CO3→0.056C5H7O2N+0.47N2+HCO3
-+1.68H2O[3]
Asthenitrifyingprocessisextremelyslowcomparedtodenitrification,twoseparatereactors
toaccommodatedifferentsetsofconditionsarerequired.
In the firstunits constructed forbiologicalnitrogen removal, thenitrifiedeffluent froman
activatedsludgeprocesswasdischargedinasecondreactor,operatedwithoutaeration.Inthis
second reactor, thedemandoforganic carbonwasoftennot satisfiedbecauseof thehighN
load and relative low carbon content of thewastewater. To increase thedenitrification rates
under such conditions, usually readily biodegradable organic compounds like methanol and
acetatewasaddedtothesecondreactor[20, 29]
.Thus,thetreatmentsystemwascomposedof
two reactors with different sludge, the first one being for organic material removal and
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35
nitrification;andthesecondone,fordenitrification.However,itwasestablishedsoonthatthe
organicmaterialpresentinthewastewatercouldbeverywellusedfornitratereduction[20,29]
.
Themodifieddesignsplacedthedenitrificationtankbeforetheaerobicstage.Therefore,the
classic bioreactor configuration to perform nitrification-denitrification consisted of an anoxic
tank followed inusedbyanaerobic tankandthesecondarysettler. In thisprocess,knownas
A/O,thedenitrificationtankdirectlyreceivesthewastewatercontainingrelativelyhighamounts
of carbon sources, and external organic material is not needed. Two recirculation flows are
traditionallyused:(1)internalrecirculationfromtheaerobiccompartmenttotheanoxictankto
supplyelectronacceptorsfordenitrification(NO2
-andNO3
-)and(2)externalrecirculationfrom
the secondary settler to the biological process inflow tomaintain a target biomass retention
time (normally higher than7days) andaproper sludge concentration. Theseprocesseshave
unaeratedzonesfordenitrificationandaeratedzoneswherenitrificationtakesplacetogether
withorganicmaterial removal.An important issue is theaeration,whichmustbeadjusted to
provide enough dissolved oxygen (DO) for nitrification (3.16 g O2 g-1 NH4
+) but avoiding
unnecessary energy consumption. The aeration requirements represent one of the main
fractionsof the treatmentcost inWWTPsperformingconventionalnitrification/denitrification
[20,28,29].InWWTPitiscommontoincludeananaerobictankbeforetheanoxic/oxicstages.This
process A2/O with separate anaerobic, anoxic, and aerobic tanks is a suitable method for
biologicalnitrogenremoval(schemashowninFigure2).DenitrificationoftheNO3
-recirculated
from a downstream aerobic tank occurs in an anoxic tank where denitrifiers can utilize the
organic matter present in the influent, avoiding the need for an additional organic carbon
source.However,theA2/Oconfigurationnormallyrequiresahighmixedliquidreturnratio(2–
4Qin)fromtheaerobiczonetotheanoxiczonetobringmoreNO3
-backfordenitrification.High
returnratioscanresultinaDOconcentrationincreaseandCODdilutionintheanoxiczone.This
inevitablydeterioratesthedenitrificationefficiency,especiallywhentheorganicmatterpresent
intheinfluentwastewaterisinsufficienttodepletetheDOpresentintherecycledmixedliquor.
Inaddition,highreturnratiosalsoleadtohigherenergyconsumptionandincreasedoperating
costs[27]
.
The denitrification potential ofwastewater is primarily a function of the available organic
carbon, usually expressed as chemical oxygen demand (COD)/nitrogen(N) or carbon/nitrogen
(C/N)[27]
.OneofthemainfactorslimitingthenitrogenremovalefficiencyinmunicipalWWTPs
is theC/Nratio.Typicalvalues indomesticwastewater rangebetween10.5and12.5andare
sufficient tocompletethedenitrificationof totalN (TN)of the influentwastewater.However,
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36
thesludgedigestionrecirculationtotheinflowoftheplantorthereceptionofN-richexternal
inputsmay increase theN loadof theWWTP.Thismeansactual lowerC/N ratios that fail to
meet the discharge quality standards[28]
. One way to get satisfactory nitrogen removal
performance forwastewaterwith a C/N ratio lower than the critical value is to introduce an
innovative nitrogen removal pathway, or treatment processes, which can support nitrogen
removal with low or zero organic carbon demand. These pathways are presented below. An
alternativewayistoaddexternalcarbonfordenitrification[27]
.
Figure2:Schematicdiagramoftheanaerobic/anoxic/oxic(A2/O)process.
2.1.2.SimultaneousNitrification/Denitrification
As seen before, the nitrification and denitrification processes are usually carried out
separatelyinaerobicandanoxiccompartments,respectively.However,asithasbeenreported,
some heterotrophic nitrifiers could denitrify nitrite and nitrate aerobically. Nitrification and
denitrificationtakeplaceconcurrentlyinasinglereactorunderaerobicconditions.Thisisoften
referred as Simultaneous Nitrification/Denitrification (SND) process. Generally, SND occurs
naturally inside microbial biofilms and flocs due to the dissolved oxygen (DO) gradient
established across the biomass. The biodegradable organic matter availability in the deep
biofilmregions,theDOconcentrationgradientsandtheflocsizearethethreemainparameters
affectingSNDperformance.Inthissense,alimitedDOlevelinthebulkliquid(0.5-1.5g-O2m-3)
favors thepresenceofSND inaerobic tanks.TheoptimalC/Nratio forSNDwascalculatedat
11.1,wherethenitrificationanddenitrificationreactionsarebalanced[28,31]
.
SNDismorecosteffectivethantheconventionalprocessbecausetheC-sourceconsumption
is22–40%lowerandthesludgeyieldisreducedby30%.DuetothelowDOlevelsetpointused,
theaerationintensityisalsoreduced.SNDisperformedinasinglereactor,whichrepresentsa
smaller footprint, and could be a good solution to upgrade WWTP without expanding the
Anaerobic Anoxic Aerobic C
Air
Sludgerecirculation
Nitraterecycle
Wastesludge
Settler
Effluent
Influent
Page 49
37
existing facilities. It could be also considered an option to treat domestic wastewater with
relativelylowC/Nand/orinorganicClimitationforautotrophicnitrifiers[28,32,33]
.
2.1.3.NremovaloverNO2
-.
Nitrite(NO2
-)isanintermediateinboth,nitrificationanddenitrificationpathways.Inthe
combinednitrification/denitrificationprocess,NH4
+isoxidizedtoNO2
-andthentoNO3
-,which
isagainconvertedtoNO2
-beforeN2gasformation.Therefore,theproductionofNO3
-isnot
requiredtocompletethewholeN-removalprocess[28]
.Thepartialnitrificationpathwaymaybe
formedbycontrollingtheNH4
+oxidationtoNO2
-(nitritation)insteadoftoNO3
-(nitratation)and
thecoupledbyreductionoftheaccumulatedNO2
-viadenitrification
[27].
AscanbeseeninEquations[4]and[5],theapplicationoftheshortcutnitrificationfollowed
by denitrification of NO2
-instead of complete nitrification/denitrification can reduce the
treatment costs due to 25% less aeration and 40% less biodegradable COD consumption.
Therefore,theprocessbecomeshighlycosteffectiveforthetreatmentofwastewaterwithlow
C/N ratio, as part of the methanol addition can be saved. Moreover, it is known that
denitrification rates for NO2
-are 1.5-2 times faster than NO3
-denitrification rates, allowing
higher removal capacities. Moreover, sludge production is reduced by 40% in shortcut
nitrification/denitrification[27,28,34,35]
.
NH4+removalviaNO3
-(nitrification/denitrification)
NH4
++2O2+4gCOD→0.5N2+H2O+H
++1.5gbiomass[4]
Shortcutnitrification/denitrification(nitritation/denitritation):�
NH4
++1.5O2+2.4gCOD→0.5N2+H2O+H
++0.9gbiomass[5]
Unfortunately,NO2
--Naccumulationisdifficulttoattain.
Thekeyfactoristolimitasmuchas
possibletheoxidationofNO2
-toNO3
-.AlthoughNOBgenerallyhavehighersubstrateutilization
rates than AOB, a forced biological conversion through the NO2
- route has been successfully
obtained by different approaches. This is always based on the different physiological
characteristics of AOB and NOB and their responses to three environmental factors: the
temperature,theDO,andtheconcentrationoffreeammonia(FA)andfreenitrousacid(FNA).
Thesefactorscanvarysignificantlyandunpredictablyinwastewaterandtreatmentprocesses,
soitisdifficulttoachieveandmaintainhighremovalvianitrite[27,28,36]
.
2.1.4.TheAnammoxprocess.
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38
The classical N removal pathway via nitrification and denitrification is costly, because the
nitrification stagemust be aerated and, if needed, organic carbon sourcesmust be added to
maintaindenitrification.Therefore,currentinterestfocusesonpathwaystoNeliminationthat
require less aeration and external carbon supply. In this context, the anaerobic ammonium
oxidizers(Anammox)arehighlyrelevant.TheseautotrophicbacteriacanoxidizeNH4
+andNO2
-
aselectronacceptortoproduceN2andasmallpartofNO3
-underanoxicconditionswithoutthe
requirementof anorganic carbon source (Eq. [6]). Thus, this anaerobic process constitutes a
‘shortcut’ in the N cycle. Future full-scale implementations of the Anammox process could
markedlyreducethespacerequirementsandcostsofNremovalfromwastewater[27,29]
.
Anaerobicammoniumoxidation�
NH4
++1.32NO2
-+0.066HCO3
-+0.13H
+→1.02N2+0.26NO3
-+0.066CH2O0.5N0.15+2.03H2O[6]
Accordingtothestoichiometry(Eq.[6]),89%oftheincomingN(NH4
+plusNO2
-)isconverted
to N2 gas, while the rest (11%) corresponds to NO3
-production. From the environmental
engineeringpointofview,thisNO3
-producedisconsideredasawasteoftheAnammoxprocess
and must be treated further. The anaerobic ammonium oxidation reaction requires a NO2
-
supply.Therefore,theprocessneedstobecoupledtoapartialnitrificationprocess,inorderto
aerobicallyoxidize60%oftheNH4
+ofthewastewatertoNO2
-.Comparedtotheconventional
biological nitrogen removal, theAnammoxprocesspresents several advantages such as: 63%
lessoxygendemandand100%savingsonanexternalCsourcefordenitrification,becauseitisa
low-oxygen consumingprocess[27, 28, 37]
.Moreover, theAnammoxprocesshas the interesting
characteristicsofverylowproductionofsludge,andverylowCO2,N
2O,andNOemissions.For
all these reasons, the Anammox process as a cost-effective and energy-saving biotechnology
hasagreatpotentialinthetreatmentofNH4
+-richwastewaterswithverylowC/Nratio,suchas
sludgetreatmenteffluents[27,28,38]
.
The main handicap to implement this process is the slow growth rate of anaerobic
ammonium-oxidizing bacteria. Long start-upperiods are required evenwhenworking at high
temperatures, limiting its application[28, 38]
. In addition, several environmental factors can
perturbtheAnammoxprocessandaffecttheN-removalefficiency.Theoptimumtemperature
foramaximumgrowthrateofanaerobicammonium-oxidizingbacteriawassetat35-37ºC,but
recent works have obtained high N-removal efficiencies in reactors operated at low
temperatures (<20ºC)[28]
.Currently, theAnammoxprocess is still confined toa few typesof
wastewaters(sludgedigestateandanimalwastewaters)[38]
.
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39
2.1.5.Alternativetreatmentprocessesorreactors.
Common biological nitrogen removal processes occur in various treatments train
configurationsinWWTPs,includingtheA/OprocessandtheA2/Oprocess.Alltheseprocesses
rely on a predenitrification zonewhere a portion of the nitrifiedwastewater is recycled and
mixedwiththeinfluenttoserveasanelectrondonorfordenitrification.Disadvantagesinclude
theneed for high recirculation rates and the additionof external carbon substratewhen the
influent C/N ratio is not high enough. To overcome this situation, advanced process control
methods,newbiologicaltreatmentprocessesandreactorssuchasthemodifiedA2/Oprocess,
themultistageA/Ostep-feedprocesshavebeendeveloped[27]
.
2.1.5.1.ModifiedA2/Oprocess.
ThemodifiedA2/Oprocessavoidstheabovedisadvantages(Figure3).InthemodifiedA
2/O
process[27]
:
1. Therecyclesludgeisdirectedtoaseparatepreanoxicbasinwherehydrolysisprocesses
canreleasebiodegradableorganiccarbon.Thiscarboncanbeusedinthedenitrification
processestakingplacedownstream.
2. The influent wastewater goes directly to the anaerobic zone of the reactor and gets
mixedwiththewastewater fromthepre-anoxictank.Partofmixedwastewater inthis
reactor is recirculated to the postanoxic zone at a ratio of 0.4Qin to provide available
organiccarbonfordenitrification.
3. Inthefirstaerobiczone,NH4
+isoxidizedtoNO2
-andNO3
-,andbotharefedcontinuously
totheso-calledpostanoxiczonefordenitrification.
4. Thetreatedwastewaterpassesthroughafinalaerobictanktominimizetheamountof
COD in the effluent, and enhances the settling ability of the sludge by minimizing
denitrification in thesecondarysettler.Pilot-scale results showedmore than88%COD
and70%TNwasremoved.
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40
Figure3:SchematicdiagramofthemodifiedA2/Oprocess.
2.1.5.2.Step-feedmultistageA/Oprocess.
The step-feedmultistage A/O process consists of two ormore denitrification–nitrification
units in serieswithwastewater distributed at several reactor points so that an internal NO3
-
recirculationisunnecessary.Aschemaofstep-feedmultistageA/OprocessisshowninFigure4.
The biodegradable organic material in the influent is utilized for denitrification, and also
simultaneousnitrification/denitrificationmayoccurinthisprocess[27]
.
Figure4:Schematicdiagramofthestep-feedingmultistageanaerobic/oxic(A/O)process.
2.2.ORGANICMATTERREMOVALFROMWASTEWATER:Anaerobicbiologicaltreatment.
The anaerobic process operates in absence of molecular oxygen in the reactor for the
growthofmicrobesandnormallyfailsinthepresenceofexcessiveoxygen.Removaloforganic
contentinwastewateriscarriedoutbyanaerobicandfacultativemicroorganismsbystabilizing
theorganicmatterintoliquid,gases(mainlymethaneandcarbondioxide)andotherstableend
productsintheabsenceofoxygen[10]
.
C
Air
Sludgerecirculation Wastesludge
Settler
Effluent Influent
Anaerobic Anoxic Aerobic Aerobic
C
Pre-anoxic
Air
Sludgerecirculation Wastesludge
Settler
Effluent
Influent
Page 53
41
Though the process was primarily developed for stabilization and volume reduction of
wastewater sludge, it was later on employed for the treatment of industrial wastewater
containing high organic wastes[10]
. Anaerobic treatment of wastewaters is nowadays widely
acceptedasaprobedtechnologyandextensivelyused[39]
.Comparedtothemostconventional
aerobicprocess,anaerobicprocessshouldbeconsideredfordomesticwastewatertreatmentas
an alternative because of a variety of reasons. Anaerobic treatment can be carried out with
technicallysimplesetups,atanyscale,andatalmostanyplace.Itproducesasmallamountof
excess,well stabilized sludge, and energy can be recovered in the form of biogas[17, 40]
. The
sludgequantitiesproducedintheanaerobicprocessaremuchsmallerthatthesludgequantity
formedwhiledecomposingthesameamountoforganicmatterunderanaerobicpathway.Only
about5-15%oftheorganiccarbonisconvertedtobiomassduringanaerobicdecompositionof
organicmatter,while inaerobicdecomposition,theequivalentnumber isabout50-60%[1]. In
addition to the energy that can be recovered from methane-rich biogas, the application of
anaerobic processes distinctly reduces the overall energy demand for municipal wastewater
treatmentbecausenoaerationenergy is required formineralizing theorganics[11]
. Complete
anaerobic treatment of domestic wastewater has the potential to achieve net energy
production while meeting stringent effluent COD standards[37]
. Anaerobic treatment of
domesticwastewater is receiving increasedattentionbecauseof the recognizedpotential for
net energy recovery and low sludge production when compared with traditional aerobic
processes[41]
.
Both aerobic and anaerobic systems are capable of achieving high organic removal
efficiency. In general, aerobic systems are suitable for the treatment of low strength
wastewaters(biodegradableCODconcentrationslessthan1000mg/L)whileanaerobicsystems
aresuitableforthetreatmentofhighstrengthwastewaters(biodegradableCODconcentrations
over4000mg/L).Anaerobicprocessesachieveorganicremovalintherange40-85%depending
onthetypeofreactorused.Theadvantagesofanaerobictreatmentoutweightheadvantages
of aerobic treatmentwhen treating influentswithhigh concentrations. In addition, anaerobic
treatment generally requires less energy with potential bioenergy and nutrient recovery.
However,compared toanaerobicsystems,aerobicsystemsachievehigher removalof soluble
biodegradableorganicmattermaterialandtheproducedbiomassisgenerallywellflocculated,
resultinginlowereffluentsuspendedsolidsconcentration.Asaresult,theeffluentqualityfrom
anaerobicsystemisgenerallyhigherthantheanaerobicsystem[1,22]
.
Amongthedrawbacksintheuseofanaerobicreactorfordomesticwastewaterarefound:
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42
- Temperature: anaerobic wastewater treatment becomes increasingly difficult as
temperaturesdropbelow20ºC[42]
.Thesedifficulties canbeattributed tochanges in
the physico-chemical nature of the wastewater and sludge and the slowing of
biochemicalreactions.Bothhaveconsequencesforthemicrobiologicalprocessesinthe
different trophic levels of anaerobic digestion: hydrolysis, acid- and acetogenesis and
methanogenesis.
- Regardingnutrients,theeffluentqualitydonotmeettherequirementforwastewater
effluenttosurfacereceivers[43,44]
.
- Biomass: their slow growth rates could create challenges in treating wastewater,
especiallyinstart-upperiods,duetowashoutoftheseslowgrowingmicroorganisms[45,
46].
2.2.1.Upflowanaerobicsludgeblanket(UASB).
Thereisalargevarietyoftypesofanaerobicreactorsfortreatmentofwastewaterincluding:
anaerobic digesters of excess sludge, septic tanks, anaerobic lagoons, rotating bed reactor,
expandedbedreactor,fluidizedbedreactor,upflowanaerobicsludgeblanket(UASB),expanded
bed granular reactor[1]. One of the most employed technologies is the UASB that has
successfullybeenusedtotreatavarietyofwastewaters[39]
.
The successof theUASB reactor relieson theestablishmentof adense sludgebed in the
bottom of the reactor where all biological processes take place. This sludge bed is basically
formed by accumulation of incoming suspended solids and bacterial growth. Under certain
conditions,bacteriacannaturallyaggregateinflocksandgranules[39]
.Thegranuleshaveahigh
density,excellentmechanicalstrength,andahighsettlingvelocity incombinationwithahigh
specific methanogenic activity. The granules form a blanket through which the influent
wastewater flows.Organic substances in thewastewateraredigestedbyanaerobicmicrobes,
whilethewastewaterflowsthroughthissludgeblanket.Asaresultofanaerobicdigestionofthe
organic substances, biogas consisting of methane, carbon dioxide, hydrogen, nitrogen,
hydrogen sulfide, etc. is generated[40]
. Theoptimaloperational conditionsofupflowvelocity,
influent COD, pH and temperature are needed for an efficient biological treatment of
wastewatertoproducebiogasintheUASBreactor[47]
.Duetoitshighbiomassconcentrations,
the conversion rate in UASB is several times higher than that in conventional anaerobic
processes[40]
.
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43
Natural turbulence caused by the influent flow rate and biogas production provides good
wastewaterbiomass-contactinUASBsystems[39]
.Neithermechanicalmixingwithinthereactor
norrecirculationofsludgeandeffluentisneeded,resultinginlittleexternalenergyrequirement
[40].
NumerousstudiesonsmallandlargescaleUASBprocesseshavebeenrapidlyrecognizedas
agoodoptioninthetreatmentofsewage[40]
.Althoughthistechnologycannotbyitselfproduce
an effluent of the quality of a convention secondary process like activated sludge, it can still
achievesignificantorganicmatterremovalratesintherangeof60-75%ofBOD5atafractionof
theconstruction,operatingandmaintenancecostsofactivatedsludge[1].
Among existing anaerobic treatment processes, the UASB process has to a large extent
proventosatisfythefactors.ThepositivefactorshavemadeUASBanattractiveoptionforthe
treatmentofmunicipalsewageindevelopingcountriesbecauseofthewarmclimaticconditions
[39, 40, 48]. However, UASB treatments also have disadvantages. The main advantages and
disadvantagesofUASBreactorsusedforthetreatmentofdomesticwastewateraredescribed
inTable3[10,39,40,49]
.
Table3.AdvantagesanddisadvantagesofUASBreactors.
UASBadvantages UASBdisadvantages
Goodremovalefficiency,evenathighloading
ratesandlowtemperatures.
Longstartuptakesbeforesteadystate
operation,duetothelowgrowthrateof
methanogenicorganisms.
Constructionandoperationrelativelysimple. Hydrogensulfideisproducedandaproper
handlingofthebiogasisrequired.
Highlyskilledpersonnelforitsoperationnot
required.
Lossofdissolvedmethaneintheeffluent(loss
ofenergyandhighglobalwarningpotential).
Processtolerantofflowvariationsorshock
loads.
Propertemperaturecontrol(15-35ºC)required
forcolderclimates.
Highstrengthwastewatercanbetreated
withnoenergypenalty.
Post-treatmentoftheeffluentisgenerally
requiredtoreachthedischargestandardsfor
organicmatter,nutrientsandpathogens.
Sludgeproductionlowercomparedto
conventionalaerobicmethods,duetothe
slowgrowthratesofanaerobicbacteria.
Lownutrientsandchemicalrequirement.
2.2.2.AnaerobicMembraneBioreactors(AnMBR).
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44
Anaerobic membrane technology brings together the advantages of anaerobic processes
with the production of solid free effluent, which provides an appropriate alternative to
complete biomass retention, enabling short hydraulic residence time (HRT) and high solid
retentiontime(SRT)[46,49,50]
.Asaconsequence,theparticulateorganicsretainedinthereactor
caneventuallybehydrolyzedanddecomposedbecauseofthelongsolidsretentiontime.Also
the AnMBR allows the anaerobic microbes proliferate without being washed out from the
process[46]
. One of the goals of anaerobic treatment processes is to maintain a long SRT
because of the slow growth rate of anaerobicmicroorganisms, especially when operating at
psychrophilic conditionsandwith low strengthwastewater, suchasdomesticwastewater[49]
.
AnMBRshavebeendemonstratedtobecapableofachievinghigheffluentquality in termsof
suspended solids, chemical oxygen demand (COD), and pathogen count, even at low
temperatures, thus demonstrating their potential formeetingmore rigorous effluent quality
requirements[41,49]
.
2.3.AnMBR+NREMOVAL.
Theeffluentsfromanaerobicreactorsrarelymeetdischargestandardsforwastewaterreuse
due to the kinetic limitations of anaerobicmetabolism. In contrast to the high COD and TSS
elimination,theremovalofnitrogenorphosphorusintheAnMBRsystemsisusuallynegligible.
The low removal of nitrogen and phosphorus is expected because both nutrients removal
processesrequiredanoxicoraerobiczone.Thiscanbebeneficialiftheeffluentistobeusedfor
agriculture or irrigation purpose. However, in most cases, this means that the downstream
treatmentisneedediftheeffluentistobereclaimed[51]
.
Theanaerobiceffluentsreactorsusuallyrequireapost-treatmentstepasameanstoadapt
the treated effluent to the requirements of the environmental legislation and protect the
receivingwaterbodies[52,53]
.Themainroleofthepost-treatmentistocompletetheremovalof
organicmatter, aswell as to remove constituents little affected by the anaerobic treatment,
such as nutrients (N and P) and pathogenic organisms (viruses, bacteria, protozoans and
helminths)[52]
.
When nitrogen removal has to be accomplished, the application of nitrification–
denitrificationprocessesaresofarselectedtocomplementtheUASBreactor[54]
.Insuchcase,
theanaerobic reactor shouldbeused to treat initiallyonly apartof the influent raw sewage
(possiblynomorethan50–70%),andtheremainingpart (30–50%)shouldbedirectedtothe
complementarybiologicaltreatment,aimingatnitrificationanddenitrification,sothatthereis
Page 57
45
enoughorganicmatterforthedenitrificationstep[52,54]
.
To couplewith nitrogen removal limitation, anaerobicmembrane bioreactors can play an
important role with post-treatment systems based on biofilters, sponge-bed filters and
sequencingbatchreactorsamongothers.
Among the different possible types of post-treatment for the removal of nitrogen, down
below,sequencingbatchsystemsandbiofiltersarepresented.
2.3.1.Sequencingbatchsystems
Sequencing batch reactors (SBR) are considered as fill and draw version of the activated
sludgeprocess.SBRsarebasicallysuspendedgrowthbiologicalwastewatertreatmentreactors,
inwhichallthemetabolicreactionsandsolid-liquidseparationtakesplaceinonetankandina
well-definedandcontinuouslyrepeatedtimesequence.[55]
.
The first activated sludge systems were composed of a single reactor that processed
sequential batches of wastewater for a certain period while aeration was applied. This was
followedbyaperiodinwhichtheaerationwasswitchedoff,whichtransformedthereactorinto
a settler. From there, the effluent was discharged and a new batch could be taken in. SBR
operatesundera seriesofperiods thatconstituteacycle. Thecyclegenerallyconsistsof fill,
react,settle,dischargeandanoptionalperiodofpause(seeFigure5)[20,55]
.
(1) Fill:awastewaterbatchisfedtothesludgemassalreadypresentinthetankfromthe
previouscycle.Duringthisphasetheaeratormayormaynotbeswitchedon.
(2) React:Reactionsforsubstrateremoval initiatedduringfillarecompletedduringreact.
The treatment is controlled by air, either on or off, to produce anoxic and aerobic
conditions. Controlling the time of mixing and/or aeration produces the degree of
treatmentrequired.
(3) Settle: sludge settling in the reactor. The entire tank acts as a clarifier without any
infloworoutflow.Aerationand/ormixersoff.
(4) Discharge:theclarifiedsupernatant(treatedeffluent)isdischargedfromthereactoras
effluentand,ifrequired,excesssludgeiswithdrawnaswell.
(5) Pause:optionalphasewhichisgenerallyrequiredwhenseveralSBRsareinoperation.
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46
Figure5:Typicaloperationalcycleofasequentialbatchreactor(SBR).
MultifunctionalSBR,allows the removalofnotonly the remainingCODbutalsonutrients.
ThecarbonandnutrientremovalefficienciesofinSBRvarywiththedurationofthecycletime
and time for each phase of the process in a cycle of operation. The cycle time dictates the
numberofcyclesperday,thevolumeofreactorrequiredandthecostoftheWWTsystemand
is basedon the strengthof thewastewater.Normally, the systemasbatchprocess doesnot
requiresecondaryclarifierandpumpingofreturnactivatedsludge[20,56,57]
.
The SBR processes are known to save more than 60% of the expenses required for
conventional activated sludgeprocess inoperating costandachievehigheffluentquality ina
veryshortaerationtime[55]
.SBRtechnologyismoreadvantageousthantheextendedaeration
process due to higher COD and N removal rates at comparatively shorter HRT. Other
advantagesattributedtoSBRapartofthegoodeffluentquality,arethesimplicityofoperation
andthelowerinvestmentcosts,duetotheabsenceofafinalsettler.Onedisadvantagethatis
oftenattributedtoSBRsystemsistheinflexibilityindealingwithflowvariations,astheSBRonly
receivesinfluentduringaminorpartofthetotalcycletime[20,58]
.
Conventional activated sludge systems are space oriented. Wastewater flow moves from
onetankintothenextonacontinuousbasisandvirtuallyalltankshaveapredeterminedliquid
volume. The SBR, on the other hand, is a time-oriented system, with pre determined flow,
energy input and tank volume varying according to some predetermined, periodic operating
strategy[56,59]
.
In its original version, the activated sludge process was operated as a batch process.
Althoughthisactivatedsludgeprocesshasbeenreplacedgraduallybyotherconfigurations, it
Phase1:Fill Phase2:React Phase3:Settle
Phase4:Discharge Phase5:Pause
Aerationon Aerationon/off Aerationoff
Aerationoff Aerationoff
Influent
Effluent
Excesssludge
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47
has survived in the form of SBR. The SBR has regained popularity over the last decades,
especiallyforapplicationtosmallerwastewaterstreams[20]
.
2.3.2.Biofilters
Biofiltration seems tobean interestingoption forefficiently removeboth residualorganic
matterandnutrientsfromdomesticwastewater[60]
.
Thesubmergedaeratedfilters(SAFs)arebiofilmsystemsinwhichabiofilmsupportmedium
issubmergedinwastewatertocreatea largecontactareaforaerobicbiologicaltreatment[61,
62].Duetotheimmobilizationofbiomassonmedia,thelossofbiomassbyshearingistheonly
mechanism for the escape of biosolids in the bioreactor effluent. The sloughed biomass has
goodsettlingcharacteristicsandcanbereadilyseparatedfromtheliquid[63]
.Asorganicmatter
and nutrients are absorbed from the wastewater, the film of biological growth grows and
thickens[64]
.
There are twomain configurations for denitrification filters commercially available: down
flow and up flow continuous backwash filters. Down flow denitrification filters operate in a
conventionalfiltrationmodeandconsistofmediaandsupportgravellayingonanunderdrain.
In up flow continuous-backwash filters, wastewater flows upward through the filter,
countercurrenttothemovementofthesandbed[65]
.
Biofiltration systems are typically robust, simple to construct and have low energy
requirements[60, 66]
. Themost salient advantages are: no problemswith bulking sludge, high
sludge age enables degradation of complex compounds and biofilm mitigates inhibition and
toxic impacts[61, 63]
. The biofilter can be used in aerated and unaeratedmodes. Thus, these
systemscanbedesigned forcarbonremoval,nitrificationand/ordenitrificationdependingon
processobjectives[61]
.
3. FINALREMARKS
Muchprogresshasbeenachievedinthelastyearsintermsofunderstandingthepollutants
elimination from waste water. This progress has been accompanied and motivated by
increasinglegislationtowardsacleanerandsaferworld.Thisrepresentsapromisingscenarioto
thewastewatertreatmentcompaniesandthetechnologydevelopersinresearchinstitutesand
universities. The current needs of the wastewater “system” point to the development of
combined processes of pollutant abatement while transforming it into useful products. In
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48
addition, this development should be accompaniedby intensified processes, getting compact
apparatus able to runwithhigh yields, and selectivity’s. Improving thebiology,mass transfer
and chemistry of the process will allow the development of this king of process in reduced
treatmenttimes.Attheend,compactandefficientprocessesallowamassiveimplementation
ofthetechnologyinaneconomicalway.
The main challenges of the nitrogen removal technology lie in the development of an
adequate combination of biological reactor and organicmatter usage,which allows: (a) COD
elimination and its non-contaminant recycling to facilitates nitrogen elimination, (b) fast and
selectivereactorand(c)economicallyfeasibleconfigurations.
Toaddresstheprobleminvolvedwiththenitrogenremovalindomesticwastewater,inthis
PhDThesisaredevelopeddifferentreactorconfigurationsanddifferentreactionwaystotreat
theeffluentofananaerobicreactor.
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49
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ScienceandBio/Technology,2015,14(4),p.649-679.
55. M. Singh and R. K. Srivastava, Sequencing batch reactor technology for biological
wastewater treatment: a review, Asia-Pacific Journal of Chemical Engineering, 2011,
6(1),p.3-13.
56. R. Pannirselvamand Y. Ibrahim,Simultaneous carbon and nutrien removal fromdairy
wastewaterinsequencingbatchreactor(SBR),2015.
57. E. Foresti, M. Zaiat and M. Vallero, Anaerobic Processes as the Core Technology for
Sustainable DomesticWastewater Treatment: Consolidated Applications, New Trends,
Perspectives, and Challenges, Reviews in Environmental Science and Bio/Technology,
2006,5(1),p.3-19.
58. R. Ganesh, P. Sousbie,M. Torrijos, N. Bernet and R. A. Ramanujam,Nitrification and
denitrification characteristics in a sequencing batch reactor treating tannery
wastewater,CleanTechnologiesandEnvironmentalPolicy,2014,17(3),p.735-745.
59. S.Vigneswaran,WasteWaterTreatmentTechnologies-VolumeII,EOLSSPubl.,2009.
60. A. Nogueira, J. Bassin, A. Cerqueira and M. Dezotti, Integration of biofiltration and
advanced oxidation processes for tertiary treatment of an oil refinery wastewater
aimingatwaterreuse,EnvironmentalScienceandPollutionResearch,20161-12.
61. A. Jácome, J. Molina, R. Novoa, J. Suárez and S. Ferreiro, Simultaneous carbon and
nitrogen removal from municipal wastewater in full-scale unaerated/aerated
submergedfilters,Waterscience&Technology,2014,69(1),p.
Page 66
54
62. A.Ramos,M.Gomez,E.HontoriaandJ.Gonzalez-Lopez,Biologicalnitrogenandphenol
removal fromsaline industrialwastewaterby submerged fixed-film reactor, Journalof
HazardousMaterials,2007,142(1),p.175-183.
63. G.Nakhla,J.ZhuandY.Cui,Liquid-solidcirculatingfluidizedbedwastewatertreatment
systemforsimultaneouscarbon,nitrogenandphosphorusremoval,2007.
64. R.L.Pehrson,W.J.FlournoyandS.B.Hubbell,Wastewatertreatmentmethod,2010.
65. A.G.Capodaglio,P.HlavínekandM.Raboni,Advancesinwastewaternitrogenremoval
bybiologicalprocesses:stateoftheartreview,RevistaAmbiente&Água,2016,11(2),
p.250-267.
66. J.Reungoat,B.Escher,M.Macova,M. J.Farré,F.X.Argaud,P.G.Dennis,W.Gernjak
andJ.Keller,BiofiltrationforAdvancedTreatmentofWastewater,2012.
Page 67
55
AimsandContents
Nitrogenremovalindomestic
wastewaterafteranaerobic
treatment
Page 69
AimsandContents
57
Outlook
Theincreasingurbangrowth,theunsustainableuseofthenaturalresourcesandthesociety
awareness of the environmental impact, highlight the necessity to develop and implement
advancedtechnologiesaimedtoprevent,mitigateandcorrectthepollutionproblemsderived
from anthropogenic origin. Currently, one key environmental problem is the wastewater
production.
Organic matter and nutrients present in domestic wastewater should be removed or
valorized to reduce its impact on the environment. Conventionalwastewater treatments are
focusedontheremovalofthesepollutionsourcesattheminimumcost.The ideaofresource
recovery fromwastewater ischangingtheconceptof theconventionalwastewatertreatment
plantsthattendtoincorporatelittlebylittleprocessesasanaerobicdigestion.
Anaerobic treatment processes are well-known to achieve high organic matter removal
efficiencieswithoutoxygenrequirement, lowbiomassproductionandenergygenerationfrom
biogas.Thegrowinginterestinanaerobictreatmentofdomesticwastewaterrequiresaparallel
approachinthedevelopmentofdownstreamtechnologiesbecausetheeffluentoftenrequires
apost-treatmenttoremovenutrients,especiallynitrogen.
Nitrogenremovalhasbecomeoneofthemostsignificantcostfactorsawastewaterfacility
faces.Tocomplywiththeregulations,facilitiesareconfrontedwithmajorplantupgradesthat
include nitrification and denitrification. These systems typically require significant space,
substantialcapitalupgrades,andimpactbothenergyandchemicaloperationalcosts.
As it was analyzed in State of the Art, the main challenges of the nitrogen removal
technology lie in the development of a fast and selective biological reactor that allows an
adequate electron donors usage with an economically feasible configuration. The
accomplishment of this goal was analyzed in this PhD Thesis by using different reactor
configurationsaswellasdifferentreactionways.
Theaimof thisPhDThesis is todevelopandevaluatedifferent treatmentprocessesofan
anaerobicreactoreffluentfedwithdomesticwastewater.
The AIM OF THIS WORK is to develop and evaluate different treatment processes of an
anaerobic reactor effluent fedwithdomesticwastewater. For this purpose, nitrogen removal
Page 70
AimsandContents
58
efficiency and environmental sustainability have been considered to comply the discharge
standardsindomesticwastewater.
Inorder toaccomplish thegeneralaimof this thesis, the followingpartialobjectiveswere
established:
• Design and construction of a SBR process to remove nitrogen of a domestic
wastewaterpreviouslytreatedinananaerobicreactorat18ºC.
- Studyofdifferentcyclesanddeterminationoftheoptimum.
• Designandconstructionofafixedfilmbioreactorforpartialandtotaldenitrification
of the effluent from an anaerobic reactor treating domestic water under
psychrophilicconditions.
- Feasibilityoftheremovalofnitratesandnitriteusingmethane,sulfideand
organic matter as electron donors to remove nitrates and nitrites at
differentHRT.
- StudyoftheinfluenceoftheNO2
-/NO3
-ratiointhefeed.
• Designandconstructionofadenitrification/nitrificationpilotplanttreatingdomestic
wastewaterafteranaerobictreatment.
- StudytheinfluenceevaluationoftheCOD/Nratioandthenitraterecycling
ratioinnitrogenremoval.
• Evaluatetheeconomicalfeasibilityofthenitrogeneliminationtechnology.
- Comparison of a conventional denitrification/nitrification and
denitritation/nitritation process as a post-treatment of membrane
anaerobiceffluent.
- Searchof the sensitiveparameter that canbemodified to get thebiggest
conversionofnitritetonitrogengasinthedenitritationprocess
Inordertoachievetheobjectivesofthisthesis,theworkwasstructuredinfivechapters.In
eachofthem,thepartialobjectivesandchallengesarepresented.Ineachchapter,aliterature
reviewwasdoneinordertoknowthemainachievementsandchallengesoftheanalyzedstudy.
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AimsandContents
59
Themaincontentofthechaptersisdescribedbelow.
InChapter1,“SBRprocess fornitrogenremovalofadomesticwastewater fromanaerobic
treatment”,theperformanceofSBRispresentedtotreattheeffluentofananaerobicreactor.
The5Lofworkingvolumewasinvestigatedatdifferentcycletimesof12h,8hand6h,at18
ºC, and the 6 h cycle time was selected as the optimal for the treatment. Results from
nitrification anddenitrificationof domesticwastewater in the SBR showedCODandnitrogen
removal efficiencies of about 73% and 81%. The process was successful in an
anoxic/aerobic/anoxic cycle sequence with the addition of methanol just before the second
anoxicstage.
In Chapter 2, “Denitrification of the AnMBR effluent with alternative electron donors in
domestic wastewater treatment”, the performance of a fixed film bioreactor for partial and
total denitrification was investigated. Wastewater after anaerobic treatment contains a
remainingCODnotenough for the conventionalheterotrophicdenitrification.As theeffluent
fromthe low-temperatureanaerobic reactorholdsmethaneandsulfide, itwasevaluated the
feasibilityofusingthemaselectrondonorstoremoveNO2
-andNO3
-atdifferentHRT,obtaining
theoptimumat2h. Inaddition, the influenceof theNO2
-/NO3
- ratio in the feedwas studied.
NitrogenremovalwasdemonstratedobtainingasuccessfulNO2
-andNO3
-eliminationwhenthe
feedwas80mgN-NOx
-/L,exceptwhenthefeedingwasformedonlybynitrate,thattheprocess
was at the limit.Methanewas themain electron donor used to removeNO2
- andNO3
-, with
morethan70%orparticipation.
In Chapter 3, “Advanced denitrification of anaerobic treatment effluent of domestic
wastewater by usingwasted gas”, the denitritation process using alternative electron donors
present in the water at 18 ºC and 2 h of HRT was investigated. Different synthetic nitrite
concentrations were supplied to the anoxic reactor to simulate the effluent of a nitritation
process.Theresultsdemonstratedthattheprocesswasabletoremovearound95%and93%of
nitritewhentheinletwas50mgNO2
--N/Land75mgNO2
--N/Lfromasimulatedrecirculationof
aerobic treatment effluent. For high inlet concentrations of NO2
-, recirculation of the gas
collected in the anoxic reactor was a successful solution, thus achieving a nitrite removal
efficiencyupperthan98%whenthenitriteconcentrationinthefeedwas95mgNO2
--N/L.
In Chapter 4, “Nitrogen removal in domestic wastewater. Effect of nitrate recycling and
COD/N ratio”, a denitrification/nitrification pilot plant was designed, built and operated to
examine the effect of the nitrate recycling and the COD/N ratio on the nitrogen and the
Page 72
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60
remainingorganicmatter removal. The systemconsistedof ananoxic reactorandanaerobic
one,withHRTsof2hand4h, respectively.The increase inthenitraterecyclingratiodidnot
suppose a significant improvement in the nitrogen removal due to the insufficient carbon
source.Theadditionofmethanolwasakeypointinthedenitrificationprocess.Themaximum
nitrogenandorganicmatterremoval(85%and96%,respectively)wasachievedwithanitrate
recyclingratioof600%andaC/Nof8.25,adjustedbymethanoladdition.Actually, insteadof
theadditionofmethanol,theenhancementoftheC/Dratiocanbemadebybypassingpartof
thefeedstreamfromapointbeforetheanaerobictreatmenttoanotherpointintheendofthis
reactor.
InChapter5,“Techno-economicalstudyofadomesticwastewater treatmentsystem”, the
techno-economicalfeasibilityofthenitrogeneliminationtechnologywithaMBRpre-treatment
wassimulated.Theinfluenceofdifferentelectrondonors(methane,organicmatterandsulfide)
on the nitrogen removal capacity was analyzed. Different scenarios have been assessed
changing the concentration of the involved components and evaluating their effect on the
nitrogenremovalcapacityaswellastheabilitytoproducebiogas intheanaerobictreatment.
These scenarios imply on the one hand, the increment of the available soluble COD for the
nitrogen elimination stage; On the other hand, different flows of biogas from the anaerobic
reactorwerepumpedtothedenitritationreactor.Thegoalwastoachieveanitrogenremoval
capacitytoreachaneffluentwith10-20mgN/L.Then,themostpromisingscenariowasstudied
in detail and it was compared to the costs associated to the WWTP with a biological MBR
anaerobictreatment.Theresultsindicatedthattheproposedprocessisfeasiblesincethefixed
andvariablescostsofbothtreatmentplantsaresimilar.
ThisworkispartoftheIPT-2011-1078-310000researchprojectwithintheINNPACTO2011
program funded by the Ministry of Economy and Competitiveness, the European Regional
DevelopmentFund,andthecompanyCadaguaS.A.
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61
Chapter2.
SBRsystemfornitrogenremoval
indomesticwastewaterfrom
anaerobictreatment.
Abstract
Thisworkpresentstheperformanceofasequencingbatchreactor(SBR)system
usedasnitrogen(N)removaltreatmentofdomesticwastewaterpreviouslytreated
withananaerobic reactor andas consequence,witha lowC/N ratio. Theaimof
the work was to determine the feasibility for the removal of nitrogen from the
domesticwastewater.A5 LofworkingvolumeSBRwas investigatedatdifferent
cycletimesof12h,8hand6h,at18ºC.ThetreatmentefficiencyofSBRvaried
with the duration of the cycle time, being optimal the anoxic/aerobic/anoxic
sequencecyclewith6hofduration.Duetotheloworganicmatterpresentinthe
domesticwastewater after anaerobic treatment, an additional supplyof external
carbonbefore the second anoxic stagewasnecessary. The additionofmethanol
was a key point in the denitrification process employed as a model for the
wastewater by-pass in wastewater treatment plants (WWTP). The removal
efficienciesobtainedwere:98%fortotalKjeldahlnitrogen(TKN)and84%fortotal
nitrogen (TN) and 77% for soluble chemical oxygen demand (COD). The reactor
showedviability,sothisprocesscanbesuccessfullyappliedasapost-treatmentof
ananaerobicreactortreatingdomesticwastewater,fortheremovalofnitrogen.
Keywords: Denitrification • Nitrification • Nitrogen removal • Organic
matter•Sequencingbatchreactor.
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Chapter2
63
1. INTRODUCTION
Conventional activated sludge treatment, commonly used to treat domestic wastewater,
causes problems such as excessive generation of sludge and involves consumption of a large
amountofenergy[1,2]
.Incontrast,anaerobicbiologicaltreatmenthasanumberofadvantages
favoringenergybalancesbecauseof thereducedsludgeproduction, , thenotrequirementof
aerationandtheenergyrecoveryasmethanegas[2-4]
.
The anaerobic reactors treating domestic wastewater can produce two main valuable
products,which can be recovered and utilized:methane and the effluent. Themethane gas,
which isproducedduringtheCODremovalcanberecoveredandtransformed intoenergy[5].
Theeffluentcontainssolubilizedorganicmatter,highammonia-nitrogenandorganic-nitrogen
concentrations. Therefore, application of a post-treatment process is necessary in order to
removenutrientsfromthewastewaterandachievethedesiredeffluentquality[2,6,7]
.Advancing
treatment of domestic wastewater requires implementing energy efficient nitrogen removal
technologies that avoid nullify the energy savings realized from the anaerobic process. This
processalsomitigatesgreenhousegasemissionsandmaintainsorreducesthefootprint[4,5]
.
Biological nutrient removal (BNR) constitutes the most economical and sustainable
technique for removing organic carbon and nitrogen, and then, to meet rigorous discharge
requirements[8-10]
.Thebiologicalnitrogen(N)removalinvolvestwoprocesses:nitrificationand
denitrification.Nitrification isanaerobicprocessperformedbyautotrophicbacteria, inwhich
ammonium(NH4
+)isoxidizedtonitrite(NO2
−),bymeansofammoniumoxidizingbacteria(AOB).
Then, nitrite is oxidized to nitrate (NO3
−) by nitrite oxidizing bacteria
[2]. Denitrification is an
anoxic process performed by a functional group of bacteria that use oxidized nitrogen as
electronacceptorinrespiration.Inthisprocess,NO3
−isreducedtoNO2
−andthentonitricoxide
(NO),nitrousoxide(N2O)andfinallytomolecularnitrogen(N2)[8,10]
.
Both nitrification and denitrification possess nitrite (NO2
−) as an intermediate. Hence, if
nitrification is stopped at nitrite (nitritation), then complete denitritation from nitrite to
nitrogengascanbeachieved.Nitritation-denitritationmaysave25%ofaerationconsumption
and40%ofchemicaloxygendemand(COD),aswellas lowbiomassproductionandincreased
kinetic. However, the difficulty to utilize nitrogen removal via nitrite lies in achieving specific
inhibitionof thenitriteoxidizingbacteriawhile retainingammoniaoxidizingbacteria, thereby
attainingnitritation[11-13]
.
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Chapter2
64
Thebiologicalprocesswithananaerobic-aerobic-anoxicsystemisafeasibleandsustainable
technologyforremovingnitrogenandorganicmatterfromdomesticwastewater.Highorganic
andammoniumremovalefficienciesareachievedbyusingthissystems,butthetotalnitrogen
(TN) removal efficiency is not high due to the shortage of carbon source available for
denitrification[9]. Organic substrates such as methanol can be used for carbon and electron
sourceforbiologicaldenitrification[14,15]
.Themaindisadvantageofusingmethanolisthesafety
issuesassociatedwith its transportation,handling, and storage[16]
.Oneof themosteffective
methods to increase theorganicmatterconcentrationof the influentwithout theadditionof
external organic substrates is achieved bymixing a fraction of the influent to the anaerobic
reactorwiththeeffluentofthatreactor.Insuchcase,theanaerobicreactorshouldbeusedto
treat initiallyonlyapartofthe influentrawsewage(possiblynomorethan50–70%),andthe
remainingpart (30–50%)shouldbedirectedto thecomplementarybiological treatment.The
useofthis“by-pass”will increasetheCODofthereactoreffluentmakingitmoreadequateto
thenextdenitrificationstage[17,18]
.
SBR is a flexible system that has been used successfully for developing the classical
nitrification anddenitrificationprocess[19]
. The SBR is a fill anddraw typemodified activated
sludgeprocessthatoperatesunderaseriesofperiodsthatconstituteacycle.Fourbasicsteps
of filling, reaction, settling and discharge phases take place sequentially in a single batch
reactor. The SBR process offers minimum operator interaction, good oxygen contact with
microorganismsandsubstrate,smallfloorspace,goodremovalefficiencyandtheoperationcan
be adjusted to obtain aerobic and anoxic conditions in the same tank[5, 6]
. In contrast to
continuoussystems,SBRshavebecomequitecommonforobtaininghighnitriteaccumulation
duetotheflexibilityofprocesscontrol[19]
.
ThemainobjectiveofthepresentstudywasthedesignandthefeasibilityofSBRprocessto
removenitrogenofadomesticwastewaterpreviouslytreatedinananaerobicreactor.
2.MATERIALSANDMETHODS
2.1.ExperimentalSetup
The lab-scale systemdeveloped for this study consistedon theonehand,of two reactors
with a total volume of 1 L. Airwas supplied through porous diffusers at the bottomof each
reactortopromotemixingandallowingagooddiffusionofoxygeninthewastewater.
Ontheotherhand,thesystemwasdevelopedwithaSBRbioreactorandtwotanks:feeding
and effluent tanks. The reactor of 6 L of total volume and 5 L of working volume, was
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Chapter2
65
completelymixedwithamechanicalstirrerandcontainedafinebubbleairdiffuser,whichwas
part of the aeration system. Two peristaltic pumpswere used for the reactor filling and the
effluentdischarge.Thetwopumps,thestirrerandtheaerationsystemwereconnectedtoan
electrictimersystem.AschematicdiagramoftheSBRplantisgiveninFigure1.Thereactorwas
keptinaroomunderacontrolledtemperatureofaround18ºC±1ºC[20]
.Thiswastheworking
temperature of a previous anaerobic reactor that produced the effluent to treat. For the
denitrification step, itwasnecessary theadditionofmethanol (1:100), suppliedwith another
peristaltic pump. The additionofmethanolwas amodel to simulate a by-pass of part of the
feedstreamfromapointbeforetheanaerobicreactor,toanotherpointjustintheendofit.The
reactorwasoperatedduring730days.
In the two parts of the study, the aeration rate was controlled through a flow meter,
maintainingthedissolvedoxygen(DO)concentrationbetween2.0-2.5mgO2/L.
Figure1.SchemeoftheSBRplant.(1)Fillingpump,suppliesthewastewaterfromanaerobictreatmentto
thereactor,(2)Compressor,responsibleforsupplyingtheairfortheaerationstep,(3)Mechanicalstirrer,
(4)SBRreactor,(5)Pumpthatdrainsthewateraftertreatment.
2.2.InoculumandFeedingCharacteristics
TheinoculumofthethreereactorsstudiedwassecondaryaerobicsludgefromtheWWTPof
Valladolid(Spain).
Thereactorswerefedwiththeeffluent fromananaerobicmembranebioreactor (AnMBR)
fedwithrawdomesticwastewaterfromthecityofValladolid(Spain).TheAnMBRpilotplantis
(1) (2)
(3)
(4)
(5)
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Chapter2
66
explained indetail inapreviouswork[20]
.Themeanconcentrationof themainparametersof
the influent feeding the denitrification/nitrification plant are given in Table 1. It is a residual
waterwithahighcontent inammoniacalnitrogenanda lowconcentrationoforganicmatter,
leadingtoaCOD/TNratioaslowas1.2.Intheinletstream(fromanaerobictreatment),sulfuris
thecorrespondingamountofsulfideoxidationwithoutquantifytheoversaturation,sothereal
valueforsulfideishigher.
Table1:Averagecompositionofthewastewaterafteranaerobictreatment.
sCOD
(mgO2/L)
TKN
(mgN/L)
NH4
+
(mgN/L)
NO2
-
(mgN/L)
NO3
-
(mgN/L)
SO4
2-
(mgS/L)
solP
(mgP/L)
100.6 81.9 77.7 0.0 0.0 8.5 9.2
2.3.AnalyticalMethods
Samples of wastewater were taken before and at the end of each treatment cycle. The
concentrationofnitrite,nitrate,sulfateandsolublephosphorusweremeasuredbyHPLC.The
ammoniumconcentrationwasdeterminedusinganammonia-selectiveelectrode:Orion,model
9512HPBNWP. The analyses of COD, TKN aswell as total and volatile suspended solids (TSS,
VSS) were determined according to standard methods suggested by the Standard methods
manual[21]
. Temperature was measured using a temperature probe. The measurement of
dissolved oxygen (DO) concentration was determined with an oximeter WTW, model oxi
330/SETandadissolvedoxygenprobeCeliOx325.
2.4.OperationStrategy
Inthefirstpartofthestudy(Section3.1),beforestartingwiththeSBRsystem,theaeration
periodwas optimized to ensure the nitrification process. To do so, two reactorswith a total
volumeof1Lwereused.Onereactorwasaeratedduring12handtheotherduring7h.These
timewasconsideredmorethanenoughtooxidizetheammoniumpresentinthewastewater.
For the rest of the work, the denitrifying/nitrifying SBR was used and operated with
successivecycles.Eachcycleconsistedof15minoffeedingstage,areactionperiod,andfinally,
thesupernatantdrawwasdischargedduringthelast15min,after30minofbiomasssettling.
For the operation cycles determination, the cycles were initiated (after the filling) with an
aerated stage and continued with an anoxic one (Sections 3.2 and 3.3). During the aeration
phase, the averageDOwas between 2-2.5mgO2/L. In the discharge stage, 3.5L of the total
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Chapter2
67
workingvolumeweredischarged,remaininginthereactor1.5L,tobetreatedinthefollowing
cycle.ThedifferentcyclesstudiedarepresentedintheTable2.
Table2:Timedistributionofthestagesofthedifferentcycles.
Stage Case1 Case2 Case3 Case4 Case5
Filling 15m 15m 15m 15m 15m
Pre-anoxic ⎯ ⎯ ⎯ ⎯ 30m
Aeration 7h 3h 5h30m 4h 3h45m
Anoxic 4h 4h 1h30 1h 45m
Sedimentation 30m 30m 30m 30m 30m
Discharge 15m 15m 15m 15m 15m
Cycletime: 12hours 8hours 8hours 6hours 6hours
Modificationsof thecycle tookplacetoenhancetheorganicmatterandnitrogenremoval
efficiencies.InCase5(Section3.4),apre-anoxicstagewasaddedandthecyclesconsistedofan
anoxic stage, aeration stage and another anoxic stage. Finally, a new cycle was achieved by
adding methanol before the last anoxic stage, to provide more organic matter to the
denitrificationstepandinthisway,enhancethereactionsinthisstage(Section3.5).
3.RESULTSANDDISCUSSION
3.1.Optimizationoftheaerationperiod.
The required time to assure the nitrification processwas study. In the Figure 2, it can be
seentheevolutionofthenitrogenspeciesduringtheaerationperiodof7and12hours.Inthe
Figure2A,where it is represented theTKNconcentration in time, it canbe seen theaverage
valuesinthefeedof90and115mgN/Lwhileintheeffluentitwasintherangeof20mgN/L.
During the first two hours of aeration, around 60% and 70% of the TKN concentration was
decreasedforthetwocasesstudied,andthefinalTKNremovalefficiencywasabout79.4%.In
aerobicconditionsammoniacalnitrogenwasnitrified, i.e. itwasusedastheenergysourceby
nitrifying bacteria leading to the formation of nitrite and nitrate. Residual ammonium was
utilizedasthenitrogensourceforthebiomasssynthesisbythebacteria.Atthesametimethat
ammoniumwasoxidized,nitriteandnitrateconcentrationsincreased,althoughthelattermore
slowly,3.5hoursvs5hours(Figure2B).Nitrateandnitriteco-existedinthereactor,buthaving
an accumulation of nitrite almost four times higher than nitrate. Nitrite was the primary
Page 80
Chapter2
68
product of nitrificationduring the aeration experiment,whichwas accumulatedup to 63mg
NO2
--N/L,while thenitrateconcentrationwasalwaysbelow15mgNO3
--N/L.Ahigher levelof
nitrite accumulation indicates a high activity of AOB, suggesting that the partial nitrification
performanceoftheaerationprocesswasgood.Bycontrast,theactivityofNOBwaslimitedin
theaerobicphase.Highnitriteaccumulationhasbeenreportedby[19,22,23]
inSBRsystems.
Theoptimumtimeconsideredfortheaerationprocesswasfourhours.Afterfourhoursof
aeration,theeffluentshowedameanconcentrationof15.0mgNH4
+/L,58.5mgNO2
--N/Land
12.3mgNO3
--N/L.Afterthistime,therewereavariationintheparameterslowerthan5%.
Figure2.(A)ProfileofTKNconcentrationduringaeration;(B)Profileofnitriteandnitrateconcentration
duringaeration.
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7 8 9 10 11 12
NOx-(m
gN/L)
Time(h)
� TKN-7h � TKN-12h ●NO2--7h ○ NO2
--12h � NO3--7h � NO3
--12h
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7 8 9 10 11 12 13
TKN(m
gN/L)
Time(h)
(A)
(B)
Page 81
Chapter2
69
3.2.Definitionofthereactoroperationcycles.
The timeof theaerobic andanoxic stageswere changedwith theaimofdetermining the
influenceofthedurationofthesestagesinthenitrogenremoval.Fourcyclesof12h(Case1),8
h(Cases2-3)and6h(Case4)werestudied,asshowninTable2.
Figure3,(A)and(B)depictthegraphiccomparisonintheTKNandNO2
-,respectively,forthe
differentcycles.Nitrateisnotrepresentedbecauseonlyinthe12hand6hcycleswasdetected
butinverylowamounts,notexceedingaconcentrationof4mgNO3
--N/L.
Figure3.ProfilesofTKN(A)andNO2
-(B)concentrationsintimeforthedifferentcycles.
0
20
40
60
80
100
120
0 2 4 6 8 10 12
TKN(m
gN/L)
Time(h)
(A)
0
10
20
30
40
50
60
0 2 4 6 8 10 12
NO2-(m
gN/L)
Time(h)
� Case1 � Case2 − Case3 � Case4
(B)
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Chapter2
70
InFigure3A,canbeobserved that theaverageTKNconcentrationdeclinedsharplyduring
thefirst6hours.NH4
+wasalmostcompletelyoxidizedin6hours.Incycleslongerthan6hours,
TKNconcentrationdecreasedlessthan4%fromthistime.Meanwhile, inFigure3B,therewas
anincreaseinNO2
-concentrationachievingitsmaximumataround4hoursofcycleandthen,it
remainedconstantorsufferedaslightdeclineinitsconcentration.
Table3showsthenitrogenremovalefficiencies foreachcycleachievingthehighestvalue,
54%,inthelastcycle.Therefore,itwasconsideredtheoptimumcyclefornitrogenremovalto
haveadurationof6h.
Table3:NitrogenconcentrationinthewastewaterbeforeandaftertheSBRprocessinthedifferent
cycles.
Parameter Case1 Case2 Case3 Case4
Ninlet
(mgN/L)
TKN 90.0 78.8 78.8 107.5
NO2
- 0.0 0.0 0.0 0.0
NO3
- 0.0 0.0 0.0 0.0
Noutlet
(mgN/L)
TKN 32.1 17.1 6.4 33.3
NO2
- 40.4 50.0 37.7 11.9
NO3
- 2.4 0.0 0.0 4.3
%TNremoved: 16.8% 14.3% 44.0% 54.0%
3.3.Studyofa6hcycle.
TheSBRprocesswitha6hcyclewasstudiedduring70days.
Figures 4 to 6 show the evolution of the soluble COD, TKN, NO2
--N and NO3
--N
concentrations during the aerobic/anoxic cycle of the operational period in the SBR. For the
casesofCODandTKNcanbeseenthatinfluentandeffluentfollowedthesametrend(Figures4
and 5). The effluent concentrations increased when the inlet stream had a higher load, and
decreasedwhenloweringtheinfluentconcentration.ThemeanremovalefficienciesofCODand
TNwere30.3%and42.9%,respectively.Theseefficiencieswereverylowsoitwasnecessaryto
consideramodificationofthecycle.
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71
Figure4.CODconcentrationintheinfluentandeffluentintime,forthe6haerobic/anoxiccycle.
Figure5.TKNconcentrationintheinfluentandeffluentintime,forthe6haerobic/anoxiccycle.
0
30
60
90
120
150
0 10 20 30 40 50 60 70
COD(m
gO2/L)
Time(d) ● In○Out
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70
TKN(m
gN/L)
Time(d) ● In○Out
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72
Figure6.Nitrite(A)andnitrate(B)concentrationintheinfluentandeffluentintime,forthe6h
aerobic/anoxiccycle.
During most of the time, the final effluent exhibited a high nitrite concentration that
proceedfromalow-yielddenitrificationprocess(Figure6A).Assaidbefore,duringtheaeration
stage,therewasnitriteaccumulationinthetankbecausemainly,partialnitrificationtookplace.
Therefore,a lowdenitrificationyieldwasobserveddueto the loworganicmatteravailable in
thewastewater after the anaerobic treatment. TheC/N ratio of 1.2was low for establish an
efficient denitrification process. Concerning the nitrate, throughout the study period, it has
beenanabsenceofthiscompound,exceptinveryfewspecificcases(Figure6B).
SolublephosphorusconcentrationdidnotsufferconsiderablechangeswiththeSBRprocess
(Figure7).Aslightremovalofabout12%wasobserved.
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70
NO2-(m
gN/L)
Time(d)
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70
NO3-(m
gN/L)
Time(d)
● In ○Out
(A)
(B)
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73
Figure7.Solublephosphorusconcentrationintheinfluentandeffluentintime,forthe6haerobic/anoxic
cycle.
3.4.Additionofapre-anoxicstage.
In order to improve the denitrification process, a pre-denitrification stage was added in the
cycle, prior to the aeration step. Thus, during the first anoxic stage of 30 minutes, residual
nitriteandnitratenoteliminatedinthepreviouscycle,thatremainsinthe1.5Lnotdischarged,
couldbereducedtonitrogenN2.
HavingtwoanoxicstagesallowsloweringtheTKNeffluentconcentrations.Thus,mostofthe
nitritesandnitratesproducedafternitrificationintheaerationstagecanbetreatedbyflowing
through the second anoxic stage. Pre-anoxic stage has been used with beneficial results to
accomplishtheremovaloforganicmatterandnitrogenbyLu,Q.etal.[24]
.
Toadaptthecycletothisnewchange,itwasnecessarytomodifythetimesofthedifferent
periods, so that the six-hour cycle was carried out as indicates the Case 5 in Table 2. The
parametersconcentrationbeforeandafterSBRtreatmentforthemodifiedcycleareshownin
theTable4.
Table4.Parametersconcentrationbeforeandafter6hoursmodifiedcycletreatment.
sCOD
(mgO2/L)
TKN
(mgN/L)
NH4
+
(mgN/L)
NO2
-
(mgN/L)
NO3
-
(mgN/L)
solP
(mgP/L)
Influent 111.3 99.5 88.4 0.0 0.0 12.3
Effluent 67.9 20.2 18.1 25.3 1.5 12.6
Almost the80%ofTKNwasremoved in thedenitrification/nitrificationprocess, resultinga
final effluentwitha concentrationof around20.2mgN/L.During the first anoxic stage,NO2
-
andNO3
-fromthepreviouscyclewasremoved.Duringtheaerationstage,NH4
+wasdecreased
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70
SolubleP(m
gP/L)
Time(d) ● In○Out
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74
from88.4to18.1mgN/LunderthesupplyofDO,whereastherewasacorrespondingincrease
ofNO2
-concentration.Inthefollowinganoxicphase,NO2
-concentrationdecreasedto25.3mg
NO2
--N/L.Nitritewastheprimaryproductof theprocess,showinganaccumulation,whilethe
nitrateconcentrationwasalwaysverylow,about1.5mgNO3
--N/L.Theremovalefficienciesof
CODandTNwere39.0%and52.8%,respectively,resultinginefficienciesimprovementofabout
20%whencomparingthecyclewithoutpre-anoxicstage.
3.5.Additionofmethanol
The anoxic/oxic/anoxic process requires sufficient degradable carbon substrate to provide
theenergysourceneededforthedenitrificationreactionsthatoccursaftertheaerationstage.
Duetothelowamountofeasilybiodegradableorganicmatteravailableinthewastewatertobe
treated in the SBR process, methanol was added as an external carbon source before the
denitrificationstage[25-27]
.Thiscompoundcouldbeusedaselectrondonorbythedenitrifying
bacteria, responsible of the nitrate and nitrite reducing to gaseous nitrogen. A solution of
methanol (1:100)wasadded in thecyclebefore the secondanoxic stage, justafter finish the
aeration.
It isnoteworthythatinsteadoftheadditionofmethanol,theincreaseoforganicmatterin
WWTPscanbemadebybypassingpartof the feedstreamfromapointbefore theanaerobic
treatmenttoanotherpointintheendofthisreactor.Withthiscourseofaction,itispossibleto
increase the soluble COD available in the liquid stream that feeds the denitrification reactor,
withoutaddinganexternalcarbonsource.Inthiswork, inordertosimulatethisbehavior,the
additionofmethanolwasemployedasextracarbonsource.
TheCOD,TKN,NO2
-andNO3
-graphicscorrespondtotheFigures8to10,respectively.This
part of the study had a duration of three months and has been divided in three stages as
indicatesTable5.
Tabla5:Summaryofthestages.(CODAnMBR:CODoftheAnMBReffluent.CODMethanol:CODsupplied
withmethanol.CODinlet:CODoftheSBRfeed).
Stage Days CODAnMBR CODMethanol CODinlet TN C/N %TNremoved %CODremoved
1 0-20 195,6 0,0 195,6 93,0 2,1 40,7% 53,7%
2 21-70 148,6 81,9 230,6 97,4 2,4 74,7% 57,8%
3 71-91 176,2 97,1 273,3 90,6 3,0 83,9% 76,7%
Untilday20,theCODinthewastewaterfromanaerobictreatmentwasapproximately196
mgO2/Landmethanolwasnotaddedtoevaluatethereactor’sreaction.Inthisperiod,theC/N
ratiowas2.TKNwasremovedalmost90%,obtainingameanconcentrationofTKNof9.6mg
N/Lintheeffluent.Thenitriteandnitrateraiseduntilconcentrationsof41mgNO2
--N/Land37
Page 87
Chapter2
75
mgNO3
--N/L.TheCODandTNefficienciesdecreasedto53.7%and40.7%,respectively,because
ofthelowerC/Nratio.
From day 21 to 70, methanol was added up to 231 mg O2/L. The C/N ratio was 2.4.
Approximately,86%ofTKNwasremoved,obtaininganeffluentwithameanconcentrationof
TKNof13.3mgN/L.Nitritewasfound intheeffluentduringthisperiod,withaconcentration
about10.6mgNO2
--N/L,achieving19mgNO2
--N/Linapoint.Itisnoteworthytheeffectofthe
additionofmethanol innitratebutmainly innitrite (Figure10).Thesecompounds suffereda
significant decrease in their concentrations when the COD was increased. The removal
efficienciesofCODandTNraisedupto57.8%and74.7%,respectively.
Finally,fromday71to91,aCODofabout273mgO2/Lwasachievedwiththeadditionof
methanol.TheC/N ratiowas3.Up to97.7%ofTKNwas removed in this section,obtaininga
mean concentration of TKN of 2.1 mg N/L in the effluent. The nitrite concentration in the
effluentwasabout11.4mgNO2
--N/L.RemovalefficienciesofCODandTNof76.7%and83.9%,
respectively,wereobtained.
Insteadofthemethanoladdition,bypassingavolumetricflowof30%oftheanaerobicfeed
from a point before the AnMBR to another point in the end of this reactor, is possible to
increase the available soluble COD 55% up to reach 273 mg O2/L. Estimation based on the
average of 501 mg O2/L of soluble COD that contains the wastewater before the anaerobic
treatment,afterthesedimentationstage[20]
.
About98%ofTKNwasremoved inthe lastsectionofthestudy.Attheendofthecycle,a
meanconcentrationofTKNof2.1mgN/Lwasobtained.About91%oftheammoniacalnitrogen
after the aerobic stage was nitrified and 7% was assimilated by heterotrophic bacteria. The
effluent after the cycle contained around 11.4mg NO2
--N/L while nitrates were occasionally
found with a concentration of 2.5mg NO3
--N /L. Therefore, nitrite was themain compound
accumulatedinthereactor.Aftercompletionofthenitrification,about79.7%and92.2%ofthe
generatednitriteandnitrate,respectively,wereremovedbydenitrification.
TheremovalefficienciesofCODandTNwiththiscycleconfigurationwere76.7%and83.9%,
respectively.AftertheSBRtreatment,theeffluentcontainedameanTNconcentrationof14.6
mgN/L.High removalpercentageswasobserved. If theseefficienciesare comparedwith the
oneswithoutmethanoladdition,therewasanimprovementofabout43%inCODremovaland
theTNremovalyieldwasdoubled.Thesesincreasesintheorganicmatterandnitrogenremoval
wereattributedtoahigherdenitrifiedactivityinthiscycleconfiguration.
Page 88
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76
Figure9.TKNconcentrationintheinfluentandeffluentintime,forthe6hanoxic/aerobic/anoxiccycle
withadditionofmethanol.
Comparingtotheliterature,Hwangetal.[27]
reached80%ofnitrogenremovalefficiencyby
using a sequencing batch biofilm reactor treating the rejectedwater from sludge. Its reactor
temperaturewas15-35ºC.Thereactorperformedanaerobic–anoxic–aerobic–anoxicsequence
andaddedmethanolat thebeginningofeachanoxicstep.Thetotalcycle timeusedwas8h,
versus 6 h in the present study. Therefore, that process needs twomore hours per cycle to
achieve the same nitrogen removal than the process developed here. In the same research,
Hwangetal.enhanced thenitrogen removalefficiencyup to91% inaanoxic–aerobic–anoxic
sequencewhenaddingmethanolasexternalcarbonsourceandNaHCO3asalkalinity,butwith
thesametotalcycletimeof8h[27]
.
Ontheotherhand,Fernandesetal.[28]
usedaSBRtreatingdomesticwastewaterwithaC/N
ratio of 3. Although it operated with a cycle time of 8 h (versus 6 h in this study), its COD
removalefficiencywashigher:83%versus77%obtainedinthepresentstudy.Onthecontrary,
thatprocessonlyachievedameanof50%ofTNremoval,versus84%inthepresentstudy.
TheSBRperformedbyChenetal.[29]
wasusedforthebioaugmentedtreatmentofmunicipal
wastewaterwith a C/N ratio of 8, achievedwith external carbon dosages. Comparing to the
present study, Chen et al. achieved better COD removal efficiency, 85.2% versus 76.7% and
80.5% of N removal versus 84% in this study. Moreover, their system required much more
amount of external carbon source to reach a C/N ratio almost 3 times higher than the
presentedinthiswork(C/N=8versusC/N=3).
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80 90
TKN(m
gN/L)
Time(d) In out
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Chapter2
77
Figure10.Nitrite(A)andnitrate(B)concentrationintheinfluentandeffluentintime,forthe6h
anoxic/aerobic/anoxiccyclewithadditionofmethanol.
4.CONCLUSIONS
A SBR process was applied to a domestic wastewater from anaerobic treatment and
therefore,witha lowconcentrationoforganicmatter.Anexperimentalstudyonapilotplant
scalewascarriedouttoascertainitssuitabilityforsimultaneousnitrificationanddenitrification.
Cycletimesof12h,8hand6hinSBRwereconsideredinthestudy,andthe6hcycletimewas
selected as the optimal for the treatment. Results from nitrification and denitrification of
domesticwastewater intheSBRshowednitrogenandCODremovalefficienciesofabout84%
0
4
8
12
16
20
24
28
32
36
40
0 10 20 30 40 50 60 70 80 90
NO3-(m
gN/L)
Time(d) In out
0
5
10
15
20
25
30
35
40
45
0 10 20 30 40 50 60 70 80 90
NO2-(m
gN/L)
Time(d) In out
(A)
(B)
Page 90
Chapter2
78
and77%,respectively.Theprocesswassuccessful inananoxic/aerobic/anoxiccyclesequence
with the addition of methanol just before the second anoxic stage. Thus, it has been
demonstratedthattheSBRprocessinasinglereactoratlowtemperatureisasuitableprocess
for the simultaneous removal of nitrogen andorganicmatter of a domesticwastewaterwith
low COD with only the addition of external carbon source. The addition of methanol was
employedasamodelforthewastewaterby-passintheWWTP.Asfutureworkitisproposedto
evaluatetheeffectofincreasingthecarbonratioonthenitrogeneliminationpotentialusingthe
mixingoftheanaerobicreactoreffluentandtherawfeed.
ACKNOWLEDGEMENTS
The authors thank the companyCadagua S.A., the EuropeanRegionalDevelopment Fund,
and the project IPT-2011-1078-310000, and the INNPACTO 2011 program of theMinistry of
EconomyandCompetitivenessforthetechnicalandfinancialsupport.
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79
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and T. Yamaguchi, Performance evaluation of the sulfur-redox-reaction–activated up-
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3. A.Moawad,U.F.Mahmoud,M.A.El-KhateebandE.El-Molla,Couplingofsequencing
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7. Y.Liu,X.Su,L.Lu,L.DingandC.Shen,Anovelapproachtoenhancebiologicalnutrient
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8. G. Liu, X. Xu, L. Zhu, S. Xing and J. Chen,Biological nutrient removal in a continuous
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9. H.Sun,H.Zhao,B.Bai,Y.Chen,Q.YangandY.Peng,Advancedremovaloforganicand
nitrogen from ammonium-rich landfill leachate using an anaerobic-aerobic system,
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12. R.N.Mohammed,S.Abu-AlhailandL.Xi-wu,Long-termoperationofanovelpilot-scale
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13. A. Al-Omari, B. Wett, I. Nopens, H. De Clippeleir, M. Han, P. Regmi, C. Bott and S.
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27. B.Hwang,Q.Lu,R.A.d.ToledoandH.Shim,Enhancednitrogenremoval fromsludge
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28. H. Fernandes,M.K. Jungles,H.Hoffmann,R.V.AntonioandR.H.R.Costa,Full-scale
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83
Chapter3.
DenitrificationoftheAnMBR
effluentwithalternative
electrondonorsindomestic
wastewatertreatment
Abstract
Theperformanceofafixedfilmbioreactorforpartialandtotaldenitrificationof
theeffluent fromananaerobicmembranebioreactor (AnMBR) treatingdomestic
water was investigated. Wastewater after anaerobic treatment, with a low C/N
ratio,containsaremainingchemicaloxygendemand(COD)whichisnotenoughfor
the conventional heterotrophic denitrification. As the effluent from the low-
temperature anaerobic reactor holds methane and sulfide dissolved and
oversaturated, itwasevaluatedthefeasibilityofusingthesereducedcompounds
aselectrondonorstoremove80mgNOx
--N/Latdifferenthydraulicretentiontimes
(HRT) obtaining the optimum at 2 h. In addition, the influence of the NO2
-/NO3
-
ratio (100%/0%; 50%/50%; 25%/75% and 0%/100%) in the feed was studied.
Satisfactory results were obtained achieving total nitrogen removal in the
denitrifyingeffluent,beingawareofthecasewith100%NO3
-inthefeed,thatwas
atthelimitoftheprocess.Methanewasthemainelectrondonorusedtoremove
thenitritesandnitrates,withmorethan70%ofparticipation.
Keywords:COD•Denitrification•Denitritation•methane•sulfide
Page 97
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85
1.INTRODUCTION
Anaerobic treatment process has been widely applied to various types of wastewater
because it has many advantages over the aerobic treatment. Among its advantages it is
noteworthyitslowenergyconsumption,reducedproductionofexcesssludgeandittransforms
the organic matter into valuable biogas. The anaerobic treatments have drawbacks such as
process sensitivity, vulnerability, odorproblems, long start-upperiod, andpost treatments to
achieve discharge standards because components such as nitrogen compounds are not
removedefficientlyinanaerobicreactors[1-4]
.Thus,theneedtoimprovethequalityofeffluents
fromanaerobicreactorshasdrivenresearcherstostudyalternativepost-treatmentsystems.
Nitrogencompoundsdischargedintotheenvironmentcaninduceseriousproblemssuchas
theeutrophicationofriversaffectingaquaticlifeanddeteriorationofwatersources,aswellas
hazardstohumanhealthandtheenvironment.Furthermore,nitritesandnitratescanalsoform
nitrosamines,potentiallycarcinogeniccompounds[5-7]
.Asaresult,developmentofeconomical
andsustainabletechniquesforreducingthenitrogencontentfromwastewaterhasattracteda
great deal of attention lately[8]. The most widely used method for nitrogen removal in
municipal wastewater treatment plants (WWTP) is the combined treatment by aerobic
autotrophic nitrification of NH4
+ to NO2
-and NO3
-, followed by anoxic heterotrophic
denitrification of the oxidized nitrogen species to N2 gas. The denitrification potential of
wastewater is mainly governed by the available biodegradable organic carbon, commonly
expressed as the C/N ratio -biodegradable (COD/N) or biological oxygen demand/nitrogen
(BOD/N) ratio-[9]. Theconventionalheterotrophicdenitrificationprocessesarequiteeffective
provided that wastewater contains adequate amount of organicmatter. However, when the
influentCOD/NO3
--Nratioislowerthan6,nitrogenremovalislikelytobelimitedbythelackof
availableorganiccarbonsource[4,9-11]
.Asanaerobicreactorsremoveasignificantfractionofthe
organicmatter,theavailableC/Nratioinwastewaterislow.So,thedenitrificationstepcanbe
achieved by adding an external carbon source, such as ethanol, methanol, or acetic acid.
However, the use of external carbon sources increases the operating cost and the sludge
production[4,12]
.
NO2
- is an intermediate in both nitrification and denitrification reaction pathways. In the
combinedconventionalnitrification/denitrificationprocess,NH4
+isoxidizedtoNO2
-andthento
NO3
-,which isagainconverted toNO2
-beforeN2gas formation.Therefore, theproductionof
NO3
- isnot required tocomplete thewholenitrogenremovalprocess.Thepartialnitrification
techniques aim to keep NO3
- out of the treatment system and promote the conversion of
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86
ammonia to nitrite (nitritation) for subsequent direct reduction of nitrite to N2 gas,
denitritation. The application of the nitritation/denitritation process instead of complete
nitrification/denitrification reduces the treatment costs thanks to 25% less aeration and 40%
lessbiodegradableCODconsumption.Therefore,theprocessbecomeshighlycosteffectivefor
thetreatmentofdomesticwastewaterwithlowC/Nratio,becausetheorganiccarbonsourcein
itistypicallylimiting.Moreover,itisknownthatdenitrificationreactionratesforNO2
-are1.5-2
timesfasterthanforNO3
-allowinghigherremovalcapacities.Furthermore,sludgeproductionis
reducedby40%inshortcutnitrification/denitrification[9,13]
.
Aswell as biogas is produced in anaerobic reactors, the effluent from a low-temperature
anaerobicsewagesystemcontainssignificantamountsofthegaseousproductsdissolvedinthe
liquid phase. Those gaseous products may be unintentionally emitted into the atmosphere
causinganegativeprocesscarbonfootprint[2,14,15]
.Methanelossbecomesespeciallyimportant
at low operational temperature processes since the solubility of this compound in the liquid
phaseinverselydependsontemperature[16]
.Methaneisagreenhousegasthathasaneffecton
globalwarming25timesstrongerthanthatofcarbondioxide.Therefore,themanagementof
dissolvedmethane isnecessaryto limitgreenhousegasemissions[2-4, 14, 15, 17, 18]
.Ontheother
hand, sulfide, which is also produced in anaerobic treatment, represents an environmental
problem,becauseofitscorrosiveproperties,odor,toxicityandCOD[4,12,19]
.
Frequently, methane and sulfide oversaturation occurs. If the effluent containing those
compounds isdischarged,methaneandsulfidewouldbereleasedtotheatmosphere.Several
authorshavereportedonanaerobiceffluents thatareoversaturatedwithdissolvedmethane,
which demonstrates that dissolved methane and sulfide concentrations can be higher than
thosepredictedbasedonHenry'slaw,ostensiblyduetotheformationofmicrobubbles[15,20]
.
Consideringtheundesirableimpactsofsulfideanddissolvedmethaneinanaerobiceffluents,
itmakessensetoevaluatetheefficacyofusingeitherorbothaselectrondonorsfornitrogen
removalwhenstringentnitrogendischargelimitsapply.Theelectrondonorstypicallypresentin
anaerobic effluents are preserved in solution as organic COD not removed during anaerobic
treatment, dissolved methane and sulfide. These compounds may be used by denitrifying
bacteriatoachievenitrogenremovalvianitriteornitrate[19]
.
Theobjectiveofthisworkwastoevaluatetheviabilityofthepartialandtotaldenitrification
of the effluent of an anaerobic membrane bioreactor (AnMBR) that treated domestic
wastewater under psychrophilic conditions, using organic matter, methane and sulfide as
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87
electrondonors.Nitrite and/ornitratewere theelectronacceptorsof thewastewaterwith a
lowC/N ratio. The remainedorganic carbon compoundswere removedby theheterotrophic
denitrification process.Methanewas oxidized to carbon dioxide, while sulfide to sulfate and
insoluble element sulfur. Nitrate and nitrite were converted to nitrogen gas that would not
causesecondarypollution.[21]
2.MATERIALSANDMETHODS
2.1.ExperimentalSetup
A schematic diagram of the denitrification plant is given in Figure 1. For the fixed film
bioreactorstudies,ananoxiccontinuousup-flowreactorwithaworkingvolumeof21Landa
total volumeof26Lwasused.The filtermadeofPVCglass consistedofa cylindrical column
withaheightof1.5manddiameterof0.15m.ThereactorwasfilledwithcorrugatedPVCrings
(withan innerdiameterof12mmand17mmof length),whichservedassupportwherethe
biomasswasattached.The temperatureofoperationwasbetween18ºCand20ºC.Theplant
was operated under continuous flow. The inoculum was formed by 10 L of not thickened
secondarysludgetakenfromtheWWTPofValladolid(Spain)and2Lofthermophilicanaerobic
sludge,coexistingnitrate/nitritereducersandmethanogenicculturesinthesystem.
Figure1:Schemeofthedenitrification/denitritationplant.(1)Fillingpump,suppliesthewastewatertobe
treated,(2)Tankwiththewaterthatdoesnotenterinprocess.(3)Gascollectionchamber,(4)
GasFlowMeter
Wastewater fromAnMBR
ORP
Effluent treated
SyntheticNO2
-/NO3-
FI
(1)
(3)
(4)
(5)
(6)
(7)
(8)
(2)
PI (9)
Page 100
Chapter3
88
Bioreactor,(5)PumpthatsupplythesyntheticNO2
-/NO3
-solution,(6)Currentoftheeffluenttreated,(7)
Oxidation/reductionpotentialmeter,(8)Flowindicator,(9)Pressuregauge.
2.2.FeedingCharacteristics
The studied reactor was fed continuously with the effluent from an AnMBR treating
domestic wastewater under psychrophilic conditions, and a medium of sodium nitrite and
sodiumnitrate,tosimulatetheeffluentfromapreviousnitrificationprocess.TheAnMBRpilot
plant is explained in detail in a previous work[2]. This feeding strategy tries to simulate the
operation of a real wastewater treatment plant using an anaerobic reactor as the first
treatmentunit.ThecharacteristicsofthewaterafteranaerobictreatmentareshowninTable1.
Table4:Averagevaluesoftheinfluentduringallthework(ontheleftinthetable).Nitritesandnitrates
concentrationineverystage(ontherightinthetable).
Parameter Inletconcentration
sCOD(mgO2/L) 116.9
TKN(mgN/L) 98.3
NH4
+(mgN/L) 88.0
Stage1 Stage2 Stage3 Stage4
NO2
-
(mgN/L)
80.080 40 20 0
NO3
-
(mgN/L) 0 40 60 80
SO4
2-(mgS/L) 10.8
solP(mgP/L) 13.8
NOx
--Ninthefeedingwaskeptaround80mg/L.Thatvaluewasassumedbecauseas itcan
beseen inTable1,theconcentrationofnitrogen intheammoniumformisabout88mgN/L.
Thisamountisoxidizedinthenitrificationstepproducingthatconcentrationofnitrogen,inthe
formofnitriteand/ornitrate.Assuminganitrificationyieldof90%,itwasgottenthevalueof80
mg NOx
--N/L as feeding concentration. Throughout the investigation NO2
-/NO3
- ratios were
changed. The flow of the N-NOx
- synthetic solution was set as 5% of the total water to be
treated to avoid excessive dilution. The wastewater to be treated in the proposed
denitrificationprocess,containaverylowC/Nratio,around1.3.
Theeffluent froma low-temperatureanaerobictreatmentcontainsaconsiderableamount
of dissolvedmethane and sulfide, which can be used as electron donors by the denitrifying
bacteria.Assuming atmospheric pressure, 15ºC and knowing thepercentageofmethaneand
sulfide in the anaerobic reactor biogas, 84% and 0.2% respectively, the concentration of
dissolvedmethaneandsulfidecanbecalculatedaccording toHenry’s law, resulting in22mg
Page 101
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89
CH4/Land9mgH2S/L respectively.Someexperimental testswereperformedtocalculate the
oversaturationofmethane(inliquidphase)inthewatertobetreated(fromAnMBR)resulting
in20-40mgCH4/L.Therefore,theamountofmethaneandsulfidedissolvedandoversaturated,
is the available quantity of these compounds in the reactor to perform the denitrification
process.
2.3.AnalyticalMethods
Samplesofwastewaterweretakenbeforeandafterfinishingthedenitrificationordenitritation
process.Theconcentrationofnitrite,nitrateandsolublephosphorusweremeasuredbyHigh
PerformanceLiquidChromatography(HPLC).Ammoniumconcentrationwasdeterminedusing
anammonia-selectiveelectrode:Orion,model9512HPBNWP.TheanalysesofChemicalOxygen
Demand(COD),TotalKjeldahlNitrogen(TKN)aswellastotalandvolatilesuspendedsolids(TSS,
VSS) were determined according to standard methods suggested by the Standard methods
manual[22]
. The measurement of dissolved oxygen concentration was determined with an
oximeterWTW,modeloxi330/SET andadissolvedoxygenprobeCeliOx325.Gasproduction
wasmeasuredvolumetricallybywaterdisplacement,anditscompositionintermsofmethane,
carbon dioxide, nitrogen, oxygen, hydrogen sulfide and hydrogen was determined by gas
chromatography (GC) (Varian CP-3800). Pressure, temperature and oxidation-reduction
potential(ORP)weremeasuredbyusingsensorsandprobes.
2.4.OperationStrategy
In the first part of the work, denitrification efficiency was studied at different hydraulic
retentiontimes(HRT)inordertoinvestigatetheoptimumtreatmenttimeconditionsfornitrite
andnitrateremoval.DuringthisstudytheconcentrationofNOx
--Ninthefeedingwasaround80
mg/LandtheratioofNO2
-/NO3
-wassetat50%/50%,correspondingtothestage2,asdescribed
inTable2.ThereactorremovalefficiencywasevaluatedatthedifferentfollowingHRT:8h,6h,
4h,2hand1.5h,bychangingthereactorfeedingflowfrom2.6L/hto14L/h.
Table5:AverageconcentrationsofNO2
--NandNO3
--NatdifferentHRT.
Stage2 8h 6h 4h 2h30 2h 1h30
NO2
-
(mgN/L)
Inlet 43.8 49.5 43.9 46.6 41.7 47.9
Outlet 0.0 0.0 0.0 0.0 0.0 2.0
NO3
-
(mgN/L)
Inlet 35.8 44.3 38.5 34.9 35.7 32.1
Outlet 0.0 0.0 0.0 0.0 0.0 2.9
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90
Once it was found the optimum HRT for the process, the second part of the work was
focused on the study of the reactor behavior when NO2
-/NO3
- ratios were changed. Stage 1
corresponds to denitritation with a feeding of about 80 mg NO2
--N/L (ratio NO2
-/NO3
-:
100%/0%).Stage2whenthefeedingwas40mgNO2
--N/Land40NO3
--N/L(50%/50%).Stage3
whenthefeedconcentrationwas20NO2
--N/Land60NO3
--N/L(25%/75%).Andinthestage4is
represented the denitrification with a feed of around 80 mg NO3
--N/L (0%/100%). The
procedurecarriedoutinthestagesisshownintherightpartoftheTable1.
3.RESULTSANDDISCUSSION
Inthisstudy,usingananoxicfixedfilmreactor,itwasperformedthenitrogenremovalfrom
realwastewaterwiththeorganicmatternotremovedintheprecedinganaerobicprocess,and
thedissolvedandoversaturatedbiogas(methaneandsulfide)availableinthewater.
3.1.Optimizationoftheresidencetime.
Thefirstpartoftheworkhadadurationof130days.Denitrificationefficiencywasstudiedat
differentHRTbychangingthefeedingflow.
As shown in theTable2, in this study the concentrationof theNOx
--N in the feedingwas
around 80 mg/L and the ratio of NO2
-/NO3
- was set at 50%/50%. The effluent was studied
decreasing theHRT from 8 to 1.5 hours (Table 2)with the aim of determining the optimum
treatmenttimeconditionsforthenitrateandnitriteremoval.
Duringthisstudyatfixedinfluentrateofnitriteandnitrate(50%/50%),withHRTdecreasing
from8to2hours,theremovalefficiencywas100%withaneffluentfreeofoxidizednitrogen
compounds.TotaleliminationofNO2
-andNO3
-wasnotdetectedwhenworkingwithHRTless
than2hours.At1.5hoursofHRT,theremovalefficiencyofnitriteandnitratewasabout96%
and90%respectively,obtaining5mgNOx
--N/Lofconcentrationaverageintheoutlet.
Judging by these results, denitrification of a domestic wastewater from a AnMBR was
feasibleanditsoptimumHRTfornitrateandnitriteremovalwas2hours.
Looking at the literature, on the one hand, ZhouWet al. by using an upflowbiofilter for
denitrificationachievedahighyieldofnitrateremovalat8hofHRT[23]
.Ontheotherhand,in
theA2OprocessproposedbyZhengWetal.,a50%ofnitrateremovalwasreachedwhenHRT
was3h[24]
anda85%at4.35h[13]
whentreatinganeffluentwithalowC/Nratio(about2.5).
However, the resulting HRT obtained in the current work is much lower than previously
reportedinliterature.Thisrepresentanadvantagewhenthinkinginscaling-uptheprocess.
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91
3.2.Nitriteandnitrateremovalatdifferentfeeds.
With theobjectiveof study theviabilityof theprocess forpartial and totaldenitrification,
theHRTwaskeptat2h,asoptimizedintheprevioussection.Thefeedflowwasfixedat10.5
L/hwhiletheratioofthenitriteandnitrateconcentrationwaschanged(rightpartofTable2).
The average concentrations of the main parameters in the inlet and outlet of the
denitrificationprocesscanbeseenintheTable3.
TKNisameasureofbothtotalorganicnitrogenandammoniacalnitrogeninwastewater.As
expected,theTKN,whoseNH4
+compositionexceeded90%,didnotvaryduringthetreatment,
becausenitrification is unlikely tohaveoccurreddue to the lowDO levels in the reactor.No
modificationofsolublephosphoruswasobservedduringtheprocess.
Inallthecases,thebioreactorwasabletoremovealltheNO2
--N,attainingefficiencyabout
100%. Theprogress of nitrite andnitrate in the reactor during thedenitrificationprocess for
different ratios isdepicted inFigure2.This figurecomprises fourgraphs,eachrepresentinga
feed.Inallthefeedscanbeseenthereactorbehaviorfortheinletof80mgNOx
--N/Lwiththe
correspondingratios.Inallthecases,nitriteand/ornitritereductionstartedwithoutanydelay
and resulted in the formation of N2. The stage 4 was the most unfavorable case of
denitrificationwith a 100%of nitrate as feeding. In fact, nitrateswere found in the effluent.
Althoughinthisstagethemeanconcentrationofnitrateintheeffluentwas4.9mgNO3
--N/L,it
canbenoticedthatpunctuallyreached19mgNO3
--N/L,highervaluethantheallowable limit.
Thisindicateanotgoodyieldoftheprocess,beinginthelimitremovalofreactor.
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92
Table6:AverageparametersfordifferentNO2-/NO3
-ratiosondenitrification.
Stage1 Stage2 Stage3 Stage4
Parameter Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet
sCOD(mgO2/L)
107.7±10.9 73.7±7.4 100.1±5.7 66.7±5.7 102.9±3.3 68.1±2.3 106.7±17.2 58.6±10.3
TKN(mgN/L)
82.2±3.1 82.0±4.6 107.2±3.6 110.8±4.9 121.7±5.0 118.8±3.9 97.8±7.3 97.4±9.8
NH4+
(mgN/L)76.4±1.2 77.7±2.4 93.5±3.8 92.9±4.1 120.4±4.9 116.0±4.2 90.6±10.6 85.8±6.6
NO2-
(mgN/L)79.1±3.0 1.3±4.0 41.7±2.0 0.2±0.9 25.9±0.7 0.3±2.1 0.0±0.0 0.2±1.5
NO3-
(mgN/L)0.0±0.0 0.3±0.6 35.7±1.7 0.2±0.6 46.1±0.9 1.0±3.8 78.4±1.6 4.6±5.4
SO42-
(mgS/L)10.7±9.5 20.4±7.5 8.7±5.0 21.4±6.0 8.8±10.4 41.1±10.9 9.9±6.3 48.3±10.2
solubleP(mgP/L)
10.3±1.0 10.8±1.3 13.9±0.8 14.5±1.4 14.6±3.6 13.0±1.4 9,8±0.8 10.1±2.0
Page 105
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93
Figure2:InfluentandeffluentofNO2-andNO3
-.(Stage1)100%NO2-;(Stage2)50%NO2
-/50%NO3-;(Stage3)25%
NO2-/75%NO3
-and(Stage4)100%NO3-.
The experimental percentages of organicmatter removal are between 35%-40%, and the
values of COD removal correspond to the biological oxygen demand (BOD) available in the
feeding wastewater. The COD removed is consumed by the bacteria in the denitrification
process.
As Table 3 shows, simultaneously to the denitrification occurrence, an increase of sulfate
concentrationwasobservedfromstage1to4.Thiscanbeexplainedbecausethedenitrifying
bacteria need more sulfide to remove nitrate than nitrite. As consequence, more sulfate is
formed. Stoichiometrically, sulfate concentration in the effluent should be between 50% and
65%higherthanexperimentaldata.Inallstagesofoperation,theanoxicsulfideoxidationtook
place via partial and total oxidation producing elemental sulfur and sulfate. The milky
appearance inside the reactor suggested theelemental sulfur production, as an intermediate
product,probablyhigherthenexpected.Theinsolubleelementalsulfurwasaccumulatedinside
thereactorinthelowersectionbecauseofitsprecipitation.Duetothedifficultyofseparating
solid sulfur from biomass elemental sulfur could not be analyzed. Therefore, sulfate
0
20
40
60
80
100
0 1 2 3 4 5 6 7 8 9 10111213141516
Concentration(m
gN/L)
Time(d)
Stage1
0
10
20
30
40
50
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Concentration(m
gN/L)
Time(d)
Stage2
0
10
20
30
40
50
0 1 2 3 4 5 6 7 8 9 10 11 12
Concentration(m
gN/L)
Time(d)
Stage3
0
20
40
60
80
100
0 4 8 12 16 20 24 28 32
Concentration(m
gN/L)
Time(d)
Stage4
● InNO2--N� InNO3
--N○ OutNO2--N � InNO3
--N
Page 106
Chapter3
94
concentration was lower than expected stoichiometrically, due to the higher formation of
elemental sulfur. It was assumed that all sulfide removed, but not recovered as sulfate,was
convertedtoelementalsulfur.Throughoutthestudy,thegasphasefromthetopofthereactor
wasanalyzedbygaschromatography,resultinginabout0%ofH2S.
As reported in the literature [4, 25], there are indications that denitrification occurs more
easily by using sulfur compounds than methane. The activity of methanotrophic
microorganisms is much lower than that of autotrophic sulfide denitrifiers. Therefore, it is
suggestedthatthefirstelectrondonorusedfordenitrificationaftertheorganicmatterwasnot
methane, but the hydrogen sulfide present in thewater after anaerobic treatment. After all
electronsfromsulfidewereconsumed,denitrification/denitritationwithmethanestarted.
3.3.Balancesofthedenitrificationwithorganicmatter,sulfideandmethane.
The mass balance of different species over the reactor gives an indication about the
functioning of the system. To get further evidence about the nature of the process, a
stoichiometricanalysisoftheconsumptionofOM,H2SandCH4wascarriedout.
Fromthereactionsthattakeplaceintheprocessofdenitrificationwiththethreedifferent
electrondonors available in thewastewater, it canbe calculated the stoichiometric needsof
eachonetoreducenitriteandnitrate(Table4).
Table7:AverageparametersfordifferentNO2-/NO3
-ratiosondenitrification.
Ratios NO2- NO3
-
OM(mgCOD/mgN) 2.4 4.0
S2-(mgS/mgN) 1.4 2.3
CH4(mgCH4/mgN) 0.4 0.7
Theconcentrationsoforganicmatter,sulfide,nitriteandnitrateinwatercanbemeasured
empiricallyasexplainedpreviouslyintheanalyticalmethodssection,andareshowninTable3.
DividingthisvaluebythecorrespondingnumberinTable4,itisobtainedtheamountofnitrites
and/ornitratesremovedwithorganicmatterandsulfideforeachstudyphase.Forexample,in
the stage1: FromTable3, (107.7 - 73.7) = 34mgCOD/L removed; 34 (mgCOD/L) / 2.4 (mg
COD/mgNO2--N)=14.2mgNO2
--N/Lareremovedwith theorganicmatteraselectrondonor.
TheseresultsarepresentedinTable5.
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95
Table8:Concentrationandpercentagecontributionofdifferentdonorsintheelimination.
Stage1 Stage2 Stage3 Stage4
NO2-/NO3
- NO2- NO3
- NO2- NO3
- NO2- NO3
- NO2- NO3
-
Initial(mgN/L) 80 0 40 40 20 60 0 80
OM 14.2-
13.9-
14.5- -
11.2
(mgNelim/L) (17.7%) (34.8%) (72.5%) (14.0%)
S2- 7.8-
6.4-
5.5 0.6-
4.3
(mgNelim/L) (9.8%) (16.0%) (27.5%) (1.0%) (5.4%)
CH4 58.0-
19.7 40-
59.4-
64.5
(mgNelim/L) (72.5%) (49.2%) (100.0%) (99.0%) (80.6%)
Nitrates and nitrites are removed firstly using the organic matter and sulfides. Once
consumed all the organic matter easily biodegradable and sulfides, nitrates and nitrites are
eliminatedbymethaneconsumption.Numbers inbrackets indicate thepercentageofnitrites
andnitratesremovedwitheachelectrondonor.
Theamountofmethanerequiredfortheprocess isobtainedbymultiplyingtheamountof
nitritesand/ornitratesremovedwiththiselectrondonor,andthestoichiometricratioofTable
4.Theseconcentrationswere24.9mgCH4/L,36.9mgCH4/L,42.2mgCH4/Land45.8mgCH4/L
respectively. So, in the stage 4, which represents the least favorable denitrification (total
denitrification), to achieve complete removal of nitrates, the amount ofmethane needed as
electrondonorwas45.8mgCH4/L.
Thismethaneavailableforthesystemwas,ontheonesidedissolvedinthewastewater,
and in theother side, desorbedwhenentering into the reactor because its oversaturated
state. As indicated in the feeding characteristics, about 22 mg CH4/L was the dissolved
methane, and between 20-40mgCH4/Lwas the oversaturated one. Therefore, therewas
enoughamountofmethaneandbalancesarejustified.Itshouldbenotedfromthestage4,
thatmethanemaynotbeenoughtocarryoutthecompletedenitrification if it isavailable
onlyattheminimumvalueofoversaturation.
Based on the results of the balances, methane was by far the most used electron by
bacteria. This canbeexplainedbecauseof thehigheramountofmethaneavailable inwater,
but in fact, theorganicmatter and sulfidewere the first compounds tobe consumed. In the
balances,itwasnottakenintoaccountthenitrogenconsumedforcellsynthesis.
Page 108
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4.CONCLUSIONS
The denitrification of domestic wastewater with a low concentration of COD could be
possible, by using the methane and sulfide that contains the water after the anaerobic
treatment.NO2- andNO3
-were the electron acceptors,while theOM, CH4 andH2Swere the
electrondonors.Theresultsoftheworkdemonstratedthatdenitritationanddenitrificationisa
feasible process for the simultaneous removal of NO2-, NO3
-, OM, CH4 and H2S for real
wastewater. Nitrogen removal was demonstrated obtaining a successful NO2- and NO3
-
eliminationwhenthefeedwas80mgN-NOx-/L,exceptwhenthefeedingwasformedonlyby
nitrate. In this case, the process was at the limit of the denitrification process, obtaining an
effluentatsomepointsupto19mgN-NO3-/L.TheoptimalHRTtoobtainboth,denitritationand
denitrificationwas 2 hours using an anoxic reactor. The amount ofmethane available in the
waterwasenoughtoachievethegoalbeingthemainelectrondonorusedwithmorethan70%
orparticipation.
ACKNOWLEDGEMENTS
TheauthorsthankthecompanyCadaguaS.A.,theEuropeanRegionalDevelopmentFund,and
the project IPT-2011-1078-310000, and the INNPACTO 2011 program of the Ministry of
EconomyandCompetitivenessforthetechnicalandfinancialsupport.
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Chapter4.Advanceddenitrificationof
anaerobictreatmenteffluentofdomesticwastewaterbyusing
wastedgas
Abstract
A pilot plant of denitritationwas operated formore than fivemonths treating
domesticwastewaterwithhighammoniumnitrogenconcentration fromanaerobic
process under ambient temperature conditions (18 ºC). The process consisted on
onebiofilterwith2hofhydraulicretentiontime(HRT)fordenitritation.Tostudythe
feasibilityofthedenitritationprocess,differentsyntheticnitriteconcentrationswere
suppliedtotheanoxicreactortosimulatetheeffluentofanitritationprocess.The
presentworkinvestigatesanadvanceddenitritationofwastewaterusingtheorganic
matter and other alternative electron donors from an anaerobic treatment:
methaneandsulfide.Thedenitrifyingbacteriawereable to treatwateratan inlet
nitriteconcentrationof75mgNO2--N/Lwithremovalefficiencyof92,9%.Whenthe
inlet nitrite concentration was higher it was necessary to recirculate the gas
obtained in the anoxic reactor to enhance the nitrite removal, achieving 98,3%of
NO2-eliminationefficiency.
Keywords:Denitritation•Domesticwastewater•Electrondonor•
Methane•Sulfide
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1. INTRODUCTION
Theanaerobictreatmentofwastewaterhasbecomethemostusedmethodfortheeffluents
because its advantages over conventional activated sludge treatment. These include that
energy balances are quite favorable due to the energy recovery as biogas instead of energy
consumption, no energy requirement for aeration, minimum sludge production, low space
requirements and a smaller footprint. On the contrary, it has some disadvantages such as
process sensitivity, possible bad odors, long start-up period and to comply with discharge
standards, effluent from anaerobic treatment require further treatment for the remaining
chemicaloxygendemand(COD)andespeciallyfornitrogenandphosphorusbecauseofitslow
pathogenandnutrientremoval[1-5].
Themethane(CH4)production in theanaerobicbiodegradationoforganicmatterdepends
onthetreatmentefficiency.Thesolubilityofmethaneintheliquidphaseofanaerobicreactors
raises with a decrease in the temperature, and increases its loss to the environment. The
amountdissolveddependsonthepartialpressureofmethaneinthebiogas,thetemperature,
and the degree of oversaturation [6, 7]. Therefore, part of the CH4 produced is lost with the
effluentandnotavailableforenergyproduction [2-4]. Inadditiontothereductioninrecovered
energy, the unintentional emission of CH4 in the atmosphere has the problem that it is a
greenhouse gaswith an effect on globalwarning 21 times stronger than carbon dioxide [7-9],
thus the resultant fugitive methane emission is potentially sufficient to impose a negative
processcarbonfootprint.Releaseofmethanemay imposeapotentialhealthandsafety issue
duetoits lowexplosivelimit(downto5%) [10,11].Apost-treatmentprocesswillberequiredin
order to avoid dissolved methane release to the atmosphere and to make anaerobic
wastewatertreatmentamoreeco-friendlytechnology[3,12].
Sulfide(H2S)productionandemissionisawell-knownprobleminanaerobicdigestion,which
causes corrosionofpipes,odornuisanceandhealthhazardsbecauseof its toxicity. Sulfide is
mainly generated anaerobically by the reduction of sulfate in wastewater through the
respiration of sulfate-reducing bacteria (SRB). Sulfate concentration, COD concentration and
HRTareamongthekeyfactorsidentifiedtoinfluencesulfideconcentration,withhighersulfate
andCODconcentrationsandlongerHRTfavoringhighersulfideproduction[10,11].
The elimination of nitrogen compounds from wastewater is based on nitrification and
denitrification. Inthefirststep,nitrification,ammoniumisoxidized intonitritebyammonium-
oxidizing bacteria (AOB), and nitrite is oxidized into nitrate during the second step by nitrite
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oxidizing bacteria [13]. The second step is denitrification, where nitrate (formed in the
nitrification step) is anoxically converted intonitrite, then intonitrousoxide,nitricoxide, and
finallyintonitrogengas,accordingwiththissequence:
NH4+→NO2
-→NO3-→NO2
-→NO→N2O→N2
Denitrifyingmicroorganismsareheterotrophic,andinanoxicconditionsusenitriteornitrate
as final electron acceptors [14-16]. The presence of an organic carbon source is needed in
heterotrophicdenitrification.WhennotenoughCODispresentinthewastewaterbeingtreated
fordenitrificationtooccur,forexampleinwastewaterswithalowCOD/Nratio,orbecauseof
highCODconsumption inprevious steps suchasnitrification, theadditionofexternal carbon
sourceisrequiredtobeaddedinthesystemtoachieveeffectiveheterotrophicdenitrification
[17,18].Operationalcostsofthebiologicalnitrogenremovalprocessaretoagreatextentrelated
totheoxygenandorganicmatterrequirementsfornitrificationanddenitrification,respectively.
Severalnewprocessesandoperationalstrategieshavearisenduringthe lastyears inorderto
reducethesecosts.Oneoftheseistheshortcuttobiologicalnitrogenremoval.Thisprocessis
based on the fact that, since nitrite is an intermediary compound in nitrification and
denitrification, it will be convenient to produce a partial nitrification up to nitrite and then
denitrificationstartingfromthisnitrite,asindicatesthefollowingsequence:
NH4+→NO2
-→NO→N2O→N2
The nitritation/denitritation process results in savings in oxygen demands during
nitrification, requires less carbon source, leading to a reduction of the organic matter
requirementsinthedenitrificationprocessandadecreaseinsurplussludgeproduction[14-16,18].
Denitrification process requires electron donors like organic carbon sources for the
heterotrophicmicrobial reaction. However, the content of readily biodegradable substrate in
wastewater isveryoftenthelimitingfactorforcompletedenitrificationevenatrelativelyhigh
C/N ratios. In these cases, external carbon sources such asmethanol need to be supplied to
achievecompleteheterotrophicdenitrification,thusincreasingtheoperatingcostoftreatment
becauseoftheacquisitionofchemicalsandthepossibleproductionofadditionalsludge[19-21].
To lower the costs of denitrification, the search for electron donors produced during the
wastewatertreatmentprocesseshasdeservedspecialattention.Methaneandsulfidecouldbe
interesting alternative electron donors for the denitrification process [22-24]. The literature
presentsoptionsinwhichthebiogasoutletlinegeneratedfromanUASBreactorwasconnected
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105
to the anoxic reactor. Thus, the biogas supplied could be used in the denitrification process.
Becauseofbiogasisproducedintheanaerobictreatmentplants,thistechnologycanproduce
low-costandefficientelectrondonorsreadilyuseablefordenitrification.
Whatisproposedinthisworkistheuseofmethaneandsulfidepresentinthewastewater
fromtheanaerobictreatment,andnotthebiogaslinedirectly.Methaneandsulfide,dissolved
and oversaturated in thewater, by entering in the anoxic reactor are going to be desorbed,
passing to the gaseous state and thus being used by denitrifying bacteria. Using these
compounds for denitritation would make nitrogen removal less expensive than introducing
chemicals.
If thisproceeding isnotenoughto removethenitrite fromthewastewater, it isproposed
therecirculationofthegasobtainedinthetopoftheanoxicreactortothelowerpartofitself.
In this way, the remained electron donors present in thewaste gas, not previously used for
denitrify,haveanotheropportunitytobeusedintheprocess.
The process combining both anaerobic treatment and nitrogen removal allows partial
conversion of organic matter into a valuable energy, while respecting the environmental
constraints as regards nitrogen and energy costs are reduced. The denitrification process
displayed can simultaneously convert nitrate,methane and sulfide from thewastewater into
dinitrogengas,carbondioxideandsulfate,respectively,usinganoxiccondition.
Theobjectiveofthisresearchwastostudythefeasibilityofthepartialdenitrificationprocess
of high ammonium nitrogen concentration wastewater using alternative electron donors
presentintheanaerobicmembranebioreactor(AnMBR)effluent:OMandCH4andS2-.Forhigh
nitrite concentrations in the feeding, itwas study the possibility of recirculate thewaste gas
from the anoxic reactor with the aim of reuse the electron donors not previously used for
denitrify.
2. MATERIALSANDMETHODS
2.1. ExperimentalSetup
The experimental study of partial denitrification process consists of one anoxic fixed-bed
bioreactorbuiltinglassPVC.Thebioreactor,anupflowcylindricalcolumn,hadaheightof2.8
m,adiameterof0.15mandaworkingvolumewasofapproximately20L.Adiagramof the
bioreactorisshowninFigure1. Inordertoserveassupportforthemedium,thereactorwas
filledwithFiltralite®withthefollowingcharacteristics:effectivesize,3.5mm;bulkdensity,825
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106
kg/m3;particledensity,1450kg/m3;particleporosity,46%.Thereactorwasfedwiththereject
water of an AnMBR that treated domestic wastewater under psychrophilic conditions in a
previous stage where the major COD was converted into biogas [5], therefore, the AnMBR
produced effluentswith low levels of readily biodegradable organicmatter. Also, a synthetic
nitritestreamfedthebioreactorsimulatingtheeffluentofanitritationprocess.NaNO2solution
waspumpedcontinuouslybyadiaphragmmeteringpumpanditwasusedasthenitritesource.
The inoculum was a mix of anoxic sludge and anaerobic digested sludge, taken from the
wastewater treatment plant of Valladolid (Spain). The biofilter was equipped with
measurementsystemsforpressure,gasflowandoxidation-reductionpotential(ORP).
Figure1:Schemeofthedenitrification/denitritationplant.(1)Fillingpump,suppliesthewastewatertobe
treated,(2)Tankwiththewaterthatdoesnotenterinprocess.(3)Gascollectionchamber,(4)
bioreactor,(5)PumpthatsupplythesyntheticNO2-solution,(6)Currentoftheeffluenttreated,(7)
oxidation/reductionpotentialmeter,(8)Flowindicator,(9)Pressuregauge.
2.2. FeedingCharacteristics
ThestudiedreactorwasfedwiththeeffluentfromanAnMBRtreatingdomesticwastewater
andthesyntheticnitritestream,tosimulatetherecirculationoftheaerobictreatmenteffluent.
TheAnMBRpilotplant isexplained indetail inapreviouswork [5].Typicalcompositionof the
wastewaterusedasinlettothecontinuousflowdenitritationreactorisgiveninTable1.When
GasFlowMeter
Wastewater fromAnMBR
ORP
Effluent treated
SyntheticNO2
-
FI
(1)
(3)
(4)
(5)
(6)
(7)
(8)
(2)
PI (9)
Recirculationofgas
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the AnMBR effluent was sampled, the sulfide contained in the wastewater was oxidized to
sulfate,andbychromatographic techniquesthiscompoundcouldbedetermined. In the inlet
stream (from anaerobic treatment), sulfur is the corresponding amount of sulfide oxidation
withoutquantifytheoversaturation,sotherealvalueforsulfidewashigher.
Table1:AveragecompositionofthewastewaterfromAnMBRbeforetreatment.
sCOD(mgO2/L)
TKN(mgN/L)
NH4+
(mgN/L)NO3
-(mgN/L)
NO2-
(mgN/L)SO4
2-
(mgS/L)solP
(mgP/L)
113.2 108.6 94.3 0.0 50-75-90 8.4 10.9
2.3. OperatingScheme
Theanoxicbioreactorwasoperatedforaperiodoffivemonthswithaninletflowof10L/h.
Considering the effective volume of the reactor it can be assumed a corresponding HRT of
approximately2hours throughout theexperiment. Temperature in theplantwasmaintained
under ambient conditions (18 ºC) using a fan coil unit in the laboratory. Four stages with
differentoperatingconditionswerestudied.
Thefeedconcentrationsofnitritewereusedwiththeintentionofsimulatetheeffluentofa
nitritation process. The nitritation process would oxidize the NH4+ available in the feeding
wastewater, that looking the Table 3 its concentration varied from 80 to 110 mg NH4+-N/L.
Stoichiometrically,theNH4+oxidizedwouldimplyanitriteconcentrationbetween62and85mg
NO2--N/Lapproximately.Toworkwithasecurityrange,itwasintroducedupto95mgNO2
--N/L.
Table2 shows thestagesofoperating.Thedifferencebetween the first threestageswas the
nitritefeedingconcentration.50,75and90mgNO2--N/Lweretheinletnitriteconcentrationfor
stages1,2and3respectively.Instage4,anewstreamwasaddedtothedenitrificationreactor.
The gas obtained from the anoxic reactor was recirculated to reintroduce in the process
electrondonorsnotpreviouslyusedfordenitrify.
Table2:Stagesofoperation.
Stage NO2-inletconcentration
1 50 mgNO2--N
2 75 mgNO2--N
3 90 mgNO2--N
4 95 mgNO2--Nwithgasrecirculation
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2.4.AnalyticalMethods
Samplesofwastewaterwerecollectedperiodicallybeforeandafter thedenitritation
process.Theconcentrationofnitrite,nitrateandsolublephosphorusweremeasuredby
High Performance Liquid Chromatography (HPLC). Ammonium concentration was
determined using an ammonia-selective electrode: Orion, model 9512HPBNWP. The
analysesofChemicalOxygenDemand(COD),TotalKjeldahlNitrogen(TKN)aswellastotal
and volatile suspended solids (TSS, VSS) were determined according to the Standard
methods for examination of water and wastewater suggested by the manual APHA-
AWWA-WPCF [25]. Themeasurementofdissolvedoxygenconcentrationwasdetermined
withanoximeterWTW,modeloxi330/SETandadissolvedoxygenprobeCeliOx325.Gas
productionwasmeasured volumetrically bywater displacement, and its composition in
termsofmethane,carbondioxide,nitrogen,oxygen,hydrogensulfideandhydrogenwas
determined by gas chromatography (GC) (Varian CP-3800). Pressure, temperature and
oxidationreductionpotential(ORP)weremeasuredbyusingsensorsandprobes.
3. RESULTSANDDISCUSSION
The reactor was operating during more than five months under the conditions
previously described in Table 2. The feasibility of using the reduced compounds of the
waterfromanAnMBRaselectrondonorsfordenitritationwasevaluatedatdifferentNO2-
concentrations in the feed stream (stages 1-3). As consequence, raising the NO2-
concentrationinthefeeding,thenitrogenloadingrate(NLR)wasincreasedfrom0.57kg
N-NO2-/m3dinthefirststageto1.03kgN-NO2
-/m3dinthestage3.AnHRTof2hourswas
remained during all the research. The anaerobically pretreated domestic sewage
presentedalowCOD/NO2--Nratio,specifically1.87inthestage1,and1.47,1.30and1.32
forthestages2,3and4,respectively.Table3summarizestheaverageconcentrationsof
themainparametersintheinletandoutletofthedenitritationprocessforthedifferent
stagesofoperation.
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109
Table3:SummaryoftheparametersaveragefordifferentNO2-concentrationsinletondenitritation.
sCOD(mgO2/L)
TKN(mgN/L)
NH4+
(mgN/L)NO3
--N(mgN/L)
NO2--N
(mgN/L)solS
(mgS/L)solP
(mgP/L)
1Inlet 97,7 114.0 79.0 0.0 52.2 12.8 8.6
Outlet 60.6 86.4 77.5 0.0 2.5 21.3 8.8
2Inlet 109.9 110.7 97.6 0.0 74.7 7.2 11.3
Outlet 65.1 109.8 95.9 0.0 5.3 29.2 11.3
3Inlet 119.7 98.5 92.4 0.0 92.2 5.8 11.8
Outlet 76.6 100.5 86.0 0.0 21.0 24.6 9.8
4Inleta 125.5 111.0 108.0 0.0 95.0 7.8 11.7
Outlet 79.3 112.4 84.2 0.0 1.6 20.0 9.1
aWithgasrecirculation.
AsitcanbeseeninTable3,theresultsindicatednitriteeliminationefficienciesof95.1%and
92.9%whentheinletconcentrationwas50and75mgNO2--N/L(stages1and2),respectively.
Inthestage3,whenthefeedwas90mgNO2--N/L,77%ofnitriteswereeliminatedobtaining
around21mgNO2--N/Lintheoutlet.TheNO2
--Nconcentrationintheeffluentincreasedwhile
theremovalefficiencydecreasedduetotheincreaseintheNLRbyraisingtheinfluentNO2--N
concentration.
Duringthestages1,2and3,thegascompositionattheoutletofthedenitritationprocess
containedmainlyN2andCH4(58%and37%,respectively).All thesulfidefromthefeedwater
wereusedforthenitritesreductionbecausetherewasnoH2Sinthegasphase.
To improvethedenitritationprocesswhenthenitrite feedingconcentrationwasabout90
mgNO2--N/L,itwasproceededtorecirculatethegascollectedinthetopofthereactortothe
lower part (stage 4). Thus, methane desorption (initially oversaturated) was favored and
denitrifyingbacteriawereabletouse itaselectrondonor.Fortunately, thisperformancewas
successful, achieving around 98.3% of nitrite elimination efficiency after the denitrifying
processwhentheNLRwas1.09kgN/m3d.
Figure2showstheconcentrationsofNO2--NandCODinthe influentandeffluent intime.
Thegraphshowsahighvariability in the feedingCODconcentrationsduringall thestagesof
the experiment due to the typical fluctuations in actual domestic sewage. In Figure 3, it is
representedtheevolutionofnitritebeforeandafterdenitritationandtheremovalpercentage
of thiscompound ineachperiodstudied,with theircorrespondingstandarddeviation. In the
stage3,itcanbeseentheaccumulationofnitriteattheendoftheprocess,revealingaprocess
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limitationtoremovenitritewhentheinletconcentrationwas90mgNO2--N/L,whileintherest
ofstagesitisclearitsalmosttotalelimination.
Figure2:ConcentrationsofNO2--NandCODintheinfluentandeffluentintime.
During theanoxicprocess, theorganicmatter showedsignificantdecreasesbetween36%
and41%,asitcanbeseeninthecolumnofsolubleCODfromtheTable3andintheFigure3.
ThesevaluesofCODremovedcorrespondtothebiologicaloxygendemand(BOD)availablein
thefeedingwastewater.Thisphenomenoncanbeexplainedbecausetheorganicmatterisone
of the electron donors used by the bacteria to denitrify. The TKN, whose NH4+ composition
exceeds70%,didnotvaryduringthetreatment,becausenitritationisunlikelytohaveoccurred
duetothelowDOlevelsinthedenitritationprocess.ComparingtheconcentrationofSO42- in
the influent and effluent of the process, it was increased due to the oxidation of the H2S
available in wastewater from anaerobic treatment, to SO42-. No modification of soluble
phosphoruswasobservedduringtheprocess.
0
20
40
60
80
100
120
140
160
0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105
NO2--NandCODconcentration(m
g/L)
Time(d)
InfluentNO2-N EffluentNO2-N InfluentCOD EffluentCOD
STAGE3 STAGE4STAGE2STAGE1
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Figure3:EvolutionofNO2-beforeandafterdenitritationandpercentageofNO2
-removalefficiency.
Knowing the OM, H2S and CH4 available in wastewater, and the stoichiometric ratios
betweenelectrondonorsandacceptors(Table4),itwaspossibletodeterminetheintervention
percentageofeachelectrondonor.ThebiogascompositionoftheAnMBRwithrespecttoCH4
andH2S,is84%and0.2%respectively.ByHenry´sLawattheoperationaltemperature,itcanbe
obtained the theoretical concentration of that compounds dissolved in the effluent of
anaerobictreatment,thereforetheinletforthedenitritationprocess.Thetheoreticalvaluesof
dissolvedmethane and sulfide calculatedwere 22.2mg CH4/L and 8.9mg H2S/L.Moreover,
dissolvedmethaneandsulfideoversaturation in theanaerobiceffluentwasobserved [5], soa
higherconcentrationofthesealternativeelectrondonorswasavailableinthewastewatertobe
usedofdenitrify.
Table4:AverageparametersfordifferentNO2-/NO3
-ratiosondenitrification.
Ratios NO2- NO3
-
OM(mgCOD/mgN) 2.4 4.0
S2-(mgS/mgN) 1.4 2.3
CH4(mgCH4/mgN) 0.4 0.7
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0
10
20
30
40
50
60
70
80
90
100
Stage1 Stage2 Stage3 Stage4
NO2-removalefficiency(%)
NO2-concentration(m
gNO2--N/L)
NO2- inlet NO2- outlet %NO2- removal
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Methanewasbyfarthemainelectrondonorused(65%)followedbyorganicmatter(22%).
Sulfidewas theelectrondonor lessused (13%)due to their lowercontent in thewater from
AnMBR. As reported in the literature [21, 26], there are indications that denitrification occurs
moreeasilybyusingsulfurcompoundsthanmethane.Itissuggestedthatdenitrificationfirstly
occurs by using organic matter and sulfide present in the wastewater after anaerobic
treatment.Then,becauseoftheactivityofmethanotrophicmicroorganismsis lowerthanthe
autotrophic denitrifiers, after all sulfide electrons were consumed, the denitritation with
methanestarted.
Methane and sulfide used as electron donors in denitrification process have several
advantages when comparing with other alternatives such as the addition of methanol or
acetate. The first two compounds are low-cost sources that can be suitable in the nitrogen
removalfromwastewatersincethattheycanbegeneratedonsitebytheanaerobicdigestionof
sludgeintheWWTP[23,27].Theprocessofmethaneoxidationcoupledtodenitrificationcanbe
appliedfornitrogenpollutioncontrol,andtooffseteutrophicationandatmosphericmethane
concentrationssimultaneously[28].
This process developedwas a part of an overall treatment plan where the NO2-, organic
matter, CH4 andH2Swere removed. For the full treatment, an aerobic reactor for nitritation
was necessary, where NH4+ was converted into NO2
-, and took place the oxidation of the
residualorganicmatter,achievingCODremovalefficiencieshigherthan80%.
4. CONCLUSIONS
Inordertotreateffectivelywastewaterafteranaerobictreatment,wheremostoftheCOD
hasbeenremoved,denitritationprocessusingalternativeelectrondonorspresentinthewater
was investigated. The results of this study demonstrated that the denitritation process
presentedinthisworkwasabletoremovearound95%and93%ofnitritewhentheinletwas
50 mg NO2--N/L and 75 mg NO2
--N/L from a simulated recirculation of aerobic treatment
effluent in 2 hours of HRT. For high inlet concentrations of nitrite, recirculation of the gas
collected in the anoxic reactor was a successful solution, thus achieving a nitrite removal
efficiencyupperthan98%whenthenitriteconcentrationinthefeedwas95mgNO2--N/L.
Specifically,denitritationisafeasibleprocessforthesimultaneousremovalofNO2-,OM,CH4
andH2S for actualwastewater and the recirculationof thegas from theanoxic reactor is an
efficacioussystemtoenhancethenitritesremoval.
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113
ACKNOWLEDGEMENTS
The authors thank the company Cadagua S.A., the European Regional Development
Fund, and the project IPT-2011-1078-310000, and the INNPACTO 2011 program of the
MinistryofEconomyandCompetitivenessforthetechnicalandfinancialsupport.
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Technology,2008,99(13),p.5335-5340.
21. J.L.R.PantojaFilho,M.H.R.Z.Damianovic,D.F.FonsecaandE.Foresti,Nitrogenand
residualorganicmatterremoval fromanaerobicreactoreffluent inafixed-bedreactor
withbiogasfordenitrification, JournalofChemicalTechnology&Biotechnology,2015,
90(12),p.2227–2233.
22. C.Costa,C.Dijkema,M.Friedrich,P.García-Encina,F.Fernández-PolancoandA. J.M.
Stams, Denitrification with methane as electron donor in oxygen-limited bioreactors,
AppliedMicrobiologyandBiotechnology,2000,53(6),p.754-762.
23. S. Islas-Lima, F. Thalasso and J. Gómez-Hernandez, Evidence of anoxic methane
oxidationcoupledtodenitrification,WaterResearch,2004,38(1),p.13-16.
24. J.RodríguezVictoriaandE.Foresti,Anovelaerobic-anoxicbiologicalfilterfornitrogen
removal from UASB effluent using biogas compounds as electron donors for
denitrification,RevistaFacultaddeIngenieríaUniversidaddeAntioquia,201172-80.
25. Apha, Standard Methods for the Examination of Water and Wastewater, American
PublicHealthAssociation,WashingtonD.C.,USA,1998.
26. Garbossa,J.A.Rodriguez,K.R.LapaandE.Foresti,Journal.
27. F.-y. Sun, W.-y. Dong, M.-f. Shao, X.-m. Lv, J. Li, L.-y. Peng and H.-j. Wang, Aerobic
methaneoxidationcoupledtodenitrificationinamembranebiofilmreactor:Treatment
performanceandtheeffectofoxygenventilation,BioresourceTechnology,2013,1452-
9.
28. J. Zhu, Q. Wang, M. Yuan, G.-Y. A. Tan, F. Sun, C. Wang, W. Wu and P.-H. Lee,
Microbiology and potential applications of aerobic methane oxidation coupled to
denitrification (AME-D) process: A review, Water Research, 2016, 90203 - 215.
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Chapter5.Nitrogenremovalindomestic
wastewater.EffectofthenitraterecyclingandtheCOD/Nratio.
Abstract
A denitrification/nitrification pilot plant was designed, built and operated to
treattheeffluentofananaerobicreactor.Theplantwasoperatedtoexaminethe
effect of the nitrate recycling and the COD/N ratio on the nitrogen and the
remainingorganicmatterremoval.Thesystemconsistedofatwostagestreatment
process:anoxicandaerobic.Thehydraulicretentiontime(HRT)ofthesystemwas
2 h for the anoxic bioreactor and 4 h for the aerobic one. The increase in the
nitrate recycling ratio did not suppose a significant improvement in thenitrogen
removalduetotheinsufficientcarbonsource.Thewastewatertobetreatedhada
C/Nratioof1.1showingalackoforganiccarbon.Theadditionofmethanolwasa
key point in the denitrification process employed as a model for the traditional
wastewater by-pass in the WWTP. The maximum nitrogen and organic matter
removal(84.7%and96%,respectively)wasachievedwithanitraterecyclingratio
of600%andaC/Nof8.25,adjustedbymethanoladdition.
Keywords:Biologicalnutrientremoval(BNR)•C/Nratio•Denitrification•
Nitrification•Organicmatter
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1. INTRODUCTION
Wastewater treatment plants (WWTPs) are defined currently to remove particulate and
dissolvedorganicfractionsand,inmoresensitiveareas,nitrogenandphosphoruscompounds.
The most conventional well-known intensive system to treat domestic wastewater is the
activatedsludgeprocess[1].However,theanaerobictreatmentofwastewaterhasbecomethe
mostusedmethodforprocessingeffluentsbecauseitsadvantagesoverconventionalactivated
sludge treatment. It requires low energy consumption, while it provides low wastage of
biological solids, and transforms the organic matter into valuable biogas [2]. Among the
disadvantagesof theanaerobic treatment,post treatmentsarenecessary inorder toachieve
dischargestandards.
AccordingtotheOfficialSpanishBulletin(BOE),thecharacteristicparametersoftheactivity,
itsemissionlimitvaluesandreferencemeasurementmethodsfordischargesfromwastewater
treatmentplantsderive fromDirective91/271/CEE transposedbyRDL11/1995,RD509/1996
and RD 2116/1998. The requirements for discharges from WWTP are 125 mg O2/L for the
chemical oxygen demand (COD) or a minimum reduction percentage of 75% (reduction in
relationtotheinfluentload),and15mgN/Lforthetotalnitrogen(TN)oraminimumreduction
percentagebetween70-80%[3,4].
Inthelastdecade,increasinglystringentenvironmentalrequirementshavebeenimposedon
nutrients discharge in receivingwaters, because excessive nutrients are considered the
primarycausesofeutrophication[5].Mostoftheeffortshavebeenfocusedonthedevelopment
of new technologies capable of obtaining better effluent quality, with special attention to
nitrogenremovalandthereductionoftreatmentcosts[6].Tocontroleutrophicationinreceiving
waterbodies,biologicalnutrientremoval(BNR)ofnitrogenhasbeenwidelyusedinwastewater
treatment practice, both for the upgrade of existing wastewater treatment facilities and the
designofnewfacilities [7].BNRconstitutes themosteconomicalandsustainable techniqueto
meetincreasinglyrigorousdischargerequirements[8,9].
BNR isachieved through twoprocesses:nitrificationanddenitrification. In thenitrification
process, under aerobic conditions, ammonium (NH4+) is converted to nitrite (NO2
-) by the
ammonium oxidizing bacteria (AOB). Then, nitrite is oxidized to nitrate (NO3-) by the nitrite
oxidizing bacteria [10]. Denitrification is an anoxic process of nitrate reduction into nitrite and
thenintomolecularnitrogengas(N2),whichisperformedbyafunctionalgroupofheterotrophs
that use nitrite and/or nitrate as the electron acceptor in respiration. Denitrification process
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requireselectrondonors likeorganiccarbonsources for theheterotrophic [8, 9, 11-13].Although
the conventional denitrification uses organic matter as electron donor for denitrify,
denitrification using alternative electron donors, as methane and sulfide, have been
experimentallyappliedtowastewatersfordenitrification[13,14].
There are different terms of denitrification such as pre-denitrification and post-
denitrification depending on the order of nitrification and denitrification. In a post-
denitrificationconfiguration,wastewaterisfedtoanitrificationsystempriortodenitrification.
This configuration leads usually to a total consumption of the COD before starting the
denitrificationprocess;thereforeanexogenouscarbonsourceshouldbesuppliedtocarryout
thepost-anoxicdenitrification[9,15].Incontrast,inmostBNRsystems,theanoxicstageislocated
upstream of the aerobic zone. Wastewater is fed directly to the denitrification system,
supplyingorganiccarbontoremovenitriteandnitrate thatare recycled fromthenitrification
system.Highdenitrificationratescanbeachievedwiththepre-anoxicregimegiventhesupply
ofreadilybiodegradablecarbon.However,it isaccompaniedwithsomedisadvantagessuchas
higher energy costs frommixed liquor recycle flows, dissolved oxygen (DO) return from the
aerobic,anddilutionofinfluentcarbon[8,15].
Afteranaerobictreatment,anitrogenremovalplantreceivesan influentcontainingmainly
the residual soluble fraction of organic carbon present in domestic wastewater and a large
fractionofthenitrogen.Therefore,theinfluentpresentsalowCOD/Nratio,whichisfavorable
tothenitrificationstagebutmaybeanobstacleforthedenitrificationstep[1].
The denitrification potential of wastewater is mainly governed by the availability of
biodegradableorganiccarbon,commonlyexpressedastheC/Nratio[6].Therefore,theC/Nratio
of the influent is one of the most critical parameters that can affect directly the biological
nitrogenremovalefficiency.Thisoccursbecausedifferentmicroorganismspopulationscompete
for substrate causing fluctuation in effectiveness of organic and nitrogen removal [12, 16].
Theoretically, the stoichiometric requirementof organic substrate for denitrification is 2.86 g
COD/gN, considering the electron transmitting balance betweenorganic substrate andNO3-.
ButsomestudiesdemonstratedthatC/Nvaluesofapproximately6-11gCOD/gNcouldallowa
propernitrogenremoval[6].InthecaseofKimetal.[15],withaC/N=8ratio,itwasobtainedan
average denitrification efficiency around 72%. Another example, Fu et al. [16], achieved a
nitrogenremovalefficiencyof90.6%whentheC/Nratiowas9.3.
The amount of biodegradable organic carbon of domestic wastewater after anaerobic
treatmentislimitedandnitrogenremovalislimitedbythelackofbioavailableelectrondonors
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forheterotrophicdenitrification[5,17].Therefore,theadditionofexternalcarbonsourcesoften
becomesnecessaryforachievinghigh-efficiencyBNR,especiallyforfacilitieswithweakinfluent
biologicaloxygendemand(BOD) and/or those facing stricteffluent limits [5, 18].Methanol is
themostcommonlyusedelectrondonor,asaresultofthehigherdenitrificationefficiency,as
indicatedbytherelativelylowermethanol-to-nitrateratio,lowercost,andbroadavailabilityin
themarket. Themain disadvantageof usingmethanol is the safety issues associatedwith its
transportation, handling, and storage. The use ofmethanol in commercial scale entails costs
andtheprocessmaynotbeviablefromaneconomicpointofview.Ithasbeenestimatedthat
an additional 25 to 31%of the capital construction cost formethanol storage, pumping, and
deliverysystemsisrequiredtomeetthesafetystandardsovertheuseofanon-flammable,non-
hazardousproduct[5,19].
One of the most effective methods to increase the organic matter concentration of the
influentwithouttheadditionofexternalorganicsubstrates isachievedbymixingafractionof
the influent to the anaerobic reactor with the effluent of that reactor. In such case, the
anaerobic reactor should be used to treat initially only a part of the influent raw sewage
(possiblynomorethan50–70%),andtheremainingpart (30–50%)shouldbedirectedtothe
complementary biological treatment. The use of this “by-pass” will increase the COD of the
reactoreffluentmakingitmoreadequatetothenextdenitrificationstage[20,21].
Among the available technologies, biofiltration has been widely deployed in urban
wastewater treatment plants. Biofiltration technology combines both physical and biological
treatmentbyusinganimmersedfiltermaterial.Duringbiofiltrationtreatment,thewastewater
issimplypassedthroughafixedbedofmedia,whichactsbothasafilterandasasupportfor
the growth of nutrient consuming bacteria. The advantages of these immersed biological
systemsresideintheircompactness(smallfootprint)andlowresidencetime[22].
Thiswork is focused on the study of the integration of denitrification/nitrification process
treatingdomesticwastewaterafteranaerobictreatment.Thespecificaimofthestudywasthe
influenceevaluationoftheCOD/Nratioandthenitraterecyclingratioinnitrogenremoval.To
do so, a denitrification/nitrification pilot plant was designed, built and operated at different
conditions.
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2.MATERIALSANDMETHODS
2.1.ExperimentalSetup
Thepilotplantconsistsoftwofixedbedbioreactorsabletodevelopthedenitrificationand
nitrification inwastewater. Both reactorswere designed as vertical cylinders. The height and
diameterof the anoxic cylinderused for denitrificationwas2.78mand0.15m, respectively,
withaworkingvolumeof20L.Theheightanddiameterofthenitrificationcylinderwas1.86m
and0.30m, respectively,with 40 L ofworking volume. The anoxic bioreactorwas filledwith
corrugatedPVCrings,whiletheaerobiconewithFiltralite®asfiltermedium.Adiagramofthe
pilotplantisshowninFigure1.Temperatureintheplantwasmaintainedat18ºC,whichisthe
working temperature of the previous anaerobic reactor [2]. The denitrifying biofilter was
equippedwithmeasurementsystemsforpressure,gas flowandoxidation-reductionpotential
(ORP), while the nitrifying biofilter with a probe to measure the dissolved oxygen and
temperature. The incoming flowwas set to 20 L/h. The denitrification reactor was operated
withaHRTof2hwhile thenitrificationoneat4h. TheseHRTwerepreviouslyoptimizedby
studying each reactor individually. The aeration rate was controlled through a flow meter,
maintaining the dissolved oxygen (DO) concentration between 2.0-2.5mgO2/L. Four aerators
werefixedonthebottomtomakethebubblesdistributeduniformly.
Theplantwasfedwiththerejectwaterofananaerobicmembranebioreactor(AnMBR)that
treated domesticwastewater under psychrophilic conditions (18 ºC) [2]. Thewastewaterwith
highconcentrationofNH4+andlowleveloforganicmatterwaspumpedtotheanoxicreactor.
Inthisfirststep,NH4+didnotchangedandpassedthroughtheaerobicreactor. Inthesecond
step,theNH4+wasoxidizedtoNO3
- inthepresenceofoxygen.Thisstreamisrecycledusinga
peristalticpump, fromtheaerobicbioreactortobethefeedtotheanoxicreactor,wherethe
denitrifyingbacteriacanusetheCODfromthefeedstream.
Due to thehighDO concentration in the recyclingwater from the aerobic bioreactor, the
organiccarbonavailableinthefeedwaterfromanaerobictreatmentwouldtendtobeoxidized
insteadofbeingusedfordenitrification.Asconsequence,denitrificationefficiencieswouldfall.
To avoid this effect as far as possible, a degassing tank was placed in the recycling line to
preventdissolvedoxygenenteringintotheanoxictank.
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Figure1:Pilotplantflowscheme.(1)Denitrificationreactor,(2)nitrificationreactor,(3)fillingpump,
suppliesthewastewaterfromanaerobictreatmenttothedenitrifyingreactor,(4)gasflowmeter,(5)
effluentfromanoxictoaerobicreactor,(6)nitraterecyclingfromaerobictoanoxicreactor,(7)degassing
tank,(8)compressor,responsibleforsupplyingtheair,(9)finaleffluent.ORP:oxidation-reduction
potentialprobe;DO:dissolvedoxygen;FI:flow-rateindicator;PI:pressureindicator.
2.2.Inoculumandfeedwastewater
The inoculum of the denitrifying bioreactor was a mix of anoxic sludge and anaerobic
digestedsludge,takenfromthewastewatertreatmentplant(WWTP)ofValladolid(Spain).The
inoculumofthenitrifyingbioreactorwassecondaryaerobicsludgefromthesameWWTP.
The studied plant was fed with the effluent from an AnMBR [2] fed with raw municipal
wastewater from the city of Valladolid (Spain). The average concentration of the main
parametersofwastewaterafteranaerobic treatmentaregiven inTable1. Itcanbeseenthat
theconcentrationofNH4+-Ndominated theTN,which leads toaCOD/Nratioas lowas1.04.
WhentheAnMBReffluentwassampled,thesulfidecontainedinthewastewaterwasoxidized
tosulfate,andbychromatographictechniquesthiscompoundcouldbedetermined.Intheinlet
stream (from anaerobic treatment), sulfur is the corresponding amount of sulfide oxidation
withoutquantifytheoversaturation,sotherealvalueforsulfidewasexpectedtobehigherthan
showed.
ORP
DO
(3)
FI
(7)
(5)
(1) (2)
(9)(4)
FI
(8)
FI
(6)
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Table1:Feedcompositionofthestudiedprocess.(Mean±standarddeviationofthemean.<QL:Lower
thanquantificationlimit).
2.3.AnalyticalMethods
Samples of wastewater were collected periodically before and after the denitrification
reactor, and after the aerobic reactor, being this stream the effluent of the process. The
concentration of nitrite, nitrate, sulfate and soluble phosphorus were measured by High
PerformanceLiquidChromatography(HPLC).Ammoniumconcentrationwasdeterminedusing
anammonia-selectiveelectrode:Orion,model9512HPBNWP.TheanalysesofChemicalOxygen
Demand(COD),TotalKjeldahlNitrogen(TKN)aswellastotalandvolatilesuspendedsolids(TSS,
VSS) were determined according to the Standard methods for examination of water and
wastewater suggestedby themanual APHA-AWWA-WPCF [23]. Themeasurement of dissolved
oxygen concentration was determined with an oximeter WTW, model oxi 330/SET and a
dissolvedoxygenprobeCeliOx325.Gasproductionfromtheanoxicbioreactorwasmeasured
volumetrically by water displacement. Gas samples were taken from the headspace of this
reactorand its composition in termsofmethane, carbondioxide,nitrogen,oxygen,hydrogen
sulfideandhydrogenwasdeterminedbygaschromatography(GC)(VarianCP-3800).Pressure,
temperature and oxidation reduction potential (ORP) were measured by using sensors and
probes.
2.4.OperationStrategy
The denitrification/nitrification experiments were run for more than five consecutive
months.Eightdifferent scenarioswerestudieduntil reach theoptimumC/Nratioandnitrate
recyclingratio(R).Eachcasewasanalyzedforaround20daysatsteadystate.Table2depicts
the recycling ratio of nitrate (R), the COD, if there was (or not) addition of external carbon
sourceandtheC/Nratioestablishedforeachcasestudied.
sCOD
(mgO2/L)TKN
(mgN/L)NH4
+
(mgN/L)NO2
-
(mgN/L)NO3
-
(mgN/L)SO4
2-
(mgS/L)solP
(mgP/L)
122.4±3.4 118.0±3.5 109.3±3.3 <QL <QL 8.7±0.2 10.7±0.3
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Table2:Characteristicsofthecasesimpliedintheoperationstrategy.
Case R Methanol SolubleCOD(mgO2/L) C/Nratio
1 Q No 106.0±2.1 1.09
2 2Q No 105.7±1.0 1.12
3 2Q Yes 286.5±3.6 2.59
4 3Q Yes 454.2±4.3 3.74
5 4Q Yes 448.7±1.5 3.94
6 5Q Yes 476.2±5.4 4.87
7 6Q Yes 574.0±3.2 5.37
8 6Q Yes 848.2±1.7 8.25
Inthecases1and2,Rwasmodified.Thesameparameterwaschangedincases4,5and6
butwithotherC/Nratiocomparedwithcases1and2.Allthesecasesareanalyzed insection
3.1.
In cases 2 and 3, R was maintained but the C/N ratio was increased by the addition of
methanol.AdifferentRwaskeptincases7and8,butwithhigherC/Nratiothancomparingto
thecases2and3.Thesecasesarediscussedinsection3.2.
3.RESULTSANDDISCUSSION
3.1.Theeffectoftherecyclingratioofnitrate.
The removal efficiency of organic matter and nitrogen in the denitrification-nitrification
systemchangingthenitraterecyclingratiowasstudied.CODconcentrationintheinfluentwas
maintainedconstantandtherecyclingRfromtheaerobicbioreactoreffluenttotheanoxicone
wasincreasedtostudyitseffect.Anincreaseintherecyclingratefromtheaerobictotheanoxic
column,providesmorenitratestothedenitrificationreactorandthus,canimprovetheoverall
nitrogenremovalandminimizetheTNconcentrationintheeffluent.
On the one hand, during the first part of the work, case 1 and 2 were experimented
analyzingtherecyclingeffect fromR=Q(Q: incomingflow)toR=2Q,beingtheCOD/Nratioof
1.09and1.12foreachcondition.
Tables 3 and 4, summarize the concentration average of the COD and the nitrogen
compounds at different nitrate recycling ratios. NH4+ concentration decreased significantly in
the anoxic reactor due to the dilution of nitrate recycling stream. The average ammonium
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removal efficiency of the overall processwas 86.1% in the case 1, and 73.6% for the case 2
(Table 4). The removal profile of NH4+-N was analogous to that of COD, indicating that the
utilizationoforganicmatterandthedegradationofNH4+occurredsimultaneously.InFigure2it
isshownthepercentageofCODandTNremoved.WiththesameCODinfluentof106mgO2/L,
theaverageCODremovalefficiencywas87.6%and74.4%foreachsituation,indicatingagood
abilitytoremovetheorganicmatter.Contrary,thetotalnitrogenremovalefficiencywaspoor
withvalueofabout20%.TheC/Nratiointhesystemwasverylow,beingalimitingfactorinthe
denitrificationprocess,whichwasnotable to remove thenitrogencompounds.NO3-was the
prominent compound of TN in the effluent and this residual nitrogenwasmainly due to the
exhaustionofthecarbonsourceofheterotrophs.
Table3:CODconcentrationintheinlet,afterthedenitrificationreactorandattheendoftheprocessfor
thedifferentconditionsevaluated.(1:Wastewaterinfluent;2:Streamfromthedenitrificationreactorto
nitrification;3:Nitrificationeffluentandtheoutletoftheplant).(Mean±standarddeviationofthe
mean).
solubleCOD(mgO2/L)
Case 1 2 3
1 106.0±2.1 44.4±1.6 13.1±1.3
2 105.7±1.0 68.5±0.4 27.1±0.2
3 286.5±3.6 76.5±0.7 22.3±0.9
4 454.2±4.3 81.9±2.4 31.9±2.8
5 448.7±1.5 72.5±2.6 20.0±3.7
6 476.2±5.4 66.7±2.1 27.7±3.3
7 574.0±3.2 68.2±1.0 21.5±0.6
8 848.2±1.7 107.0±0.6 33.8±0.2
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Table4:Evolutionofnitrogencompoundsduringthedenitrification/nitrificationprocess.(1:wastewaterinfluentfromAnMBR;2:Streamfromthedenitrificationreactorto
nitrification;3:Nitrificationeffluentandtheoutletoftheplant).(Mean±standarddeviationofthemean.<QL:Lowerthanquantificationlimit).
TKN(mgN/L) NH4+(mgN/L) NO2
-(mgN/L) NO3-(mgN/L)
Case 1 2 3 1 2 3 1 2 3 1 2 3
1 96.9±1.0 33.6±0.6 13.4±0.3 93.0±1.0 32.3±0.6 12.9±0.3 <QL <QL <QL <QL 32.5±0.3 61.2±0.6
2 94.2±1.0 68.1±0.4 28.4±0.2 94.0±1.9 57.6±1.2 24.8±0.5 <QL <QL <QL <QL 28.6±0.6 49.3±1.0
3 110.5±3.6 52.8±0.7 20.6±0.9 107.3±0.6 48.7±1.2 17.8±0.4 <QL 2.3±0.1 <QL <QL 24.8±0.3 48.8±0.2
4 121.4±1.4 42.9±1.5 17.9±1.0 119.0±1.3 38.1±1.3 15.2±0.9 <QL 1.1±0.1 <QL <QL 25.0±0.5 34.0±0.8
5 114.0±0.5 49.8±0.4 22.5±0.3 111.0±0.5 44.9±0.5 20.8±0.4 <QL 0.6±0.1 <QL <QL 10.2±1.1 23.2±1.3
6 97.7±1.0 37.7±0.7 8.6±0.8 85.8±1.5 21.0±1.4 5.4±0.9 <QL 0.6±0.1 <QL <QL 32.8±1.0 33.8±1.5
7 106.9±0.4 18.1±0.9 <QL 102.5±0.1 12.0±0.4 <QL <QL <QL <QL <QL 28.9±0.2 32.5±0.1
8 102.8±1.7 33.5±0.6 7.2±0.2 98.9±0.3 29.8±0.2 5.7±0.5 <QL <QL <QL <QL 3.9±0.05 6.1±0.04
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Ontheotherhand,cases4,5and6wereanalyzedpumpingmethanol to thesystem. It is
interesting to note the cases 4, 5 and 6, where the COD concentration in the feed was
approximatelyconstant (460mgO2/L). In thesesituations, theC/Nratiowasadjustedaround
4.1 by the addition ofmethanol. This adjustmentwas done to increase the available organic
matterinthefeedforthedenitrificationprocess.Inthosecases,thenitraterecyclingratiowas
changedasfollows:R=3Q,4Qand5Q.Forthisreason,theresultsshowedahigherpercentage
ofTNremoval than thecases1and2,withaTNremovalof57.3%,59.7%and56.2%for the
cases4,5and6,respectively,ascanbeseeninFigure2.
Figure2:CODandTNremovalpercentagesafterthedenitrification/nitrificationprocess.
ContrarytoexpectationswithrespectTNandCOD,therewasnoappreciableimprovement
in the removal efficiencieswith an increase in the nitrate recycling rate for cases 4, 5 and 6
(Figure2).Inthecasescomparedinthispartofthestudy,thesameamountoforganicmatter
for denitrifying was available. By increasing the recycling ratio of nitrate, the nitrate load
suppliedtotheanoxicreactorwasincreased.Thereweremoreelectronacceptorsforthesame
amount of electron donors. Therefore, increasing R in the system, did not provoke an
enhancementintheyieldoftheprocess,becauseofthelackoforganicmatterinthefeed.For
thewastewaterstudied,witha lowC/N,ahighernitraterecyclingratiowasnotbeneficial for
nitrogen removal and it could be economically non-profitable. The enhancement in the TN
removal efficiencies among cases 4, 5 and 6 versus cases 1-2, was due to the addition of
methanol, which provided organic material to be used by denitrifying bacteria. The results
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8
%CODandTN
removal
%CODremoved %TNremoved
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129
obtained are in agreement with the results reported by Fongsatitkul et al.[24]
, showing no
improvement on the COD removal with respect the influence of R. On the other side, this
authorobservedamodest improvementof4-5%TKNremovalwhenRdoubledfromQto2Q,
butnofurther increaseatarecyclingratioof4Q. InthecaseofChenetal.[25]
,at lowCOD/N
ratioof3.0, theN removalefficiencydecreasedwhenR increased,due to the limitedcarbon
sourcesinanoxiczones,andonlyathighCOD/Nratioof5.5,theNremovalefficiencysteadily
increasedwithR.
In Figure 3 is depicted the evolution of TKN and NO3--N concentration in the different
situationsstudiedinthework.Intheleftcolumnisrepresentedthefeedandintherightone,
theeffluentafterdenitrification/nitrificationprocess.Itcanbeobservedacleardecreaseinthe
TKNeffluent compared to the inlet concentration in all the analyzed cases, indicating a good
nitrification yield. The case 2, the most unfavorable case in terms of operating conditions,
shows theworstyieldofnitrificationandaTNremoval.Thegraphicsof thecases4,5and6,
shownoconsiderabledifferencesbetweenthem.
Figure3:Comparisonofnitrogencompoundconcentrationsinthedifferentcases,beforeandafter
denitrification/nitrificationtreatment.
SometimesincompletedenitrificationcanproduceN2O,whichisanintermediaryproductin
denitrification processes. This can be problematic as N2O is a potent greenhouse gas and
contributes to increasing the earth's temperature and destructing the ozone layer[26]
. Gas
samplestakenfromthebioreactorshowedconcentrationslowerthan9mg/LofN2Ogasinits
headspace,correspondingtolessthan10%oftheNremoved.
0
20
40
60
80
100
120
140
Concentra
tion(m
gN/L)
TKN NO3-
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
InletInlet
InletInlet Inlet
Outlet
Outlet
Outlet
Outlet
Outlet
Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8
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3.2.TheeffectofincreasingtheCOD/Nratio.
The removal efficiencyofnutrient andorganic carbon in thedenitrification-nitrification
systemwithdifferentCOD/Nratioswasalsostudied. Inawastewatertreatmentplant,partof
the stream that feeds the anaerobic reactor is derived through a bypass, to the stream that
feedsthedenitrificationreactor.Withthiscourseofaction,itispossibletoincreasethesoluble
COD available in the liquid stream that feeds the denitrification reactor, andminimizing the
adding of external carbon sources. In this work, methanol was employed as extra carbon
source, in order to simulate the increment of the denitritation potential by increasing the
concentrationoforganicmatteravailableinthesystem.
TheC/Nratioofthewastewaterafteranaerobictreatmentwasaround1.1(cases1and2),
showing a lack of carbon source to promote the denitrification process. On the one side,
comparing the cases 2 and 3, methanol was added to enhance the denitrification step
increasingtheC/Nratio from1.1 to2.6,whilenitraterecyclingratiowasmaintainedat200%
(R=2Q).ThecorrespondingremovalefficienciesofTNweredoubledfrom17.6%to38.7%ascan
be seen in Figure 2.With regard to organicmatter, the removal efficiency of CODenhanced
from74.4%to92.2%,withaconcentrationeffluentof27.1mgO2/Linthecase2,and22.3mg
O2/L in the case 3 (Table 3). As depicted in Figure 3, NO3--N in the effluent of the
denitrification/nitrification process remained almostwith the same concentration. Looking at
TKN, theeffluentconcentration in thecase3wasaround25% lower thancase2,despite the
factthatincase3thefeedingconcentrationwasalmost15%higherthanincase2.
On the other side, looking at cases 7 and 8, nitrate recycling ratio from the aerated
bioreactorwasmaintainedat600%(R=6Q)andmethanolwasaddedtoincreasetheCODinthe
feeding.Inthecase7,theCODwas574.0mgO2/LandtheC/Nratiowas5.37,asindicatedin
Table2.Moreamountofmethanolwasadded in case8,where848.2mgO2/Lwas the inlet
COD,changingtheCOD/TNratiofrom5.37to8.25. Incomparisontocase7,withthisraise in
the concentration of COD in the feed, the nitrogen removal efficiency shown a substantial
improvementfrom69.6%to84.7%(Figure2),obtainingeffluentswith32.5mgN/Land13.3mg
N/Loftotalnitrogenincases7and8,respectively(Table4).ThetwosituationsgotahighCOD
removalofaround96.1%.Figure3showsthehighdecreaseintheNO3--Nconcentrationcolumn
afterthedenitrification/nitrificationprocess.
Summarizing, the greater the influent C/Nwas, the better the TN removal was obtained.
SimilarobservationweredonebyHanetal.[27]
,Wangetal.[28]
andKumaretal.[12]
.Therefore,
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131
basedintheresults,thedenitrificationcapacityofthesystemwasaffectedbytheavailabilityof
CODpresentintheinfluentandtheadditionofCODwasaveryimportantpointinthenitrogen
removal.
Consideringon theonehand, that theCODof thewastewaterbefore theAnMBR reactor
(afterthesedimentationtank)is610mgO2/L[2],andontheotherhand,theby-passof50%of
influent raw sewage, the requirements of methanol to achieve 848 mg O2/L would be
diminishedin33.6%.
Fuetal.foundremovalefficienciesof96.2%forCODand83%forTN,withratherlongerHRT
than the achieved in this work: 1.5 days versus 6 hours. The process they developed was a
modifiedmembranebioreactorwith twoparts for theanoxicandaerobiccompartments that
treatedsyntheticwastewaterwithaC/Nratioof9.3[16]
.
Azhdarpooretal.[29]
obtained92%and86%ofCODandTNremoval,respectivelywithaSBR
configuration but with a synthetic wastewater with a C/N ratio much higher than the
experimented in this work (C/N=19 versus C/N=8.3) and 8 hours of TRH (versus 6 h in this
study).
Amongthecasesstudiedinthework,incases7and8tookplacethelargestincreasesinthe
TNremovalefficiency.Morespecifically, theremovalefficiencyofTNwas increasedby40.7%
between the cases 1 and 3; 35.1% of TN removal efficiency increase was observed when
comparingthecases3and5;and29.5%wastheincreaseintheTNremovalefficiencybetween
thecases5and8.
Therewasno significantdifference in thephosphorus concentrationbetween the influent
and effluent in any case. The wastewater would require one specific treatment for its
elimination.
Thusthedenitrification-nitrificationsystemcouldachievealong-termstabilityforremovalof
nitrogen with the addition of methanol, obtaining an effluent that likely complies with the
legislativerequirementsfordischargeintowaters,asregardsorganicmatterandnitrogen[3].
The results obtained in this work showed a big improvement over the processes already
developed by other authors and described in the literature. Similar values of COD and TN
removalwereachieved to thosedeveloped in literaturebutusingshorter residence timeand
lowerCOD,whichimpliesasconsequence,theuseofsmallerequipmentandaloweraddition
ofchemicals.
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132
As futurework it is proposed to evaluate the effect of increasing the carbon ratio on the
nitrogeneliminationpotentialusing themixingof theanaerobic reactoreffluentand the raw
feed.
4.CONCLUSIONS
The developed process is an interesting alternative to eliminate the nitrogen and organic
matter present in the wastewater from an anaerobic reactor, with very low C/N ratios. The
proposedsystemwasadenitrification/nitrification integratedprocesswithashortHRTof2h
fortheanoxicbioreactorand4hfortheaerobicone.
ThesuccessfulresultsofthesystemtoremoveCODandTNfromdomesticwastewaterafter
anaerobic treatment could be achieved mainly due to the addition of methanol. Methanol
increased the molar ratio of C/N in the wastewater accelerating the nitrification and
denitrificationrates,beingthekeypoint inthenitrogenremoval. Ontheotherhand,despite
nitrate recycling did not suppose a significant improvement in the process, it improved the
homogeneous distribution of microbial communities in the reactors increasing the removal
efficiencyofnitrogen.
The optimal nitrogen and organicmatter removalwere 84.7% and 96%, respectively. The
optimizedprocesswasperformedunderanitraterecyclingratioofsix timesthe feeding flow
(600%) and addition of methanol until obtaining an inlet C/N ratio of 8.25 and a COD
concentrationof almost 850mgO2/L.As result of the combined impacts, itwasobtained an
effluent thatmet the requirements ofwastewater discharge, in terms of organicmatter and
nitrogencontent.
ItisnoteworthythattheenhancementoftheC/Dratiocanbemadebybypassingpartofthe
feedstream from a point before the anaerobic treatment to another point in the end of this
reactor. In this way, it is provided to the denitrification process a feed with a higher
concentrationinorganicmatter,andtherefore,theexternalcarbonsourceneedisreduced.
ACKNOWLEDGEMENTS
The authors thank the companyCadagua S.A., the EuropeanRegionalDevelopment Fund,
the project IPT-2011-1078-310000, and the INNPACTO 2011 program of the Ministry of
EconomyandCompetitivenessforthetechnicalandfinancialsupport.
Page 145
133
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Chapter6.Techno-economicalstudyofa
domesticwastewatertreatment
system.
Abstract
The techno-economical feasibility of the membrane anaerobic treatment of
wastewater eliminating nitrogen has been simulated. The processwas simulated
using experimental data analyzing the influence of different electron donors
(methane, organic matter and sulfide) on the nitrogen elimination capacity.
Differentscenarioshavebeenassessedchangingtheconcentrationoftheinvolved
components and evaluating their effect on the nitrogen elimination capacity as
wellas theability toproducebiogas in theanaerobic treatment.Thesescenarios
implyontheonehand,theincrementoftheavailablesolubleCODforthenitrogen
elimination stage. The COD feed to the reactorwas adjusted at values between
15% and 30% assuming different mixing ratios with the influent stream of the
anaerobicreactor.Ontheotherhand,differentflowsofbiogasfromtheanaerobic
reactor were pumped to the denitritation reactor. The goal was to achieve a
nitrogen elimination capacity to reach an effluentwith 10-20mgN/L. Then, the
most promising scenariowas studied in detail and itwas compared to the costs
associated to the WWTP with a biological anaerobic treatment using a MBR
system.Theresultsindicatedthattheproposedprocessisfeasiblesincethefixed
andvariablescostsofbothtreatmentplantsaresimilar.
Keywords:COD•Biogas••
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1. INTRODUCTION
BesidestheremovalofCOD,nutrientremoval,especiallytheremovalofnitrogen(N),isalso
ofincreasingconcernduringthewastewatertreatmentprocess[1].Nitrification–denitrification,
which is the most common biological nitrogen removal (BNR) method in conventional
wastewater treatment plants (WWTP), is an energy intensive process that couples chemical
oxygen demand (COD) and nitrogenous oxygen demand (NOD) removal. High NOD increases
theneedforoxygensupplyandaeration,whichisthedominanttheenergyconsumingprocess
(∼50%)intypicalWWTPswithNremoval[1,2]
.
In the absence of a suitable electron acceptor, a consortia of microorganisms convert
organicmatter tomethane (CH4) and carbon dioxide (CO2),which can be used as biogas for
either heat or electricity generation. Several life cycle assessments have confirmed that
anaerobicdigestionisasustainablewaste-to-energysystemfromtheprospectsofbothenergy
productionandgreenhousegas(GHG)emissions[3,4]
.Comparedtoothertechniquesforenergy
recovery, anaerobic digestion is amaturemethod that is alreadywidely used inWWTPs for
recovering energy in the form of methane-rich biogas produced during digestion of primary
sludgeandbiomassgeneratedduringconventionalaerobictreatment[1].Generallyconsidered
as an unfavorable byproduct ofwastewater treatment,waste biomass from activated sludge
processes can also be thought as a raw material for energy production[1, 5]
. Advanced
wastewater treatment plants are nowmaking significant progress towards energy neutrality
through installation of, among others, anaerobic digestion and nitritation–denitritation
processes.
Oneoftheusefuloutcomesofaprocesssimulation isthatdifferentworkingscenarioscan
be evaluated. The results of these simulations can be used to create a holistic view of the
system. In addition, it is possible todetermine the responseof the systemwhen theprocess
parameters are varied. This is one of the most convenient ways to perform an economical
feasibilityassessmentofaprocess.
Themodelemployedinthesimulationperformedinthisresearchworkisabletodetermine
theoverallnitrogeneliminationcapacityofanitritation-denitritationsystemdependingonthe
quality of the influent under different working scenarios. This result would point the right
designoftheprocessaswellastheeffluentcharacteristics.
Aiming to employ realistic values for the study, the operation parameters used in the
simulationweregottenfrompreviousexperimentations.
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Theobjectiveofthisworkwastoevaluatetheeconomicalfeasibilityofthenitrogenelimination
technologydeveloped inthis thesis. Inaddition, theworkwas focused in findingthesensitive
parameterthatcanbemodifiedtogetthebiggestconversionofnitritetonitrogengas inthe
denitritationprocess.
2. MATERIALSANDMETHODS.
ThecompositionoftheinfluentwatertothesystemisshowninTable1.
Table1:Compositionoftheinfluentwater.
Parameter Concentration
TotalCOD(mgO2/L) 771
SolubleCOD(mgO2/L) 491
TSS(g/L) 0.14
VSS(g/L) 0.12
NO2--N(mgN/L) 1.52
NO3--N(mgN/L) 1.56
TKN(mgN/L) 93.29
NH4+-N(mgN/L) 69.54
SO42--S(mgS/L) 16.88
PO43--S(mgP/L) 9.82
A schemaof the simulated set up is depicted in Figure 1. The system is composed of: an
anaerobicmembranebioreactor(AnMBR)[6],adenitritationreactorandanitritationreactor.A
fractionofthenitritationreactoreffluentisrecycledtothedenitritationreactor.
Figure1:Schemeofthesimulatedsetup.
AnaerobicInfluent
AnaerobicEffluent
Biogas
EffluentDenit.Influent
Denit.Effluent
Recycling
ANAEROBICMEMBRANEBIOREACTOR
By-pass
DENITRITATIONREACTOR
NITRITATIONREACTOR
N2
H2S
Page 153
Chapter6
141
Thedenitritationstageisfedwiththeeffluentfromananaerobicmembranereactor,so,the
studyofthisreactorwasalsoincludedinthemodel.Avariationintheprocessflowdiagram,like
theadditionofaby-passtothefirststagetoincreasetheorganicmattercontentofitseffluent,
could affect the quality of the effluent. Consequently, this change would also affect the
simulationofthenextoperationintheprocess.Itisalsoimportanttopointthatthesimulation
wasperformedconsideringstoichiometricreactionsofthecomponentsinvolvedintheprocess.
Theammoniuminthewastewaterisoxidizedintonitriteinthenitritationprocess.Then,the
nitriteistransformedtonitrogeninthedenitritationreactorusingdifferentelectrondonors[7-9]
.
Itwasconsideredtocarryout thedenitritationwiththeresidualorganicmatter,sulfide[10, 11]
andmethane[12, 13]
present in thewater since this process is performed after the anaerobic
treatment.Differentsulfideandmethanesourceswereconsideredandtheircontributionswere
evaluatedon the capacity of overall denitritation of the system. Sulfide andmethane canbe
usedasendogenouselectrondonorssourceforbiologicaldenitrificationofwastewater.
The autotrophic denitrification employing sulfide and the heterotrophic denitrification
employingmethanecouldbe insufficienttoconverttheentireamountofnitritegotten inthe
initial process into nitrogen. In this case, it is necessary to add organic matter as source of
electrondonors.Themainsourceofsulfideandmethaneisgottenfromtheliquideffluentfrom
theanaerobicreactor[14,15]
,wherethosecomponentsaredissolvedandoversaturated[16]
.This
phenomenon takes place because the organic matter is transformed into biogas in the
anaerobicprocess,whichiscomposedofsulfideandmethaneamongothergases.Thesulfide
concentration in the influent stream to the denitrification process can be determined by
calculating the amount of sulfide produced during the anaerobic digestion by the sulfate
reducingbacteria[17, 18]
. The sulfurmassbalancedetermined that the sulfur concentrationas
sulfideinthebiogasisnotequivalenttothesulfateoxidationintheanaerobicprocess.So,this
concentrationshouldbereferredtothesulfideoccludedintheanaerobicprocesseffluent.On
theotherhand,theheterotrophicdenitritationiscarriedoutusingthemethaneoccludedinthe
influent. The net methane production in the anaerobic process was estimated employing
experimental data. This amount is lower than the theoretical amount of methane produced
fromtheeliminatedorganicmatter.Thisdifferencecanbeattributedtothemethanewhichis
occluded in the liquid influent, in the same way that it was analyzed for sulfide. The
concentrationoftheseavailablecomponentstoperformthedenitrificationwillbedetermined
bycharacterizationofthe influentwatertotheanaerobicprocessand itsoperationregarding
thecapacitiesof:organicmattereliminationand sulfate to sulfide reduction.Considering the
explained above, the denitrification capacity of the system will be determined by the
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142
concentrationofsulfide,methaneandorganicmatterintheinfluent.Nevertheless,thiscapacity
can be modified by changing the concentration of organic matter in the influent water or
adding the methane and sulfide produced as biogas in the anaerobic reactor as source of
electron donors. This can be achieved by connecting the biogas produced in the anaerobic
reactor to the denitrification reactor (see the dashed line in Fig 1). During the experimental
stage,itwasonlypossibletocarryoutthedenitrificationwhensyntheticnitriteswereaddedto
thefeed(Chapter3and4). Itwasnotpossibletogetthepartialnitritation inthenitrification
reactor(Chapter5).So,inthisstudy,itiscomparedbothprocesses:nitritation/denitritationand
nitrification/denitrification.
The different scenarios were simulated to study its influence in the
denitritation/denitrificationprocess.
2.1.Massandenergybalances:
Massandenergybalancescalculationwereconductedinordertostudytheinfluenceofthe
differenteffects.Themainequationsusedinthestudyarethefollowing:
CODeffect:
!"#$%'()$*$+%$,%- = '()/0/1234561778910: + '()4<=/>> (1)
!"#$%?-@+#. B"#-@#+$% = C(DE − C 750/81778910: + ((H + 'IJ + IKL)N105:.6/=/65:< +
C(DE − C 21O3P1NQ5:R:R1216<681N/0/123456453S/> (2)
T%-U#V+U-@-VWXBV"?YU-? = T%-U#V+U-@-VWX723OZ[\ ∙ ^1816:256/8453S/> (3)
T%-U#V+U-@-VWX723OZ[\ = 'IJBV"?YU#+"@723O453S/> ∙ _' O1:R/01 ∙ ^ (4)
Where ^1816:256/8453S/> is theelectrical efficiencyof biogas. Electric energy is considered
1/3ofthethermalenergy,sothisvalueis0.33;_'`isthemethanegrosscalorificvalue(9530
Kcal/Nm3);^isayieldof90%becauseitisconsidered10%ofenergyloss.
EffectofRecyclingbiogas:
%b-UXU%-?c+"W$d =e∙ Zfghijhklmn ∙ Z[\/pqmnrhfstusfvjhwstwxhsf
y53S/>=23N96:530zwfvx{xjwf|stwjxthfw ∙ %Z[\s{|sf}xr (5)
Where~ isthefeedflowofthesimulatedprocess(20000m3/d));'39:81:pqmn istheNOx
--N
concentration after denitritation/denitrification process; 'IJ/C(D1>:1�953O1:2562/:53 is the
stoichiometric ratiofromthereaction.
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3. RESULTSANDDISCUSSION.
3.1. CODEffect
Oneofthemosteffectivemethodstoincreasethedenitrificationcapacitybyincreasingthe
organicmatterconcentrationoftheinfluentisachievedbymixingafractionoftheinfluentto
theanaerobicreactorwiththeeffluentof thatreactor.Theuseof this“by-pass”will increase
theCODofthereactoreffluentmakingitmoreadequatetothenextdenitrificationstage.
Differentscenarioswereevaluated.Thevolumetricflowbypassedtotheanaerobicreactor
aresupposedtovaryin5%,7%and10%ofthetotalfeedtotheanaerobicreactor.Inthisway,
fromEq.1itispossibletoincreasetheavailablesolubleCODin15%,21%and30%respectively,
in comparison to the initial, when there was not by-pass. So, from Eq.2 the denitritation
potentialcanbeincreasedin4.5%,6.3%and9%respectively,dependingonthebypassedflow.
ThisbehaviorisdepictedinFigure2.
Inthedenitrificationcase(Figure2),thedenitrificationpotentialchangewithavailableCOD
is small because the relationship acceptors/donors is higher. So, the effect of increasing the
availableorganicmatteronthedenitrificationpotentialinthiscaseisslight,withvaluesof1.6%,
2.2%and3.2%(Eq.2).
The total capacity of denitritation/denitrification is calculated as the sum of the
denitritation/denitrificationcapacitiesoforganicmatter,methaneandsulfidesavailable(Eq.2).
Theincrementinthepotentialisobtainedbecauseoftheincreaseoftheorganicmatter.Ithas
tobeconsideredthatinthosecases,theconcentrationofsulfideandmethaneisreducedwhen
the bypassed flow is increased. This is an expected behavior since the bypass implies the
reductionof the treated flow in theanaerobic reactorand itwillbe translated in less soluble
CODavailablefortheproductionofbiogas(methaneandsulfide).
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Chapter6
144
Figure2:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialdependingonthe
amountofCODintheinfluent.
Theadditionofaby-passtotheanaerobicreactorinvolvesothereffects,whichdonotaffect
theeconomicfeasibilityoftheproposedmethod.Animportantadvantageoftheprocessisthe
reductionof theequipmentsizewhich is translated in less initial investment in theanaerobic
process(equipmentandinfrastructure)andalsolessoperationcostsinthemembranereactor.
On theotherhand,oneof theweakpointsof thisproposal is the reductionof theproduced
biogasbecauseof the lower flowtreated (Figure3).This reduction impliesa reduction in the
amountofelectricenergyproducedbythesystem(Eq.3andEq.4).Ithastobeconsideredthat
oneofthemostimportantcostsintheWWTPistheelectricity.So,theelectricenergyhastobe
consideredasacontrolparametertodeterminetheeconomicfeasibilityoftheprocess.So,itis
analyzedthechangeinenergyproductionlinkedtoeachoftheproposedimprovements.
Inbothcasespresented inFigure3, the reduction in theelectricenergy (Eq.3andEq.4) is
obtainedbecauseof a reduction in the feed flow to theanaerobic reactor. So, the reduction
valueislinkedtotheflowreductionvalueof5%,7%and10%.
20
25
30
35
40
45
50
55
0 5 10 15 20 25 30
Denitritatio
n/Denitrificatio
nPo
tential(mgN/L)
SolubleCODincrement(%)
Denitritation Denitrification
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145
Figure3:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialswiththeelectric
energyproducedemployingthebiogasproducedintheanaerobicprocess.
3.2. Effectoftherecyclingofbiogas
The methane and sulfide produced as biogas in the anaerobic process can be also
considered as an electron donor source[19, 20]
. The addition of a biogas recycle in the
denitrificationreactorwouldincreasethemethaneandsulfideconcentrationinsidethereactor,
enhancingthenitrogeneliminationcapacityasnitritesornitrates.
The simulations of this scenario considered the maximum elimination of nitrogen (final
concentrationof0,10,15and20mgN/L)usingaslowaspossibleamountofbiogas.Theeffect
ofbiogasrecyclingisdepictedinFigure4forthedenitritation/denitrificationprocess,anditwas
calculatedfollowingEq.5.Itcanbeseenthatthedenitritationpotential(Eq.2)canbeincreased
in21.2%,31.7%and42.3%byrecycling4.7%,7.0%and9.3%respectively,ofthetotalavailable
biogas.Inthecaseofthedenitrificationprocess,therecycledbiogasflowwasincreasedupto
15.4%. In this case, the nitrogen elimination capacity can be increased in 44.3%, 66.4% and
88.5% by recycling 7.7%, 11.6% and 15.4% respectively, of the total available biogas, in
comparisontothenorecyclesystem.
20
25
30
35
40
45
50
55
315 320 325 330 335 340 345 350 355
Denitritatio
n/Denitrificatio
nPo
tential(mgN/L)
ElectricEnergyProduced(kW/h)
Denitritation Denitrification
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Figure4:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialswiththepercentage
ofrecycledbiogas.
Ithastobepointedthatthecasesanalyzedinthissectiondonotincludetheby-passtothe
anaerobic reactor to increase the influentCODto thedenitritation/denitrification reactor.So,
theeffectobservedisattributedexclusivelytotheincrementofmethaneandsulfide.
Theincrementintherecycledbiogasflowdiminishestheproductionofelectricenergyfrom
the produced biogas. In Figure 5, it is depicted how the increment in the potential of
denitritationanddenitrification(Eq.2)affectstheeconomicfeasibilityoftheprocesses(Eq.4).
ItcanbeseeninFigure5thattheamountofelectricenergyproducedinbothcasesislower
at higher denitritation/denitrification potentials. The highest loss of produced energy is
observed in the denitrification process with reduction rates of 8.3%, 13.1% and 18.2%. The
denitritationreactorshowedreductionratesof4.9%,7.5%and10.3%.
20
30
40
50
60
70
0 2 4 6 8 10 12 14 16
Denitritatio
n/Denitrificatio
nPo
tential(mgN/L)
PercentageofRecycledBiogas(%)
Denitritation Denitrification
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Figure5:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialswiththeelectric
energyproducedusingthebiogasgeneratedintheanaerobicreactor.
3.3. EffectofcombinedCODandmethane
Thescenariospreviouslyevaluatedcanbecombinedatthesametimetogetbetterresults.
Asdepicted inFigure6, thestudyof theeffectofbothvariables in the twoanalyzedsystems
shows that the increment of the available soluble COD has a higher effect on the nitrogen
removalpotentialwhenthebiogasadditionislower.
On the other hand, in the denitrification case, the effect of the COD isminimum, so the
combinedeffectismainlyduetothemethane.So,thecombinationofthetwovariablesdonot
showsbigchanges.
20
30
40
50
60
70
295 305 315 325 335 345 355
Denitritatio
n/Denitrificatio
nPo
tential(mgN/L)
ElectricEnergyProduced(kW/h)
Denitritation Denitrification
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Figure6:Relationshipbetweenthedenitritation(a)anddenitrification(b)potentialswiththepercentage
ofbiogasrecycledinfunctionofthesolubleCODincrement.
Both strategies imply the reduction in the energy production from biogas. So, their
combination should contemplate the addition of this reduction (depicted in Figure 7). In the
denitrification case, considering that the methane/nitrate ratio is higher than the
methane/nitriteratio,theneedsofbiogaswillbehighertoreachcertainnitrogenremoval,so,
theenergybalanceislessfavorable.
20
25
30
35
40
45
50
55
60
65
70
75
0 2 4 6 8 10 12 14 16 18
Denitritatio
n/Denitrificatio
nPo
tential(mgN/L)
BiogasRecycledtoDenitritation/Denitrification(%)
sCODincrement(0%) sCODincrement(15%) sCODincrement(21%) sCODincrement(30%)
sCODincrement(0%) sCODincrement(15%) sCODincrement(21%) CODsincrement(30%)Denitrification:
Denitritation:
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Figure7:Relationshipbetweenthedenitritation(a)anddenitrification(b)potentialswiththeproduced
electricenergy.
3.4. Economicevaluation
Once the nitrogen removal capacity of both systems was evaluated, the optimum option
shouldbedecided.Thekeyfordecidingthatisthechoicebetweentheanalyzedscenariosthat
maximizesthenitrogenelimination(nitrates/nitritestonitrogen)andkeepsashighaspossible
theelectricenergyproductionfrombiogas.Inanycase,respectingthequalityrequiredforthe
effluentsbytheLaw.
Themaximumconcentrationofnitratesandnitrates that canbe reduced (considering the
potentialsevaluatedabove)dependsontheflowoftherecyclestream.Thisrecyclingflowwill
affectalsothedesignofthenextstagesoftheprocess.
Thesimulationwasdoneconsideringthefollowingparameters:
• FeedFlowtotheprocess:20.000m3/d.
• Ammoniacalnitrogenconcentration:95mgN/L.
20
25
30
35
40
45
50
55
60
65
70
75
260 280 300 320 340 360
Denitritatio
n/DenitrificationPotential(mgN/L)
Electricenergy produced(kW/h)
sCODincrement(0%) sCODincrement(15%) sCODincrement(21%) sCODincrement(30%)
sCODincrement(0%) sCODincrement(15%) sCODincrement(21%) sCODincrement(30%)
Denitritation:
Denitrification:
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• Ammoniacalnitrogenconversiontonitrites/nitrates:95%.
• Concentrationofnitrites/nitratesintheeffluent:≤8mgN/L.
Assumingtheseworkingconditions,theoptimumprocesswillbecomposedof:ananaerobic
biological reactor without derivation of the flow; followed by a denitritation system with a
recycling of biogas of 20.5% respect to the total amount of produced biogas; and finally, a
nitritationstagewitharecyclingof1.2timetheflowoftheinfluent.
The working option employing the stages of complete nitrification and denitrification were
discardedbecause its needs for aeration are high,which is translated in an increment in the
overallenergeticcostsof12%.
Oncetheoptimumworkinglinewasdecided, itwasdoneandeconomicstudyofthecosts
associatedwith the selectedprocess. Itwasalso includeda comparison toaWWTPwith the
sametreatmentcapacity,whichwillbeusedtodeterminetheviabilityoftheproposedprocess.
TheoperationschemesofthetwoevaluatedfacilitiesaredepictedinFigures8and9.
Figure8:Schemaoftheproposedfacility.
TheschemapresentedinFigure8iscomposedofthefollowingprocessunits:
• Water line: pretreatment, biological anaerobic reactor with membrane tank,
membranecleaningdeposit,denitritation reactor,nitritation reactoranddisinfection
withUV.
• Sludgeline:thickening,anaerobicdigestion,dehydrationandresiduestreatment.
• Gasline:gasometerandelectricenergygeneration.
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Figure9:SchemaofaconventionalWWTPwithMBRtreatment.
TheschemapresentedinFigure9,foraconventionalWWTPiscomposedof:
• Water line: pretreatment, biological anaerobic reactor with membrane tank,
membranecleaningdepositanddisinfectionwithUV.
• Sludgeline:thickening,anaerobicdigestion,dehydrationandresiduestreatment.
• Gasline:gasometerandelectricenergygeneration.
Thedesigndatatocarryouttheeconomicstudyarelistedasfollows:
• Designflow:20.000m3/d.
• A conventional WWTP was taken as reference with membrane bioreactor (MBR)
technology.
• The employed membranes in the MBR system, conventional plant as well as in the
AnMBRwere assumed to be supplied byGeneral Electric (membranes of PVDF, non-
ionicandhydrophilic)[21]
.Theconfigurationofthemembranewasenforcedfiberwith
flow direction out-in and a nominal pore diameter of 0.04microns. The commercial
membraneemployedis“Zeewed500”.
• The initial investmentcostaffects theamortizationof the facility. Itwasconsidereda
periodof50yearsforbuildingand20yearsforequipmentasamortizationtime.
Cost analysis are based on actual costs. The fixed and variable costs of the facility were
studied inan independentway. In the fixedcostsare include: theamortizationof the facility,
the fixedcostofenergy, theprocesscontrol, themaintenanceandconservation, salariesand
othercostssuchasinsurances,taxesorrents.
Thefixedcostsofbothfacilitiesarequitesimilar,beingalittlelowerthecostsassociatedto
theconventionaltreatment.Themainreasonofthat,istheamortizationofthefacilities(Figure
10).
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Figure10:Fixedcostsincludingtheamortizationofthefacilitywiththeproposedprocess(a)and
conventionalplant(b).
Thecostassociatedtotheamortization(thegradualchargingtoexpenseofthecostoverthe
usefullifeoftheasset)ofthefacilityisthehighestamountofcostinthetotalcosts.Thesecosts
arehigherfortheproposedfacility(Figure10a)thanforthetraditional(Figure10b).Themain
reason of that, is the required building of two extra stages, the denitritation and nitritation,
which are not included in a traditional process. In this sense, the increment in the biogas
productionobtainedwiththeproposedtechnologyintheanaerobicreactormakeitnecessary
to install bigger equipment in the gas line than in the traditional facility. Nevertheless, in
comparisonto theaerobicsystems, theanaerobicsystemsproduce lessamountofsludge,so
theequipmentinvolvedinthisstagewillbesmallerandso,theconstructioncosts.
Therestofthecostsaresimilarinbothworkingprocedures.Asmalldifferencecanbenotice
inthesectionofprocesscontrol.Intheproposedprocess,thiscostishigherbecausethereare
threeadditionalstagesthatshouldbecontrolled.
Afterthecostsassociatedtotheamortizationinthefixedcosts,thepersonnelcostfollowed
by the maintenance and facility conservation are the highest fixed costs associated to the
facilities,ascanbeseeninFigure11.
0 10 20 30 40 50 60 70 80 90 100
PERSONNEL
MAINTENANCEANDCONSERVATION
MISCELLANEOUS
PROCESSCONTROL
FIXEDTERMOFENERGY
AMORTIZATIONOFTHEFACILITY
PERCENTAGEVS.TOTALFIXEDCOSTS(%)0 10 20 30 40 50 60 70 80 90 100
PERSONNEL
MAINTENANCEANDCONSERVATION
MISCELLANEOUS
PROCESSCONTROL
FIXEDTERMOFENERGY
AMORTIZATIONOFTHEFACILITY
PERCENTAGEVS.TOTALFIXEDCOSTS(%)
(b)
(a)
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0 10 20 30 40 50 60 70 80 90 100
PERSONNEL
MAINTENANCEANDCONSERVATION
MISCELLANEOUS
PROCESSCONTROL
FIXEDTERMOFENERGY
PERCENTAGEVS.TOTALFIXEDCOSTS(%)
0 10 20 30 40 50 60 70 80 90 100
PERSONNEL
MAINTENANCEANDCONSERVATION
MISCELLANEOUS
PROCESSCONTROL
FIXEDTERMOFENERGY
PERCENTAGEVS.TOTALFIXEDCOSTS (%)
(b)
Figure11:Fixedcostsexcludingtheamortizationcostsfortheproposedprocess(a)andaconventional
WWTP(b).
Thevariablecostsincludethereplacementofmembranes(themembranesofthebiological
processaswellas thereplacementof theUVdisinfection lamps), theconsumptionofelectric
energy,transportationanddischargeofresiduesandconsumptionofchemicalreagents.
The variable costs (Figure 12) associated to the proposed process are slightly lower than
thoserequiredinthetraditionalprocess.Theconsumptionofelectricenergyinoneofthemain
reasonsofthiscostdifference.Itshouldbementionedthattheelectricenergyconsumptionin
the proposed method is higher but the generation of electricity is also higher. It should be
notedthatintheproposedmethodtherearetwosourcesofbiogas,themembraneanaerobic
reactor and the anaerobic digestion of sludge. In the conventional option there is only one
sourceofbiogas,theanaerobicdigestionofsludge.So,theoverallenergybalanceshowslower
energyconsumptionfortheproposedmethodthantheconventionalone.
The other important cost to analyze is the transportation and discharge of the residues.
Focusing inthesludgeproductionandconsideringthatthesludgeproduction inananaerobic
reactor is lower than in an aerobic one, the proposed method has the advantage over the
conventionalprocessofreducingthetransportationanddischargecosts.
(a)
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Figure12:Variablecostsoftheproposedfacility(a)andtheconventionalWWTP(b).
4. CONCLUSIONS
TheincrementintheavailablesolubleCODtocarryouttheprocessdoesnotimplythesame
amount of increments in the denitritation and denitrification potentials. A COD increment of
15%,21%and30%meansan increase in thedenitritationpotential between4.5%,6.3%and
9%,thedenitrificationpotentialisincreasebetween1.6%,2.2%and3.2%withthesamechange
inCOD.
Theuseofaby-passtotheanaerobicreactortoincreasethesolubleCODinthedenitritation
reactor provokes a reduction in the amount of biogas producedwhich affects directly to the
economicviabilityoftheproposedprocess.
It was demonstrated that the increment of available methane in the reactor is themost
promising alternative to increase the denitrification/denitritation potential in both aspects:
technicalandeconomical.
When comparing to a conventionalWWTP, it was concluded that the fixed costs of both
alternatives are similar. However, the proposedmethod in this researchwork shows slightly
higher costs than the conventional process. These differences are associated mainly to the
amortization of the facility and equipment and the addition of new stages to the process. In
terms of variable costs, the proposed method showed lower costs than the conventional
process. In this case, the difference lies in the higher amount of produced energy and lower
requirementofresiduesaccommodation.
(a) (b
)0 10 20 30 40 50 60 70 80 90 100
REAGENTS
TRANSPORTATIONANDDISCHARGE
CONSUMPTIONOFELECTRICENERGY
REPLACEMENTOFMEMBRANES
PERCENTAGEVS.TOTALVARIABLECOSTS(%)
(a)
0 10 20 30 40 50 60 70 80 90 100
REAGENTS
TRANSPORTATIONANDDISCHARGE
CONSUMPTIONOFELECTRICENERGY
REPLACEMENTOFMEMBRANES
PERCENTAGEVS.TOTALVARIABLECOSTS(%)
(b)
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ACKNOWLEDGEMENTS
The authors thank the companyCadagua S.A., the EuropeanRegionalDevelopment Fund,
and the project IPT-2011-1078-310000, and the INNPACTO 2011 program of theMinistry of
Economy and Competitiveness for the technical and financial support. The authors
acknowledge Dr. Jorge Ignacio Pérez Pérez (Department of Civil Engineering, University of
Granada).
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References
1. H.Gao,Y.D.SchersonandG.F.Wells,Towardsenergyneutralwastewatertreatment:
methodologyandstateof theart,EnvironmentalScience:Processes& Impacts,2014,
16(6),p.1223-1246.
2. M. Zessner, C. Lampert, H. Kroiss and S. Lindtner, Cost comparison of wastewater
treatmentinDanubiancountries,Waterscience&Technology,2010,62(2),p.
3. S. Evangelisti, P. Lettieri, D. Borello and R. Clift, Life cycle assessment of energy from
waste via anaerobic digestion: A UK case study, Waste Management, 2014, 34(1),
p.226-237.
4. M. Grosso, C. Nava, R. Testori, L. Rigamonti and F. Viganò, The implementation of
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WasteManagement&Research,2012,30(9suppl),p.78-87.
5. L.Appels,J.Baeyens,J.DegrèveandR.Dewil,Principlesandpotentialoftheanaerobic
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34(6),p.755-781.
6. J.Gouveia,F.Plaza,G.Garralon,F.Fdz-PolancoandM.Peña,Long-termoperationofa
pilot scale anaerobic membrane bioreactor (AnMBR) for the treatment of municipal
wastewater under psychrophilic conditions, Bioresource Technology, 2015, 185225 -
233.
7. W.Zeng, L. Li, Y. Yang, S.WangandY.Peng,Nitritationanddenitritationofdomestic
wastewater using a continuous anaerobic–anoxic–aerobic (A2O) process at ambient
temperatures,BioresourceTechnology,2010,101(21),p.8074-8082.
8. S. Aslan and M. Dahab, Nitritation and denitritation of ammonium-rich wastewater
using fluidized-bed biofilm reactors, Journal of Hazardous Materials, 2008, 156(1–3),
p.56-63.
9. W. Zeng, X. Wang, B. Li, X. Bai and Y. Peng, Nitritation and denitrifying phosphorus
removalvianitritepathwayfromdomesticwastewater inacontinuousMUCTprocess,
BioresourceTechnology,2013,143187-195.
10. B.Moraes and E. Foresti,Determination of the intrinsic kinetic parameters of sulfide-
oxidizing autotrophic denitrification in differential reactors containing immobilized
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11. M. B. de S, J. Orrú, C. de Andrade, D. Fonseca and E. Foresti, Shortcut Nitrification-
Denitrification CoupledWith Sulfide Oxidation In A Single Reactor, J Microb Biochem
Technol,2014,6087-095.
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12. A.Sánchez,L.Rodríguez-Hernández,D.Buntner,A.L.Esteban-García,I.TejeroandJ.M.
Garrido,Denitrificationcoupledwithmethaneoxidationinamembranebioreactorafter
methanogenic pre-treatment of wastewater, Journal of Chemical Technology &
Biotechnology,2016n/a–n/a.
13. O.Modin,K.FukushiandK.Yamamoto,Denitrificationwithmethaneasexternalcarbon
source,WaterResearch,2007,41(12),p.2726-2738.
14. S.UemuraandH.Harada,TreatmentofsewagebyaUASBreactorundermoderateto
lowtemperatureconditions,BioresourceTechnology,2000,72(3),p.275-282.
15. J. Cookney, A. McLeod, V. Mathioudakis, P. Ncube, A. Soares, B. Jefferson and E. J.
McAdam, Dissolved methane recovery from anaerobic effluents using hollow fibre
membranecontactors,JournalofMembraneScience,2016,502141-150.
16. J.Gouveia,F.Plaza,G.Garralon,F.Fdz-PolancoandM.Peña,Anovelconfigurationfor
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17. I. Angelidaki, L. Ellegaard and B. K. Ahring, A comprehensive model of anaerobic
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18. S.MontalvoandL.Guerrero,Tratamientoanaerobioderesiduos.ProduccióndeBiogás,
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19. J.C.Lackey,B.Peppley,P.ChampagneandA.Maier,Compositionandusesofanaerobic
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Management&Research,2015,33(8),p.767-771.
20. A.Wellinger,J.D.MurphyandD.Baxter,Thebiogashandbook:science,productionand
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Conclusions
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Thegeneral conclusionsof this PhDThesis arepresentedbelow. The specific conclusionsof
eachresearcharepresentedinthechapterconclusions.
InthisPhDthesishasbeenextensivelystudiedtheprocessofnitrificationanddenitrification
toremoveammoniacalnitrogenfromtheeffluentofanAnMBRreactorthattreateddomestic
wastewaterat18ºC.
A SBR process was applied to ascertain its suitability for simultaneous nitrification and
denitrification.Cycletimesof12h,8hand6hinSBRwereconsideredinthestudy,andthe6h
cycle time was selected as the optimal for the treatment. The process was successful in an
anoxic/aerobic/anoxic cycle sequence with the addition of methanol just before the second
anoxic stage. Thus, it has beendemonstrated that the SBRprocess in a single reactor at low
temperatureisasuitableprocessforthesimultaneousremovalofnitrogenandorganicmatter
ofadomesticwastewaterwithlowCODwithonlytheadditionofexternalcarbonsource.The
additionofmethanolwasakeypointinthedenitrificationprocessemployedasamodelforthe
wastewaterby-passintheWWTP.
The denitrification of domestic wastewater with a low concentration of COD could be
possible by using the methane and sulfide that contains the water after the anaerobic
treatment.NO2- andNO3
-were the electron acceptors,while theOM, CH4 andH2Swere the
electrondonors.Afixedfilmanoxicbioreactorforpartialandtotaldenitrificationwasstudied.
From the one hand, nitrogen removal was demonstrated obtaining a successful NO2- and
NO3-eliminationwhenthefeedwas80mgN-NOx
-/L,exceptwhenthefeedingwasformedonly
bynitrate. Inthiscase,theprocesswasatthelimitofthedenitrificationprocess.Theoptimal
HRTtoobtainboth,denitritationanddenitrificationwas2h.Theamountofmethaneavailable
in thewaterwasenough to achieve the goal being themainelectrondonorusedwithmore
than70%orparticipation.
Ontheotherhand,whenonlypartialdenitrificationwasstudiedinthesameplantandthe
sameHRTof2h,theresultsdemonstratedverygooddenitritationyieldsforthenitriteremoval
up to 75 mg NO2--N/L. For high inlet concentrations of nitrite, the recirculation of the gas
collected in the anoxic reactor was a successful solution, thus achieving a nitrite removal
efficiency upper than 98% when the nitrite concentration in the feed was 95 mg NO2--N/L.
Specifically, denitritation is a feasibleprocess for the simultaneous removalofNO2-,OM,CH4
andH2S for actualwastewater and the recirculation of the gas from the anoxic reactor is an
efficacioussystemtoenhancethenitritesremoval.
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Conclusions
162
A denitrification/nitrification integrated system with a short HRT of 2 h for the anoxic
bioreactor and 4 h for the aerobic onewas studied. The plantwas operated to examine the
effectofthenitraterecyclingandtheCOD/Nratioonthenitrogenandtheremainingorganic
matter removal. The successful results of the system to remove COD and TN from domestic
wastewater after anaerobic treatment could be achieved mainly due to the addition of
methanol. Methanol increased the molar ratio of C/N in the wastewater accelerating the
nitrificationanddenitrificationrates,beingthekeypointinthenitrogenremoval.Ontheother
hand, despite nitrate recycling did not suppose a significant improvement in the process, it
improved the homogeneous distribution ofmicrobial communities in the reactors increasing
the removal efficiency of nitrogen. As result of the combined impacts, it was obtained an
effluent thatmet the requirements ofwastewater discharge, in terms of organicmatter and
nitrogencontent.
Itisnoteworthythatinsteadoftheadditionofmethanol,theenhancementoftheC/Dratio
canbemade (at least partially) bybypassingpart of the feedstream fromapoint before the
anaerobictreatmenttoanotherpointintheendofthisreactor.Inthisway,itisprovidedtothe
denitrificationprocessafeedwithahigherconcentrationinorganicmatter.
Finally,atechno-economical feasibilityofthedomesticwastewatertreatmentconsisting in
ananaerobicmembranereactorfollowedbyanitrogenremovalplantwassimulated.Different
scenarios have been assessed changing the concentration of the involved components and
evaluating their effect on the nitrogen elimination capacity as well as the ability to produce
biogasintheanaerobictreatment.TheincrementintheavailablesolubleCODtocarryoutthe
process impliedmore increment in thedenitritationpotential than in thedenitrificationone.
Theuseofaby-passtotheanaerobicreactorto increasethesolubleCODinthedenitritation
reactor provokes a reduction in the amount of biogas producedwhich affects directly to the
economicviabilityoftheproposedprocess.Itwasdemonstratedthattheincrementofavailable
methane in the reactor is the most promising alternative to increase the
denitrification/denitritationpotentialinbothaspects:technicalandeconomical.
Then, themostpromisingscenariowasstudied indetailand itwascompared to thecosts
associated to the WWTP with a biological anaerobic treatment using a MBR system. When
comparinganAnMBR+Denitritation/NitritationplanttoaconventionalWWTP,itwasconcluded
that the fixed costs of both alternatives are similar. However, the proposed method in this
researchworkshowsslightlyhighercoststhantheconventionalprocess.Thesedifferencesare
associatedmainly to the amortization of the facility and equipment and the addition of new
stagestotheprocess.Intermsofvariablecosts,theproposedmethodshowedlowercoststhan
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Conclusions
163
the conventional process. In this case, the difference lies in the higher amount of produced
energy and lower requirement of residues accommodation. The results indicated that the
proposedprocessisfeasiblesincethefixedandvariablescostsofbothtreatmentplants.
FutureWork
From the studies developed in this PhD, it can be concluded that the field of nitrogen
removal in domestic wastewater is an interesting area with several interesting topics to
address.Themaintopicstobedevelopedintheresearchofnitrogenremovalarepresentedin
thenextparagraphs.
ItwasobservedthattheC/Nratiointhefeedisoneofthemostcriticalparametersthatcan
affect directly the biological nitrogen removal efficiency. As the amount of biodegradable
organic carbon of domestic wastewater after anaerobic treatment is limited, the addition of
external carbon sources such as methanol, often becomes necessary for achieving high-
efficiencyBNR. Itwouldbe interestingtoevaluatetheeffectof increasingtheC/Nratiousing
themixing of the anaerobic reactor effluent and the raw feed. That is bypassing part of the
feedstream from a point before the anaerobic treatment to another point in the end of this
reactor. In this way, it is provided to the denitrification process a feed with a higher
concentrationinorganicmatter.
Inthedenitrification/nitrification integratedplant(Chapter6) itwasnopossibletoachieve
nitritationintheaerobicreactor.ReducingtheaerationtoprovidelessDOinthereactor,orthe
HRTnotonlydidnot increasedthenitriteproduction,butnitrateyieldgotworse. Itwouldbe
importanttostudythewaytoreachpartialnitritationtoshortcutthedenitrificationreactions.
Methane and sulfide from anaerobic biogaswas considered as an electron donor source.
BasedontheresultsobtainedinChapter5,recyclingofthegascollectedintheanoxicreactor
wasasuccessfulsolutiontoachievehighnitriteremovalefficiencyforfeedconcentrationsof95
mgNO2--N/L.Thegascollectedcontaineddesorbedmethaneanditcanbeusedfordenitrifyby
its recycling.Whenthe integratedsystemwasperformedthisproceedingwasnotpossible to
carry out because most of the methane remained occluded in the liquid stream at low
temperatures. Itwouldberelevanttofindthewaytoenhancethemethanedesorptiontobe
ableofcollect it inthetopoftheanoxicreactortorecirculateit, improvingthedenitrification
yields.
Anotherway of operation that allows the use of sulfide andmethane from the biogas as
electrondonorsource,istheadditionoftheanaerobicbiogasinthedenitrificationreactor.The
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Conclusions
164
biogas would increase the methane and sulfide concentration inside the reactor, and the
nitrite/nitrateremovalcapacitywouldalsoincrease.
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Agradecimientos
Acknowledgements
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167
Enprimer lugarquieroagradeceraMaríasu labordedirección.Suapoyoyconfianzaenmi
trabajo,asícomosucapacidadparaguiarmisideashansidounaporteinvaluable,nosolamente
eneldesarrollodelatesis,sinotambiénenmiformacióncomoinvestigadora.
Quieroagradecera laUniversidaddeValladolid,alMinisteriodeEconomiayCompetitividad
deEspañayalaempresaCadaguaS.A.,porhabermebrindadolaoportunidadyfinanciaciónde
integrarmealsistemadeinvestigación.
A todoelDepartamentode IngenieríaQuímicayTecnologíadelMedioAmbientede laUVa
porformarmecomoprofesional.AMar,Polanco,Pana,Dani,Araceli…
QuisieraexpresarmimássinceroagradecimientoaAliciaGómezGonzález,porsuimportante
aporteyparticipaciónactivaeneldesarrollodeestatesis.Debodestacarporencimadetodosu
disponibilidad,dedicaciónyconfianza.
Amiscompañerosyamigosdelgrupodeinvestigación,loscualeshansidoyseguiránsiendo
parteimportanteenmivida:Rebeca,Ieva,Natalia,Isra,JuanCarlos,Osvaldo,Alma,Sara,Dimas,
Inés… Por el buen ambiente en el laboratorio y calidad en el desempeño de su trabajo. En
especialaJaime,Joao,Roberto,porecharmeunamanosiemprequelohenecesitado.
Gracias a Anita, Begoña, Flor, Miriam, Cynthia, Sheila, Laura, Vito, Rebeca, Teresa … por
compartirtantosmomentosdentroyfueradeldepartamento,porsuamistad.
AmisamigasdeVillambrozydePalencia…porsertandivertidasyserungranapoyo.
Amifamilia.Amispadres,graciasporhabersidolaguíaenelcaminodelavida,porhaberme
ayudadoalevantardespuésdelostropiezos,pordármelotodosincondiciones.Soyloquesoy
graciasavosotros.Graciaspapá,porqueanimarmeaempezareldoctorado.Estésdondeestés
meguíasy séquecuidasde tuschicas,nuestroángelde laguarda.Graciasmamá,por ser la
personamásfuertequeconozco,porestarsiempreahíyanimarmesiempre.GraciasVirginia,
portugenerosidad,cariñoyamistad.Endefinitiva,graciasporsermihermanayporhaberme
dadounasobrinatanpreciosa,Carla,quetrajolaluzenlaoscuridad.Graciasfamiliaporvuestro
amorincondicional.Nohabríallegadohastaaquísinvosotros.
QuieroagradeceraDanilo,porsuamor, sucariño,suapoyo,suánimo,sugranayudaen la
vidapersonalyprofesional…ysuentusiasmoenconseguirquellegaraestatesis.Graciasporser
miotramitad.