Integrated application of upflow anaerobic sludge blanket reactor for the treatment of wastewaters

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 9

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Integrated application of upflow anaerobic sludge blanketreactor for the treatment of wastewaters

Muhammad Asif Latif, Rumana Ghufran, Zularisam Abdul Wahid, Anwar Ahmad*

Faculty of Civil Engineering & Earth Resources, University Malaysia Pahang (UMP), Lebuhraya Tun Razak, 26300 Gambang, Kuantan,

Pahang, Malaysia

a r t i c l e i n f o

Article history:

Received 27 December 2010

Received in revised form

24 May 2011

Accepted 31 May 2011

Available online 12 June 2011

Keywords:

UASB reactor

Industrial wastewater

Agro wastewater

Municipal wastewater

COD

Biogas

* Corresponding author. Tel.: þ60 9 5493012;E-mail address: [email protected]

0043-1354/$ e see front matter ª 2011 Elsevdoi:10.1016/j.watres.2011.05.049

a b s t r a c t

The UASB process among other treatment methods has been recognized as a core method

of an advanced technology for environmental protection. This paper highlights the treat-

ment of seven types of wastewaters i.e. palm oil mill effluent (POME), distillery wastewater,

slaughterhouse wastewater, piggery wastewater, dairy wastewater, fishery wastewater

andmunicipal wastewater (black and gray) by UASB process. The purpose of this study is to

explore the pollution load of these wastewaters and their treatment potential use in upflow

anaerobic sludge blanket process. The general characterization of wastewater, treatment

in UASB reactor with operational parameters and reactor performance in terms of COD

removal and biogas production are thoroughly discussed in the paper. The concrete data

illustrates the reactor configuration, thus giving maximum awareness about upflow

anaerobic sludge blanket reactor for further research. The future aspects for research

needs are also outlined.

ª 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46841.1. Characterization and environmental impacts of wastewaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4684

2. Treatment potential of UASB process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46853. Operation and performance of UASB reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4688

3.1. Organic loading rates and COD removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46883.2. Flow rate and hydraulic retention time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46923.3. Upflow velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46933.4. pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46933.5. Operating temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46933.6. Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4694

4. Research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46945. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4695

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4695References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4695

fax: þ60 9 5492998.k (A. Ahmad).ier Ltd. All rights reserved.

Nomenclature

AF anaerobic film

AS activated sludge

BOD biochemical oxygen demand (g L�1)

COD chemical oxygen demand (g L�1)

CODdiss dissolved chemical oxygen demand (g L�1)

CRT cell residence time (days)

CUASB upflow anaerobic sludge blanket at Canakkale

FOG fat, oil and grease (g L�1)

HRT hydraulic retention time (days)

IUASB upflow anaerobic sludge blanket at Istanbul

L length (cm)

OLR organic loading rate (kgCODm�3 d�1)

Q flow rate (L d�1)

SBR sequencing batch reactor

SCOD soluble chemical oxygen demand (g L�1)

SS suspended solids (g L�1)

T-P total phosphorous (g L�1)

T-N total nitrogen (g L�1)

TKN total Kjeldahl nitrogen (g L�1)

TS total solids (g L�1)

TSS total suspended solids (g L�1)

TUASB upflow anaerobic sludge blanket at Tekirdag

TVS total volatile solids (g L�1)

UASB upflow anaerobic sludge blanket reactor

V volume (L)

VS volatile solids (g L�1)

Vs superficial velocity (mh�1)

VSS volatile suspended solids (g L�1)

Vup upflow velocity (mh�1)

W width (cm)

WW wastewater

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 94684

1. Introduction

At the beginning of 21st century, the world is facing environ-

mental crisis in terms of water quality and global warming,

caused by continuous population growth, industrialization,

food production practices, increased living standards and

poor water use strategies. The rapid industrialization, urban-

ization, and population growth resulted in increasing volumes

of untreated domestic and industrial wastewater being dis-

charged into the rivers and canals and consequently deterio-

rating surface and groundwater quality. The polluted water

quality would prolong to affect the groundwater, threatening

to drinkingwater safety and thus the health of urban and rural

residents along with adverse effect on ecosystem, particularly

aquatic life and biospheres. The lack of wastewater manage-

ment has direct impact on biological diversity of the aquatic

ecosystems, disrupting the fundamental integrity of our life

support systems, onwhich a wide range of sectors from urban

development to food production and industry depend. It is

essential to consider the wastewater management as a part of

integrated, ecosystem based management to operate across

sectors and borders, freshwater and marine.

The wastewater is a mixture of sewage, agricultural

drainage, industrial waste effluents and hospitals discharge.

Untreated wastewater may contain different range of patho-

gens including bacteria, parasites, and viruses, toxic chemicals

such as heavy metals and organic chemicals from agriculture,

industrial and domestic sources (Andrew et al., 1997; Drechsel

and Evans, 2010). In order to minimize the environmental

contaminants and health hazards, the treatment of these

pollutants needs to be brought down to permissible limits for

safedisposalofwastewater (Pootsetal.,1978;Manjuetal., 1998).

1.1. Characterization and environmental impacts ofwastewaters

The production of palm oil from the fruit Elaeis guineensis is the

main industry in Southeast Asia (Ma, 2000). The effluent during

and after processing contains large amount of free and dis-

solvedoil and fatty acids, crudeoil solids, starches, proteinsand

plant tissues (Cheah et al., 1998). Similarly, distillery effluents

are highly polluted and fall under medium to high-strength

wastewaters as many kinds of raw materials are used for

different typesofalcohols (Inceetal., 2005). Effluent fromawine

distillery consists first and foremost of organic acidswitha lofty

soluble biodegradable chemical oxygen demand (COD) fraction

of 98% (Moosbrugger et al., 1993). The seasonal nature of

distillery industries creates specific problems for the treatment

processes in terms ofwine distillerywastewater (Coetzee et al.,

2004; Eusebio et al., 2004). Moreover, slaughterhouse waste-

water holds high amount of suspended and colloidal compo-

nents in the form of fats, proteins and cellulose, which have

adverse effects on the environment (Lettinga et al., 1997; Nunez

and Martınez, 1999). These organic matters can be treated by

means of anaerobic digestion as they have high concentrations

of biodegradable organic contents, sufficient alkalinity, and

suitable concentrations of phosphorus, nitrogen, and micro-

nutrients for the bacterial growth (Masse and Masse, 2001).

Single-phase, upflow anaerobic sludge blanket (UASB) type

anaerobic digesters are considered to be impractical, because

the fat present may form thick foam inside the reactor,

compromising the operation (Chen and Shyu, 1998; del Pozo

et al., 2000; Torkian et al., 2003; Barreto, 2004). Moreover, accu-

mulations of suspended solids lead to a reduction in meth-

anogenic activity and biomass washout (Sayed and de Zeeuw,

1988; Hansen and West, 1992). Discharge of slaughterhouse

wastewater without treatment is degrading the aquatic envi-

ronment andpolluting the irrigationwater (Michael et al., 1988).

Piggery wastes are also distinguished as rich in organic

matters and pathogenic organisms. The disposal of piggery

wastes without ample treatment can have a drastic effect on

the environment and human health (Sanchez et al., 2005).

This waste is a mixture of manure (feces and urine) and food

waste from instance swill and sugar cane molasses (Sanchez

et al., 2001). However, UASB reactor has rarely been used for

the treatment of piggery waste because of rich nitrogenous

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 9 4685

compounds, the active biomass densification, granular

process, and consequently themicroorganism retention in the

reactor difficult. The UASB process seems to be very efficient

for the treatment of carbohydrate containing wastewaters.

Limited work has been done on the application of this reactor

for the treatment of piggery wastes (Sanchez et al., 2005).

Dairy wastewater is generated either from milk or cheese-

producing industries. Dairy wastewater, from the milk

industry has COD of 30 g L�1 whereas, in case of cheese-

producing industries the generated wastewater generally

contains surplus cheese whey with a COD value of 50 g L�1

(Nemerow, 1987). Dairy wastewater is generally characterized

by its relatively high temperature and variation in organic

contents. Due to very small and medium size dairy industries

which normally do not have an economic incentive to further

use of cheese whey, it is necessary to choose the whey as

waste stream (Mockaitis et al., 2005). Due to high organic

contents, whey causes several environmental problems for

surface water and soil while disposing (Patel and Madamwar,

1998; Kosseva et al., 2003). Dairy wastewaters may also cause

serious problems in terms of organic load on the local

municipal wastewater treatment plant (Papachristou and

Lafazanis, 1997). Dairy wastewater has been widely treated

using coagulation/flocculation and sedimentation process.

The main shortcomings of these methods are high coagulant

cost, large amount of sludge production, and the poor removal

of dissolved COD. Therefore, biological treatment is usually

recommended for such wastewaters (Vidal et al., 2000).

Several studies have been done with the aim of adapting

anaerobic high-rate digesters, especially UASB reactors for the

treatment of dairywastewater. Conventionally, UASB reactors

are inoculated with granular sludge that has high methano-

genic activity. It has been proved that it was not possible to

maintain granular biomasswithdairywastewater in long term

operation (Marqueset al., 1990;YangandAnderson, 1993). Full-

scale UASB reactor has been successfully engaged for the

treatment of dairywastewater (Malina andPohland, 1992). The

use of a laboratory-scale hybrid UASB reactor for treatment of

dairy wastewater at an operational temperature of 30 �C was

previously investigatedbyOzturket al. (1993). The treatmentof

multiple fat containing wastewaters should be done in UASB

reactors inoculated with flocculent biomass that has a high

hydrolytic and acidogenic capacity (Sayed, 1987).

In fishery wastewaters the contaminants are undefined

mixtures of mostly organic substances. It is difficult to char-

acterize the extent of the problem created by this wastewater

as it depends on the effluent potency, wastewater discharge

rate and engrossing capacity of the receiving water bodies

(Gonzalez, 1996). Moreover, wastewaters generated from

fishmeal industry contain high organic suspended solids,

proteins and salinity close to seawater (Vidal et al., 1997).

Recently, anaerobic granular sludge treatment of fishmeal

wastewaters is supposed to be an optimal process. However,

few studies have been performed so far on the application of

anaerobic granular sludge to the removal of organic pollutants

in highly saline wastewaters (Jeison et al., 2008; Lei et al.,

2008). During aerobic treatment of hyper saline wastewaters

that is under high saline conditions, cell plasmolysis, defi-

ciency of filamentous micro-organisms and lack of protozoa

will occur simultaneously (Lei et al., 2008). Although, findings

still have many disagreements. The sludge degranulation was

really observed while treating saline tannery soaked liquor in

UASB reactor (Huang et al., 2009).

Black water contains half the load of organic material in

domestic wastewater with amajor fraction of the nutrients as

nitrogen and phosphorus (Otterpohl et al., 1999; Kujawa-

Roeleveld and Zeeman, 2006). The risk of dispersion of

diseases, due to exposure to micro-organisms in the water,

will be a critical phase if the water is to be reused for toilet

flushing or irrigation. In contrast, gray water has a high

potential of reuse because it contains the major fraction

(approximately 70%) of domestic wastewater and remains

relatively less polluted (Leal et al., 2007). There is a risk that

micro-organisms in the water will be increased in the form of

aerosols that are generated when the toilets are flushed

(Feachem et al., 1983; Christova-Boal et al., 1996; Albrechtsen,

1998). Both inhaling and hand to mouth contact can be

dangerous (Ince et al., 2005). The resulting effluent stream

from the process is polluted with a COD of 20e30 mg L�1 and

pH of 3e4 (Wolmarans and de Villiers, 2002). In warm climate

countries, the high-rate anaerobic process (like UASB) shows

satisfactory treatment performance, even for diluted

domestic wastewater, with many advantages, including

reduction of green house gas emissions, reduced excess

sludge productions, stabilized sludge and low space require-

ments as compared to conventional digestion systems (van

Lier and Huibers, 2004). General characterization of different

wastewaters used in UASB reactor is shown in Table 1.

During the treatment of complex wastewaters containing

significant amounts of fat (e.g. slaughterhouse, dairy), the

continuous operation of UASB reactors has shown to cause

problems of scum and sludge layer on top of the reactors with

succeeding biomass washout (Hwu, 1997; Petruy, 1999; Nadais

et al., 2005a,b). The high COD accumulation in sludge bed has

also been reported to lead unstable performance of reactors on

long run. Research on anaerobic degradation of complex fat

containing wastewater showed that the initial removal

mechanism was mainly adsorption (Riffat and Dague, 1995;

Hwu, 1997; Nadais et al., 2003). The sharp adsorption of

organic matter occurs in the sludge bed but is not followed by

an immediate biological degradation of the adsorbed organic

matter since the kinetics of biological degradation is much

slower than the adsorption phenomena (Nadais, 2002). As

a consequence heavy accumulation of organic matter in

continuous treatment systems has been observed both at lab

and full-scale UASB reactors treating complex fat containing

substrates. According to Jeganathan et al. (2006) the factor that

most influenceshigh-rate reactorperformance is theFOG (fats,

oils and greases) accumulation rather than the FOG concen-

tration in the reactor feed. The substrates that pose more

problems in the anaerobic degradation of dairywastewater are

the fatty matters and the long chain fatty acids resulting from

milk fat hydrolysis, especially oleic acid (Petruy, 1999).

2. Treatment potential of UASB process

Anaerobic treatment of wastewaters is nowadays widely

accepted as a proved technology and extensively used. One of

themain factors leading to the success of anaerobic treatment

Table 1 e General properties of wastewaters used for UASB process.

Type ofwastewater

COD BOD TS TSS VSS T-P T-N pH Oil &grease

Reference

Palm oil mill effluent 30e70 11e30 30e65 9e25 as SS e e 0.5e0.9 3.5e4.5 5e13 as oil Borja et al. (1996)

95 22 35 12 as SS e e e 4.35 10.6 Chaisri et al. (2007)

50 25 BOD3 40.5 18 as SS 34 as

TVS

0.02 0.75 4.7 e Ma (1999)

80 21 BOD5 70 45 37 e e 5 11 Siang (2006)

Distillery WW 100e120 30 51.5e100 e 2.8 e e 3e4.1 e Nataraj et al. (2006)

Wine distillery WW 3.1e48 0.21e8.0 11.4e32 2.4e5.0 1.2e2.8 0.24e65.7 0.1e64 3.53e5.4 e Bustamante et al. (2005)

Vinasse 97.5 as SCOD 42.23 3.9 e e e e 4.4 e Martin et al. (2002)

Raw spent wash 37.5 e 2.82 e e 0.24 2.02 4.2 e Ramana et al. (2002)

Molasses WW 80.5 e 109 e 2.5 e 1.8 5.2 e Jimenez and Borja (1997)

Alcohol Distillery 11e33 6e16 e e e 0.3e0.7 0.12e0.25 4e7 3 Ince et al. (2005)

Dairy WW 3.38 1.94 1.56 0.83 0.75 0.022 0.051 as TKN 7.9 0.26 Tawfik et al. (2008)

5.4e77.3 e 3.9e58.9 3.1e48.7

as VS

0.5e5.6 4.3e8.7 0.4e5.7 Kalyuzhnyi et al. (1996)

74.5 e e 9.38 as SS 8.3 0.124 0.15 as TKN 3.92 e Erguder et al. (2001)

Fishery WW 0.17 0.12 e 25 e e e 6.85 e Huang et al. (2009)

Fish canning 3.3 1.73 5.99 e e e 0.21 as TKN 6.4 1.0 as FOG Prasertsan et al. (1994)

1.5 0.82 5 e e e 0.13 as TKN 6.9 0.66 as FOG

Slaughterhouse WW 1.5e2.2 0.49e0.65 e 40e50

as SS

e 0.012e0.02 0.12e0.18

as TKN

6.8e7.1 0.05e0.1

as grease

Sayed et al. (1984)

Poultry slaughter WW 7.3 5.5 2.7 0.94 0.82 0.01 0.08 6.6 0.31 Chavez et al. (2005)

Slaughterhouse WW 6.2 2.3 e 6.3 5.3 0.04 0.21 as

Organic N

6.6 0.6 Caixeta et al. (2002)

Slaughterhouse WW 4.4 e 3.9 0.40 as SS 0.60 as VS e e 6.8 e Seif and Moursy (2001)

Piggery manure and WW 4.8e12.6 56e58 1.9e43.2 1.72e33.1 0.2e1.52 0.8 as

Organic N

5e5.9 e Sanchez et al. (1995)

10.19 e 7.21 1.64 1.17 0.42 0.34 6 e Sanchez et al. (2005)

65 59 53 38 3 6.8 7.8 e Angelidaki et al. (2002)

Muncipal WW 2.6 1.1 e 0.85 e 0.02 0.13 as TKN 6.35 0.26 as FOG Halalsheh et al. (2008)

0.53 0.24 e 0.26 as SS e e 0.046 as TKN e e Tandukar et al. (2005)

0.45 0.22 e 0.19 e 0.0034 0.05 as TKN e e Moawad et al. (2009)

All values are in g L�1 except pH.

water

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wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 9 4687

was the introduction of high-rate reactors in which biomass

retention and liquid retention are uncoupled (Lettinga et al.,

1980; de Zeeuw, 1988). High-rate anaerobic reactors, that can

retain biomass, have a high treatment capacity and hence low

site area requirement (Droste, 1997). Several processes have

thus been developed to operate anaerobic digestion reactors,

each of them having several advantages. Musee et al. (2006)

identified waste-generating mechanisms, analyzed the cau-

ses, and then derived options for feasible waste minimization

alternatives. One of the most common is the UASB process

that has successfully been used to treat a variety of waste-

waters, but is often limited by poor biodegradability of

complex organic substrates (Goodwin and Stuart, 1994;

Seghezzo et al., 1998; Goodwin et al., 2001; Wolmarans and

de Villiers, 2002; Coetzee et al., 2004).

The UASB reactor exhibits positive features, such as high

organic loading rates (OLRs), short hydraulic retention time

(HRT) and a low energy demand (Borja and Banks, 1994;

Metcalf and Eddy, 2003). Anaerobic sludge in UASB reactors

spontaneously immobilizes into well settling granular sludge.

It has been widely adopted for treatment of medium to high-

strength industrial wastewaters (Lettinga and Hulshoff Pol,

1991; Fang et al., 1995). UASB reactor was developed by

Lettinga et al. (1980) whereby this system has been successful

in treating a wide range of industrial effluents including

those with inhibitory compounds. The underlying principle

of the UASB operation is to have an anaerobic sludge which

exhibits good settling properties (Lettinga, 1995) and effi-

ciently retains complex microbial consortium without the

need for immobilization on a carrier material (for example, as

a biofilm) by formation of biological granules with good

settling characteristics. Performance depends on the mean

cell residence time and reactor volume depends on the

hydraulic residence time, therefore, UASB reactor can effi-

ciently convert organic compounds of wastewater into

methane in small ‘high-rate’ reactors. Approximately 60% of

the thousands of anaerobic full-scale treatment facilities

worldwide are now based on the UASB design concept,

treating a various range of industrial wastewaters (Jantsch

et al., 2002; Karim and Gupta, 2003). Moreover, previous

research studies also indicate the feasibility of this process to

treat domestic effluents (Behling et al., 1997; Singh and

Viraraghavan, 2000). The key feature of this system is the

microbial aggregation into a symbiotic multilayer structure

called a granule and retention of highly active biomass with

good settling abilities in the reactor (Schmidt and Ahring,

1996). Improved process knowledge and operational details

on formation of stable granules have made the possibility of

high organic loadings and resulting in a more sustainable

operation. The long hydraulic retention times are known to

be unfavorable for sludge granulation in UASB reactors

(Alphenaar et al., 1993) whereas, very short hydraulic reten-

tion times give rise to possibility of biomass washout. Both

scenarios are unfavorable to good performance of the UASB

reactor, although granulation has been reported to be

necessary for successful domestic wastewater treatment in

UASB reactors (Aiyuk and Verstraete, 2004; van Haandel

et al., 2006).

The success of the UASB reactor also relies on the estab-

lishment of a dense sludge bed 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. In upflow anaerobic systems,

under certain conditions, it was also observed that bacteria

can naturally aggregate in flocks and granules (Hulshoff Pol

et al., 1983; Hulshoff Pol, 1989). These dense aggregates are

not susceptible to washout from the system under practical

reactor conditions. Retention of active sludge, either granular

or flocculent, within the UASB reactor enables good treat-

ment performance at high organic loading rates. The main

reason for the success of the UASB reactor is its relatively

high treatment capacity compared to other systems (Driessen

and Yspeert, 1999). Natural turbulence caused by the influent

flow rate and biogas production provides good wastewater-

biomass contact in UASB systems (Heertjes and van der

Meer, 1978). Therefore, less reactor volume and space are

required while, at the same time, high grade energy is

produced as biogas. Several configurations can be imagined

for a wastewater treatment plant including a UASB reactor. In

any case, there must be a sand trap, screens for coarse

material, and drying beds for the sludge. The UASB reactor

may replace the primary settler, the anaerobic sludge

digester, the aerobic step (activated sludge, trickling filter,

etc.), and the secondary settler of a conventional aerobic

treatment plant. However, the effluent from UASB reactors

usually needs further treatment, in order to remove remnant

organic matter, nutrients and pathogens. This post-

treatment can be accomplished in conventional aerobic

systems like stabilization ponds, activated sludge plants, and

others. The economics of anaerobic treatment in UASB

reactors were thoroughly discussed by Lettinga et al. (1983).

The advantages and disadvantage of UASB reactors are

shown in Table 2.

In particular, the UASB reactor is a reliable and simple

technology for wastewater treatment (van Haandel and

Lettinga, 1994). Several full-scale plants have been in opera-

tion and many more are presently under construction, espe-

cially under tropical or subtropical conditions (van Haandel

et al., 2006). The UASB system has become the most widely

applied reactor technology for high-rate anaerobic treatment

of industrial effluents. Its relative high treatment capacity

compared to other systems permits the use of compact and

economic wastewater treatment plants. Compared to aerobic

system, it has slow growth rate, mainly associated with

methanogenic bacteria. Therefore, it requires a long solids

retention time, and also only a small portion of the degradable

organic waste is being synthesized to new cells. So far, UASB

process technique has been applied for the treatment of palm

oil mill effluent, distillery wastewater, slaughterhouse

wastewater, piggery wastewater, dairy wastewater, fishmeal

process wastewater, municipal wastewater, potato waste

leachate, coffee production wastewater, petrochemical

wastewater, low strength wastewaters like real cotton pro-

cessingwastewater and synthetic wastewater. The key design

parameters of UASB reactor used by different researchers are

shown in Table 3. The most commonly used operational

parameters like pH, mixing; operational temperature,

hydraulic retention time and organic loading rates are

extensively discussed in Table 4 along with COD removal and

biogas production.

Table 2eAdvantages and disadvantages of UASB reactor.

Advantages1. Good removal efficiency can be achieved in the system,

even at high loading rates and low temperatures.

2. The construction and operation of these reactors is relatively

simple and low demand for foreign exchange due to

possible local production of construction material, plant

components, spare parts and low maintenance.

3. Anaerobic treatment can easily be applied on either

a very large or a very small scale.

4. When high loading rates are accommodated, the area

needed for the reactor is small thus reducing the capital cost.

5. As far as no heating of the influent is needed to reach

the working temperature and all plant operations can be

done by gravity, the energy consumption of the reactor

is less. Moreover, energy is produced during the process in

the form of methane.

6. Reduction of CO2 emissions due to low demand for foreign

(fossil) energy and surplus energy production.

7. Much less bio-solids waste generated compared with

aerobic process because much of the energy in the

wastewater is converted to a gaseous form and resulting

in very little energy left for new cell growth.

8. The sludge production is low, when compared to aerobic

methods, due to the slow growth rates of anaerobic bacteria.

The sludge is well stabilized for final disposal and has good

dewatering characteristics. It can be preserved for long

periods of time without a significant reduction of activity,

allowing its use as inoculum for the startup of new reactors.

9. Can handle organic shock loads effectively.

10. Low nutrients and chemical requirement especially in the

case of sewage, an adequate and stable pH can be

maintained without the addition of chemicals.

11. Macronutrients (nitrogen and phosphorus) and

micronutrients are also available in sewage, while toxic

compounds are absent.

Disadvantages1. Pathogens are only partially removed, except helminthes

eggs, which are effectively captured in the sludge bed.

Nutrients removal is not complete and therefore a

post-treatment is required.

2. Due to the low growth rate of methanogenic organisms,

longer startup takes before steady state operation, if activated

sludge is not sufficiently available.

3. Hydrogen sulphide is produced during the anaerobic

process, especially when there are high concentrations of

sulfate in the influent. A proper handling of the biogas is

required to avoid bad smell and corrosion.

4. Post-treatment of the anaerobic effluent is generally

required to reach the surface water discharge standards

for organic matter, nutrients and pathogens.

5. Proper temperature control (15e35 �C) required for

colder climates.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 94688

3. Operation and performance of UASBreactor

3.1. Organic loading rates and COD removal

Organic loading rate is an important parameter significantly

affecting microbial ecology and performance of UASB

systems. This parameter integrates reactor characteristics,

operational characteristics, and bacterial mass and activity

into the volume ofmedia (Torkian et al., 2003). Various studies

have proven that higher OLRs will reduce COD removal effi-

ciency in wastewater treatment systems (Patel and

Madamwar, 2002; Torkian et al., 2003; Sanchez et al., 2005).

However, gas production will increase with OLR until a stage

when methanogens could not work quick enough to convert

acetic acid to methane. Moreover, organic loading rate can

also be related to substrate concentration and HRT, thus

a good balance between these two parameters has to be

obtained for good digester operation. Short HRT will reduce

the time of contact between substrate and biomass.

Palm oil mill effluent treatment has been successful with

UASB reactors, achieving COD removal efficiency up to 98.4%

with the highest operating OLR of 10.63 kgCODm�3 d�1 (Borja

and Banks, 1994). However, reactor operated under overload

conditions with high volatile fatty acid content became

unstable after 15 days. Due to high amount of POME discharge

daily frommilling process, it is necessary to operate treatment

system at higher OLR. Borja et al. (1996) implemented a two-

stage UASB system for POME treatment with the objective of

preventing inhibition of granule formation at higher OLRs

without removing solids from POME prior to treatment. This

method is desirable since suspended solids in POME have high

potential for gas production while extra costs for sludge

disposal can be avoided. Results from this study showed the

feasibility of separating anaerobic digestion into two stages

(acidogensis and methanogenesis) using a pair of UASB reac-

tors. The methanogenic reactor was found to adapt quickly

with the feed from the acidogenic reactor and also tolerate

higher OLRs. It was suggested that OLR of 30 kgCODm�3 d�1

could ensure an overall of 90% COD reduction and efficient

methane conversion. UASB reactor is advantageous for its

ability to treat wastewater with low suspended solid content

(Kalyuzhnyi et al., 1998) and provide higher methane

production (Kalyuzhnyi et al., 1996). Whereas, the packing

material in anaerobic filter reactors clogged because of sus-

pended solids and resulted in less biogas production (Stronach

et al., 1987). Moreover, suspended and colloidal components

of POME in the form of fat, protein, and cellulose have an

adverse impact on UASB reactor performance and can cause

deterioration of microbial activities and washout of the active

biomass (Borja and Banks, 1994; Torkian et al., 2003). However,

the reactor might face long startup periods if seed sludge is

not granulated. A study by Goodwin et al. (1992) has proved

that reactors seeded with granulated sludge can achieve high

performance levels within a shorter startup period. It could

also acclimatize quickly to gradual increase of OLR

(Kalyuzhnyi et al., 1996).

The use of two identical UASB reactors by Goodwin and

Stuart (1994) operated in parallel as duplicates for 327 days

for the treatment of malt whisky pot ale, achieved COD

reductions of up to 90% for at influent concentrations of

3.5e5.2 g L�1. When the OLRs of 15 kgCODm�3 d�1 and above

were used, the COD removal efficiency dropped to less than

20% in one of the duplicate reactors. A mesophilic two-stage

system consisting of an anaerobic filter (AF) and an UASB

reactor was found suitable for anaerobic digestion of distillery

waste, enabling better conditions for themethanogenic phase

(Blonskaja et al., 2003). An advanced version of UASB system

was reported by Driessen and Yspeert (1999), wherein they

Table 3 e UASB reactor specifications and working parameters.

Type ofwastewater

Phase Reactorvolume (L)

Diameter(cm)

Height(cm)

Flowrate (L d�1)a

HRT(d)

Upflowvelocity (mh�1)a

Samplingports

Reference

Palm oil

mill effluent

Two 12 13 90 11.76 1 e 6 Borja et al. (1996)

5 9 78 4.9 1 e 6

Single 10 9e12 140 3 3.33 e 4 Chaisri et al. (2007)

Single 14 18.5 52 383.26b 0.53 0.59 3 Siang (2006)

Distillery

WW

Single 1.05 6.2 34.8 0.5 2 e e Goodwin et al. (2001)

Two 10.2 9 160 4 2.5 e 15 Laubscher et al. (2001)

Three 2.3 5 83 1.84 1.25 2 e Keyser et al. (2003),c

IUASB-Single 143,000 1102 1500 47,667 3 e e Ince et al. (2005)

TUASB-Single 476,000 2010 1500 119,000 4 e e

CUASB-Single 190,000 1270 1500 38,000 5 e e

Dairy WW Phase 1 12.3 15 70 3.5 3.5 e 4 Luostarinen and

Rintala (2005)Phase 2 3.2 9 50 2.13 1.5 e 3

Single 31.7 15.4 170 e 0.11 Nadais et al. (2005a,b)

Intermittent 6 9.4 86.4 12 0.5 e e Nadais et al. (2005a)

Single 5 10 70 5 1 e 5 Tawfik et al. (2008)

Fishery WW Single 7.85 10 100 e e 5 Huang et al. (2009)

Slaughterhouse

WW

Single 31,840 260 600 96,485 0.33 e 5 Sayed et al. (1984)

Single 3 6.7 85 24 0.13 e e Chavez et al. (2005)

Three 7.2 15 41 7.9e12.4 0.91e0.58 e 3 Caixeta et al. (2002)

Single 1000 e e 10,000 0.1 0.33e1.0 9 Torkian et al. (2003)

Two 2 8 15 0.59 3.4 e 6 Ruiz et al. (1997)

Piggery WW Single 5 15 30 1 5 e e Sanchez et al. (2005)

Single 3.78 6W, 6L 105 6 0.63 2 as Vs 6 Huang et al. (2005)

Single 800� 103 940 1160 e e e e Miranda et al. (2005)

Two 1 3.9 84 3 0.33 e 4 Hendriksen and

Ahring (1996)2 5.7 78 4.37 0.46 e 4

Municipal WW

and waste

Two 2.5 6 100 2 1.25 e 5 A�gda�g and

Sponza (2005)

Single 40 16 240 121 0.33 e e El-Gohary and

Nasr (1999)Two 25 11 300 119 0.21 e e

Single 55 28 89 172 0.32 e 1 Behling et al. (1997)

Single 8 10 100 11.4 0.7 e 5 Singh and

Viraraghavan (1998)

Threed 46 15W, 25L 125 e 0.17e0.13 0.31e0.43 4 Moawad et al. (2009)

Single 15.7 10 200 80 0.196 0.426 7 Uemura and

Harada (2000)

Single 2.3 5 90 6 0.33 1 e Aiyuk and

Verstraete (2004)

a Some values calculated by Eqs. (2) and (3).

b Total flow rate (influent and recycled).

c Design adopted from Trnovec and Britz (1998).

d Only UASB data.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 9 4689

used an internal circulation reactor characterized by biogas

separation in two stages within a reactor with a high height/

diameter ratio and the gas driven internal effluent circulation.

This systemcould handle high upflow liquid and gas velocities

making possible treatment of low strength effluents at short

hydraulic retention times as well as treating high-strength

effluents from brewery at very high volumetric loading rates

up to 35 kgCODm�3 d�1.

A three-phased UASB reactor used by Caixeta et al. (2002)

for slaughterhouse wastewater treatment at an OLR of

2.7e10.8 kgCODm�3 d�1 and average COD removal efficien-

cies of 85, 84 and 80% and BOD5 of approximately 95% at

three different HRT of 22, 18 and 14 h, respectively. Syutsubo

et al. (1997) reported a COD loading of 30 kgCODm�3 d�1 with

a COD removal efficiency of 85% at sludge loading rates

(SLRs) up to 3.7 gCOD g�1 VSSd�1 using thermophilic reactors

(Syutsubo et al., 1998). Organic loading rates (OLRs) up to

104 kgCODm�3 d�1 have been reported for anaerobic diges-

tion of sugar substrate under thermophilic conditions

(Wiegant and Lettinga, 1985). Torkian et al. (2003) concluded

that results under steady state condition where OLRs were

between 13 and 39 kgSCODm�3 d�1 and HRT of 2e7 h.

Removal efficiencies in the range of 75e90% were achieved at

feed SCOD concentrations of 3e4.5 g L�1. According to Soto

et al. (1997), excellent stability and high treatment effi-

ciency can be achieved with hydraulic residence times as low

as 2 h at an OLR of 6 kgCODm�3 d�1 with the percent COD

removals being 92e95%. Sayed et al. (1987) treated effluent

Table 4 e Operation and performance of UASB reactor with various wastewaters.

Type ofwastewater

Reactortype

Phase InfluentCOD(g L�1)

OLR(kgCODm�3 d�1)

HRT(days)

Temperature(�C)

CODremoval

(%)

Biogas(L d�1)

CH4

(L d�1)AverageCH4%

Reference

POME POME UASB Single 42.5 10.63 4 35 96 11.5 6.9 60 Borja and Banks (1994)

POME UASB Two 30.6 30 1.02 35 90 10 7 70 Borja et al. (1996)

POME UASB Single 50 15.5 3.33 28 80.5 14 7 50 Chaisri et al. (2007)

Distillery WW Recalcitrant

distillery WW

UASB Single 10 19 0.53 55 <67 6.4 3.5 55 Harada et al. (1996)

Distillery WW AF-UASB Two 8.51e16.8a 4 19e10 36� 1.5 47 0.091e0.39 0.06e0.26 65 Blonskaja et al. (2003)

13.6b 2.2 20 36� 1.5 93 5.3 3.45 65

Malt whisky

distillery pot ale

UASB Single 21.05c 10.2 2.1 35 93 4.7 e e Goodwin et al. (2001)

32.86d 4.69 7 35 88 1.3 e e

Malt whisky WW UASB Two 20.92 17.2 1.22 35� 1.5 92 310e 238f 77f Uzal et al. (2003)

Grape wine

distillery WW

UASB Single 30 18 1.67 34e36 90� 3 e e e Wolmarans and

de Villiers (2002)

Grain distillation WW UASB Two 5.1g 18.4 0.28 35 90� 3 e e e Laubscher et al., 2001

Winery effluent UASB Three 6.4 5.1 1.25 35 86 2.3h e e Keyser et al. (2003)

Raki & Cognac

distilleries

IUASB Single 33 11 3 36� 1 85 0.078i 0.045 74 Ince et al. (2005)

TUASB 32 8.5 4 36� 1 60e80 0.078i 0.045 74

CUASB 23 4.5 5 36� 1 70e80 0.071i 0.041 74

Dairy WW Dairy WW UASB Single 37 6.2 6 35 98 e e e Gavala et al. (1999)

Dairy palour WW UASB-

Septic

Phase 1 0.63 0.179 3.5 20 73 e e e Sari and Jukka (2005)

Phase 2 0.36 0.24 1.5 10 64 e e e

Dairy & Domestic WW UASB-AS Single 2.01 3.4 1 35 69 e e e Tawfik et al. (2008)

Dairy WW UASB Single 79 7.5 0.66 35� 1 74 e 16 e Nadais et al. (2005a,b)

Dairy WW UASB Intermittent 13.5 22 2 35� 1 97� 1 e 54 e Nadais et al. (2005a)

Digested

cowdung slurry

UASB Single 1.8 13.5 0.13 30� 2 >90 e e e Ramasamy et al. (2004)

Dairy WW UASB Continuous 12.48 12.48 1 35� 1 90 e e e Nadais et al. (2006)

Intermittent 12.48 12.48 1 35� 1 90 e e e

Cheese whey UASB Two 55.1 11.1 4.95 e 95 e 23.4j e Erguder et al. (2001)

Dairy manure UASB Two 17.8 8.9 2 35� 1 87.3 e 0.27k e Garcıa et al. (2008)

Fishery WW Mixed sardine and

tuna canning

UASB Single 2.72 8 0.33 e 80e90 e e e Palenzuela-Rollon

et al. (2002)

Slaughterhouse

WW

Slaughterhouse waste UASB Single 1.2 3.5 0.33 20 70 10,000 6500 65e70 Sayed et al. (1984)

Poultry slaughter WW UASB Single 5.5 28.7 0.19 as CRT 24.7 95 e e e Chavez et al. (2005),l

Slaughterhouse waste UASB Three 4.2 4.6 0.92 35 89 11.9 e e Caixeta et al. (2002)

6.5 8.7 0.75 90 10.9 e e

6.3 10.8 0.58 86 10.6 e e

Slaughterhouse WW UASB Five 2.87 30 0.1 33 90 e 280m e Torkian et al. (2003)

Slaughterhouse WW UASB-AFo Two 7.6 2.23 3.4 37 93 1.03 0.6 70.6 Ruiz et al. (1997)

Piggery WW Piggery waste UASB Single 8.12 1.62 5 30e35 75 4.1 2.37n 57.8 Sanchez et al. (2005)

Pre-settled piggery WW UASB-AS Two 2 3.17 0.63 30� 1 91 1.51 0.6 39.7 Huang et al. (2005)

(continued on next page)

water

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45

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4690

Table 4 (continued )

Type ofwastewater

Reactortype

Phase InfluentCOD(g L�1)

OLR(kgCODm�3 d�1)

HRT(days)

Temperature(�C)

CODremoval

(%)

Biogas(L d�1)

CH4

d�1)AverageCH4%

Reference

Municipal WW Municipal landfill

leachate

UASB-CSTR Twop 20 16 4.5 37� 3 79 9.5 60.05 A�gda�g and Sponza (2005)

Domestic WW UASB Single 0.39 1.21 0.32 30 85 26 e Behling et al. (1997)

Domestic WW UASB-SBRo Three 0.37 2.93 0.125 e 57 e e Moawad et al. (2009)

Municipal WW UASB-AS Fiveq 0.56 0.09 0.17 e 85 e e von Sperling et al. (2001)

Municipal WW UASB Single 3.2 1.05 0.42 20� 1 86 1.97 79 Singh and Viraraghavan (1998)

Sewage WW UASB Single 0.15e0.5 0.77e2.55 0.196 25e13 68� 4 e e Uemura and Harada (2000)

Sewage WW AF-UASBo Two 0.47e1.23 1.4e3.7 0.33 12e23 <50 e e Sawajneh et al. (2010)

a Acidogenic phase.

b Methanogenic phase.

c With 70% pot ale.

d With 100% pot ale.

e At influent of COD 16 g L�1.

f Stoichiometric calculations for CH4.

g At controlled conditions.

h At OLR of 6.3 kgCODm�3 d�1.

i As per gVSS.

j LCH4 L�1 of cheese whey.

k LCH4 g�1CODremoved.

l All data in terms of BOD.

m As per SCOD.

n Per liter of influent.

o Only UASB data.

p 1st run data, total 3 runs.

q Phase 3 data.

water

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45

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4691

(L

5.7

e

e

e

1.1

3.5

e

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 94692

from meat processing plant in a granular UASB reactor and

achieved COD removal efficiency of 55e85% with HRT of

0.5e0.6 days at volumetric loading rate of 11 kgCODm�3 d�1.

Dague and Pidaparti (1992) concluded that operation of

reactor with hydraulic retention time of 8.8 days and OLR of

0.33 kgBOD5m�3 d�1 yielded a BOD5 removal efficiency of

85e90% and biogas production of 0.51 m3CH4 kg�1CODremoved.

Two-phased UASB-septic tank used by Luostarinen and

Rintala (2005) with high removal of organic matter for onsite

treatment of synthetic black water (OLR 0.301 kgCODm�3 d�1)

and dairy parlor wastewater (OLR 0.191 kgCODm�3 d�1) at low

temperatures (10e20 �C). Moreover, CODdiss removal was

around 70% at 15 �C and 10 �C indicating good biological

activity of the reactor sludge. Gavala et al. (1999) concluded

that an OLR of 6.2 gCOD L�1 d�1 (diluted to 37 gCOD L�1, with

an HRT to 6 d) may be safely used for treating dairy waste-

water and could be increased up to 7.5 gCOD L�1 d�1. Above

that OLR, reduced performance is observed; while for non-

diluted dairy wastewater, an HRT in excess of 30 d is required.

According to Palenzuela-Rollon et al. (2002) the application

of UASB system is a promising treatment option for fish pro-

cessing wastewater. They determined the performance of

USAB reactor for the treatment of mixed sardine and tuna

canning effluent at varying lipid levels. They stated that at low

lipid level (203e261 mg L�1, 9% of total COD) approximately

78% COD removal and 61% COD conversion tomethane can be

achieved with an OLR of 2.3 gCOD L�1 d�1 and at HRT of

7.2� 2.8 h. In the case of high-lipid wastewater a two-step

UASB was recommended where the total COD removal and

conversion to methane were 92% and 47%, respectively. Punal

and Lema (1999) have used a 380 m3 UASB reactor for the

treatment of fish-canning factory wastewater. The waste-

water was a mixed effluent of tuna, sardine and mussel pro-

cessing. The total alkalinity of more than 3 gCaCO3 L�1 was

maintained to operate the system properly and allow biomass

to resist load shocks. An HRT of 2 days was maintained and

the OLRwas varied from 1 kgCODm�3 d�1 to 8 kgCODm�3 d�1.

The efficiency of the system is dependent on the nature of the

wastewater as shown in Table 4. The organic loading rate can

be calculated by the following equation,

OLR ¼ CODin

HRT(1)

where OLR¼Organic loading rate (gCOD L�1 d�1), CODin¼ In-

fluent COD (g L�1), HRT¼Hydraulic retention time (days).

3.2. Flow rate and hydraulic retention time

Flow rate is also an important operating parameter which

upholds the hydraulic retention time. A lot of data has been

published regarding flow rate which also can justify by Eq. (2).

Borja et al. (1996) worked for POME at a maximum flow rate of

11.76 L d�1 and 4.9 L d�1 at 24 h HRT for two-stage UASB

reactor. The difference in flow rate at same HRT is due to

change in volume of the reactor from 12 to 5 L. Experiment

carried out by Siang (2006) for POME, where low HRT of 12.7 h

was worked out at recycling mode of UASB and maintained

a flow rate of 383.26 L d�1. This low HRT might be due to less

height of the reactor (52 cm) andbigger diameter (18.5 cm). The

purpose of describing height/diameter combination is that,

duringupflowanaerobicprocess if diameterwill be toobig then

there is a chance of liquid channeling in the reactor. Moreover,

because of channeling, the influent stream may not be in full

contact with reactor biomass which will result in low conver-

sion of organic matter into fatty acids and finally biogas

production. So, bigger reactor diameter does not encourage

more biogas production except sludge washout because of

poor mixing within the reactor. On the other hand, compara-

tively more height may encourage substrate mixing which

leads to proper contact of influent with micro-organisms

which results in more organic matter conversion into biogas.

Chaisri et al. (2007) and Laubscher et al. (2001) used almost 10 L

UASB reactors with flow rates of 3 L d�1 and 4 L d�1 respec-

tively. The difference in flow rate is due to the change in HRT

only which might be designed according to the type of waste-

water, where, in first case POME was used as a substrate and

distillerywastewater for later one. Ince et al. (2005) worked out

at three full-scale UASB reactors at 3e4 d HRTwhile change in

flow rates is due to the different reactor volumes. Torkian et al.

(2003) used a pilot scale UASB reactor for the treatment of

slaughterhouse wastewater. They kept flow rate of 1000 L d�1

at only 2.4 h HRT. Tawfik et al. (2008) studied on dairy waste-

water and used a flow rate of 5 L d�1 and at 1 d HRT while

reactor height was also sufficient to run experiment at higher

HRTs.

In another study, Sanchez et al. (2005) used piggery

wastewater as substrate and worked at very less flow rate of

1 L d�1 while keeping HRT of 5 days. They used 30 cm high and

15 cm diameter UASB reactor. This higher HRT for lower

height might has different reasons like, This long HRT for less

height might has different reasons like, the use of raw

wastewater rich in organic contents, or may be less dilution

has been done for startup of UASB reactor, or, they used very

less or no nutrients for running the experiment. Huang et al.

(2005) used 105 cm high rectangular UASB reactor and

worked only at 15 h HRT with a flow rate of 6 L d�1.

The success of UASB reactor also done by using municipal

wastewater, Moawad et al. (2009) worked at two hydraulic

retention times 4 h and 3 h but they did not mention the flow

rate in their study. By using Eq. (2), the flow rate could be

270e353 L d�1 while they used rectangular UASB rector. A�gda�g

and Sponza (2005) studied degradation of municipal waste-

water at two-stage UASB reactor of same volume (2.5 L) and

designed flow rate was 2 L d�1 at 1.25 d HRT. The low flow rate

accompanied due to less reactor volume (2.8 L). Complete

detail of UASB reactor configuration with flow rate and HRT

designed by various researchers is shown in Table 3. Some

data regarding flow rate is calculated by Eq. (2).

By following Eq. (2), it is clear that flow rate is inversely

proportional to HRT but it is understood that volume has

direct relation with flow rate. For designing purpose, we can

calculate the flow rate by following equation.

Q ¼ VHRT

(2)

where Q¼ Flow rate of influent stream (L d�1), V¼Volume of

the reactor (L), HRT¼Hydraulic retention time (days).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 9 4693

3.3. Upflow velocity

The upflow velocity (Vup) is also an important operational

parameter in upflow digesters. It maintains the mixing and

hydraulic retention time of the substrate and biomass. A few

researches are done on upflow velocity as limited data had

shown by few researchers. Upflow velocity is directly

proportional to reactor height and inversely proportional to

hydraulic retention time at fix HRT or reactor volume, as

shown in Eq. (3). It determines the appropriate mixing of

biomass with the height of the reactor with or without chan-

neling. The permissible limit of upflow velocity is

0.5e1.5 mh�1 as described by many researchers. Siang (2006)

maintained a 0.59 mh�1 Vup at HRT of 13 h. Keyser et al.

(2003) reported a 2 mh�1 Vup at HRT of 1.25 days which is

quite higher than limits but they used distillery wastewater as

substrate and little fluctuations might be possible with

different types of wastewaters. While treating dairy waste-

water, Nadais et al. (2005a,b) reported a 0.11 mh�1Vup without

showing hydraulic retention time. Torkian et al. (2003) treated

slaughterhouse wastewater and applied Vup from 0.33 to

1.0 mh�1 while keeping a constant HRT of 2.4 h. They used

a pilot scale UASB reactor of 1 m3 capacity and height was not

shown in their study.

While treating municipal wastewater in UASB reactor,

Uemura and Harada (2000) reported 0.426 mh�1 Vup at 4.7 h of

HRT. Recently, Moawad et al. (2009) used upflow velocities of

0.31e0.43 mh�1 at HRT of 4e3 h. They used a rectangular

shaped UASB reactor for the treatment of municipal waste-

water. Upflow velocity can be determined by Eq. (3) on the

basis of HRT and height of the reactor.

Vup ¼ hHRT

(3)

where Vup¼Upflow velocity of influent stream (mh�1),

h¼ height of the reactor (m), HRT¼Hydraulic retention time

(h).

The upflow velocity can also be calculated by flow rate and

cross-sectional area of the reactor,

Vup ¼ QA

(4)

where Vup¼Upflow velocity of influent stream (mh�1),

Q¼ Flow rate of influent stream (m3 h�1), A¼ Reactor’s cross-

sectional area (m2).

3.4. pH

The microbial community in the anaerobic digester is sensi-

tive to the changes of pH and methanogens are affected to

a greater extent (Grady et al., 1999). An investigation by

Beccari et al. (1996) confirmed thatmethanogenesis is strongly

affected by pH. As such, methanogenic activity will decrease

when pH in the digester deviates from the optimum value.

OptimumpH for most microbial growth is between 6.8 and 7.2

while the pH values less than 4 and more than 9.5 are not

tolerable (Gerardi, 2006). Several cases of reactor failure have

been reported in various studies of wastewater treatment due

to accumulation of high concentration of volatile fatty acid,

causing a drop in pH which inhibited methanogenesis (Patel

and Madamwar, 2002; Parawira et al., 2006). Thus, volatile

fatty acid concentration is an important parameter tomonitor

to guarantee reactor performance (Buyukkamaci and Filibeli,

2004). It was found that digester could tolerate acetic acid

concentrations up to 4000 mg L�1 without inhibition of gas

production (Stafford, 1982). To control the level of volatile

fatty acid in the system, alkalinity has to be maintained by

recirculation of treated effluent (Borja et al., 1996; Najafpour

et al., 2006) to the digester or addition of lime and bicar-

bonate salt (Gerardi, 2003).

While treating slaughterhouse wastewater in UASB

reactor, the pH should be essentially constant, varying

between 7.5 and 8.5. The pH of the slaughterhouse influent is

normally corrected to 7.0 but it has little variation due to acid

forming bacteria. Caixeta et al. (2002) maintained the pH of

influent stream by adding sodium bicarbonate in first run.

However, in further two runs the influent was fed to the

reactor without any pH adjustment. The slaughterhouse

wastewater exhibited high buffering capacity without

requiring pH correction anymore. Sandberg and Ahring (1992)

investigated the influence of high pH on anaerobic degrada-

tion of fish processing wastewater in a UASB reactor.

According to Boone and Xun (1987) most methanogenic

bacteria have optima for growth between pH 7 and 8, whereas

VFA degrading bacteria have lower pH optima. The optimal pH

for mesophilic biogas reactor is 6.7e7.4 (Clark and Speece,

1971). A study by Sandberg and Ahring (1992) demonstrated

that fish condensate can be treated well in a UASB reactor

from pH 7.3 to 8.2. When the pHwas increased slowly to 8.0 or

more 15e17% drop in COD removal occurred. Acetate was the

only carbon source in the condensate that accumulated upon

increasing the pH. More than 99% of VFA and TMA in process

wastewater were degraded up to pH 7.9. It was concluded that

gradual pH increment was essential in order to achieve the

necessary acclimatization of the granules and to prevent

disintegration of the granules and that the pH should not

exceed 8.2. Aspe et al. (2001) modeled the ammonia-induced

inhibition phenomenon of anaerobic digestion and

concluded that methanogenesis was themost inhibited stage.

3.5. Operating temperature

Temperature is an important operating parameter for anaer-

obic degradation process. The influence of temperature on

microbial growth and biodegradation rate can be described by

the Arrhenius equation (Batstone et al., 2002; Hao et al., 2002;

Siegrist et al., 2002). Operation of anaerobic reactors under

thermophilic conditions offers a number of advantages such

as increased reaction rates and improved biodegradability of

organic compounds (Rintala, 1997; Kim et al., 2002). However,

startup and operation of a thermophilic reactor is cumber-

some due to the high sensitivity of thermophilic micro-

organisms to variations in OLR, influent composition, reactor

pH, and other factors. It is generally assumed that a transition

from mesophilic to thermophilic conditions is accompanied

by a significant (over 80%) and lengthy (over 4 days) decrease

in methane production due to adaptation of methanogens to

thermophilic temperatures (van Lier et al., 1992; Visser et al.,

1993) Nevertheless, mesophilic methanogenic populations

were shown to tolerate short-term temperature are increases

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 94694

(Speece and Kem, 1970; Ahn and Forster, 2002) or sludge

exchange between mesophilic and thermophilic reactors

(Song et al., 2004).

The cost benefit analysis for POME treatment system that

utilizes biogas for electricity generation and digester effluent

for land application also showed a faster payback (Yeoh, 2004).

POME is discharged at temperatures around 80e90 �C(Zinatizadeh et al., 2006) which actually makes treatment at

both mesophilic and thermophilic temperatures feasible

especially in tropical countries like Malaysia. Various studies

have been conducted to investigate the feasibility of operating

wastewater treatment systems in the thermophilic tempera-

ture range such as sugar, high-strength wastewater (Wiegant

et al., 1985; Wiegant and Lettinga, 1985) and POME (Cail and

Barford, 1985; Choorit and Wisarnwan, 2007). It is reported

that operation at thermophilic temperature gives better

results than mesophilic temperature after startup because

methane producing bacteria produced at mesophilic temper-

ature facilitate in high methane production at thermophilic

temperature ranges (Cail and Barford, 1985).

High production of methane was also observed from the

treatment of sugar wastewater in this higher temperature

range. Effect of temperature on the performance of anaerobic

digestion was investigated. Yu et al. (2002) found that

substrate degradation rate and biogas production rate at 55 �Cwere higher than operation at 37 �C. Studies have reported

that thermophilic digesters are able to tolerate higher OLRs

and operate at shorter HRT while producing more biogas (Ahn

and Forster, 2002; Kim et al., 2006; Yilmaz et al., 2008).

However, failure to control temperature increase can result in

biomass washout (Lau and Fang, 1997) with accumulation of

volatile fatty acid due to inhibition ofmethanogenesis. At high

temperatures, production of volatile fatty acid is higher

compared to mesophilic temperature range (Yu et al., 2002).

Many operators prefer to have digesters operating in meso-

philic temperature due to better process stability. Neverthe-

less, investigation on digester stability by Kim et al. (2002)

proved that disadvantages of thermophilic digesters can be

resolved by keeping microbial consortia in close proximity.

Full-scale thermophilic (50e55 �C) anaerobic digestion of

wastewater from an alcohol distillery was reported by

Vlissidis and Zouboulis (1993). More than 60% removal of COD

was achieved with 76% of biogas comprising of methane

thus making it a valuable fuel. Stevens and Schulte (1979)

studied the effect of the temperature at solids retention

times of 6e55 days at organic volumetric loading rates of

0.61e4.81 kgVSm�3 d�1, in a complete mixed anaerobic

digester. They concluded that at organic rates in the range of

0.61e1.80 kg VSm�3 d�1 and temperatures lower than 25 �C,the operation is proceeded satisfactorily. Low temperature

digestion was found to require twice as long retention time as

with satisfactory production and composition of gas. In

another study, Sanchez et al. (2001) observed the effect of

temperature and substrate concentration on the anaerobic

batch digestion of piggery wastewater. The study compared

the process at mesophilic temperature (35 �C) with tempera-

tures in the range of 16.8e29.5 �C, and influent concentrations

in the range of 3.3e26.3 gTCOD L�1. The process at mesophilic

temperature was more stable than at an ambient tempera-

ture, obtaining higher values of removal efficiency.

3.6. Mixing

Mixing provides good contact between microbes and

substrates, reduces resistance to mass transfer, minimizes

buildup of inhibitory intermediates and stabilizes environ-

mental conditions (Grady et al., 1999). When mixing is inef-

ficient, overall rate of process will be impaired by pockets of

material at different stages of digestion, whereby every stage

has a different pH and temperature (Stafford, 1982). Mixing

can be accomplished through mechanical mixing, biogas

recirculation or through slurry recirculation (Karim et al.,

2005a). Investigations have been done to observe the effects

of mixing to the performance of anaerobic digesters. It was

found that mixing improved the performance of digesters

treating waste with higher concentration (Karim et al., 2005b)

while slurry recirculation showed better results compared to

impeller and biogas recirculation mixing mode (Karim et al.,

2005c).

Mixing also improved the gas production as compared to

unmixed digesters (Karim et al., 2005b). Intermittent mixing

is advantageous over vigorous mixing (Stafford, 1982;

Kaparaju et al., 2008), where this has been adopted widely

in large-scale municipal and farm waste digesters (Stafford,

1982). Sludge granules are formed due to fluidization (Guiot

et al., 1992). Fluidization is achieved by mixing of the sludge

by the flow and gas release. Rapid mixing is not encouraged

as methanogens can be less efficient in this mode of opera-

tion (Gerardi, 2003). However, Karim et al. (2005b) mentioned

that mixing during startup is not beneficial due to the fact

that digester pH will be lowered, resulting in performance

instability as well as leading to a prolonged startup period.

Mixing in palm oil mills which depend on biogas produced

(Ma and Ong, 1985) is less efficient compared to mechanical

mixing as digesters are not perfectly mixed. The upflow

reactors with big diameters can face the problem of chan-

neling where upflow velocity, sometimes, cannot improve

the mixing of more viscous substrate. So, mixing becomes

the important functional parameter for such cases. Further

investigation on the effects of mixing should be commenced

to obtain a suitable mode of mixing for better digester

performance.

4. Research needs

The application of UASB reactors for the treatment of waste-

waters is limited so far to regions with constant and relatively

warm temperature conditions. The success of UASB reactors

is mainly dependent on OLR, HRT and operating tempera-

tures. Operating temperature can be a fixed parameter with

minor fluctuations but the key factors (OLR and HRT) deter-

mine the ultimate amount of hydrolysis and methanogenesis

in a UASB system especially at early stages. Organic loading

rate is still a challenge for researchers to produce maximum

biogas and high COD removal. The COD removal or biogas

production is inhibited by the accumulation of fatty acid

where, sometimes, system becomes unstable and results in

sludge washout. Accumulation of undegraded SS may also

induce a reduction in the methanogenic activity of the sludge,

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 8 3e4 6 9 9 4695

a deterioration of bacterial aggregates, and the formation of

scum layers, leading to overloading of the reactor. When too

long SRTs are necessary and, therefore, only low loading rates

can be accommodated, a two-step system with additional

sludge stabilization is to be considered. Despite considerable

work devoted to the clarification of the mechanisms of sus-

pended solids removal and hydrolysis, both the physical and

the biological processes in the first step of a two-stage

anaerobic treatment need further research. The removal of

suspended solids will depend on factors like HRT, Vup, and

sludge bed distinctiveness, and also on the characteristics of

the suspended solids themselves.

Mathematical modeling of the system, including physical

and biological processes, can help to expand more approach

into the process, and will certainly provide a balance for the

adequate management of the sludge retention in UASB reac-

tors. A model should also provide a basis for deciding for one

or two-stage anaerobic systems according to wastewater

characteristics and atmospheric conditions. Mathematical

modeling can be extremely valuable in orienting future

research on UASB technology, and can serve as a design tool

for the expansion, relocate, and distribution of anaerobic

technology for direct wastewater treatment. The treatment of

different wastewater although can permit wastewaters to

flow along with the normal surface water but biogas produc-

tion should also be keep in account. The cost benefit ratio of

the UASB reactor technology can be further decreased if more

biogas will be produced.

5. Conclusions

Upflow anaerobic sludge blanket reactor is an efficient

wastewater treatment technology that connects anticipated

anaerobic decomposition to lessen the waste volume and

produce biogas. It has been broadly applied to the treatment of

wastewaters from agricultural and industrial operations.

Depending on the starting point, the waste stream may

contain inhibitory or toxic substances such as ammonia,

sulfide, heavy metals and organics. Accumulation of these

substances may cause reactor suffering or failure, as pointed

out by reduced biogas production or methane content.

Because of the difference in anaerobic micro-organisms,

wastewater composition, and experimental methods and

conditions, results from previous investigations on inhibition

of anaerobic processes vary considerably. The reactor failure

is also mainly concerned with operating parameters such as

OLR, HRT, pH, food tomicro-organisms ratio, VFA to alkalinity

ratio and the flow rate. On spot and time to time analysis of

incoming wastewater stream and biomass within the reactor

are important for both lab and large-scale UASB reactors.

Acknowledgment

This work is carried out by the Faculty of Civil Engineering and

Earth Resources of University Malaysia Pahang (Malaysia). It is

the part of a Research Management Center (RMC) research

project on integrated application and design of upflow

anaerobic sludge blanket reactor funded by University

Malaysia Pahang. Information provided by the library staff is

gratefully acknowledged.

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