Advanced treatment for municipal wastewater reuse in agriculture. UV disinfection: parasite removal and by-product formation

Desalination 152 (2002) 315–324

0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved

Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries:Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean.Sponsored by the European Desalination Society and Alexandria University Desalination Studies and TechnologyCenter, Sharm El Sheikh, Egypt, May 4–6, 2002.

*Corresponding author.

Advanced treatment for municipal wastewater reuse inagriculture. UV disinfection: parasite removal and by-product

formation

Lorenzo Libertia*, Michele Notarnicolab, Domenico Petruzzellib

Department of Civil and Environmental Engineering, Polytechnic University of Bari, viale del Turismo 8,74100 Taranto, Italy

aTel. +39 (080) 5963-368; Fax +39 (080) 5963-282; email: [email protected]. +39 (080) 5963-477; Fax +39 080 5963 635; emails: [email protected], [email protected]

Received 30 March 2002; accepted 12 April 2002

Abstract

This paper reports the experimental results of a pilot-scale (100 m3/h) investigation, carried out at the West Bari(S. Italy) municipal wastewater treatment plant, focused on parasite removal and disinfection by-product (DBP)formation during the UV disinfection of clarified (CL) and clarified-filtered (F) secondary municipal effluents atdoses necessary for achieving the Italian microbial limit for unrestricted reuse of wastewater in agriculture (2 CFU/100ml of total coliforms). The investigation demonstrated that parasites like Giardia lamblia cysts andCryptosporidium parvum oocysts were both significantly affected by UV radiation and that potential UV-promotedformation of DBPs (nitro-phenols and N-nitroso-amines) did not occur according to GC/MS and LC/MS analyticalevidences. O&M costs ranged from € 17.5 up to € 35/1000 m3 for effluent F and CL respectively.

Keywords: Cryptosporidium; Disinfection; Disinfection by-products; Giardia; Treatment costs; UV rays; Wastewaterreuse

316 L. Liberti et al. / Desalination 152 (2002) 315–324

1. Introduction

Municipal wastewater contains a variety ofpathogenic organisms of human origin. Diseasescaused by these pathogens can occur as a resultof ingestion of untreated or improperly treatedwater, ingestion of infected aquatic food species,skin contact with contaminated water and withimproperly disinfected wastewater effluent in reuseapplication. Such diseases are more likely in coun-tries characterised by scarce rainfall, lack of fresh-water resources and high groundwater salinity as inthe Mediterranean basin where agricultural reuseof municipal wastewater is becoming a compulsorychoice for water resources manage-ment.

Various schemes of advanced (or tertiary)treatment have been proposed in the last twodecades with the so called “full Title 22” scheme[1], i.e. secondary effluent further submitted toclariflocculation, sand filtration and final disinfec-tion, most often adopted so far. The increasingoccurrence of bio-resistant microorganisms and newpathogenic species, in particular, makes advanceddisinfection a key-step for municipal wastewaterreuse in agriculture [2].

Actually, municipal wastewater effluents arecommonly disinfected by chlorination. However,protozoa like Cryptosporidium parvum andGiardia lamblia and helminths like Nematodes,of particular concern causing lethal diseases inimmunocompromised populations, have demon-strated to be resistant to chlorine-based disinfectionprocedures [3,4]. Furthermore, chlorine is knownto raise serious toxic effects on living organisms.In fact, it can react with organics contained inmunicipal wastewater to form various toxic chlo-rinated hydrocarbons, such as trihalomethanes andrelated disinfection by-products (DBP), known asanimal carcinogen and suspected to be carcino-genic towards human being [5,6].

Accordingly, attempts are under way world-wide to address the effectiveness of safer alternativedisinfectants able to meet the stringent microbialstandards usually required for wastewater recla-

mation and reuse in agriculture [7].In this context, a 3-year R&D project partially

supported by the European Community within theframework of the Avicenne Iniziative was initiatedin 1996 on various technical and health care aspectsof advanced treatment for wastewater reuse.

The Italian investigation, carried out by meansof a 100 m3/h disinfection pilot plant purposelydesigned, built and operated at West Bari (S. Italy)municipal wastewater treatment plant (3000 m3/h),was specifically aimed at comparing pathogenremoval, disinfection by-product formation andcosts of 3 alternative disinfectants, namely UV rays,peracetic acid and ozone. For each disinfectant,the influence of wastewater quality was investigatedby disinfecting 3 different municipal effluents,namely secondary (II), following activated sludgeoxidation and sedimentation, clarified (CL),following also post-precipitation with aluminumpolychloride, clarified-filtered (F), further submittedto sand filtration.

The general results of the investigation as wellas those concerning peracetic acid and ozone dis-infection have been already published [8–11].Partial results of the UV investigation referringto bacterial inactivation effectiveness have beendescribed in a previous note [12].

This paper reports further on UV disinfectionof clarified and clarified-filtered municipal effluents.The investigation was carried out at the disinfectingdoses necessary for achieving the Italian microbialstandard for unrestricted agriculture reuse(2 CFU/100ml of total coliforms, based on thewell known State of California Wastewater Recla-mation Criteria, 1978) with specific objectives of:• evaluating the effect of UV disinfection towards

selected parasitic pathogens often occurringin municipal wastewater (Nematodes eggs,Giardia lamblia cysts, Cryptosporidiumparvum oocysts)

• searching for eventual UV promoted DBPformation (i.e., nitro-phenols and N-nitroso-amines)

• drawing economic estimates.

L. Liberti et al. / Desalination 152 (2002) 315–324 317

2. Materials and methods

The investigation was carried out by means ofthe 100 m3/h pilot plant (Fig. 1) with appropriateconfiguration for comparing the performance of UVradiation on 2 different municipal wastewatereffluents, namely clarified (CL), dischargeddirectly from WBMP and clarified-filtered (F),obtained submitting CL to pressure sand filtration(MF) in the pilot plant. UV disinfection occurredin a non-contact apparatus (UVA), wherein thewater flows inside Teflon tubes surroundedexternally by low pressure Hg vapor lamps.

The comparison was performed at UV dosesrequired for meeting the Italian microbial standard(2 CFU/100ml of total coliforms) previously foundto be 100 and 160 mWs/cm2 for feed F and CLrespectively. On the contrary, the max dose achiev-able in the conditions investigated (430 mWs/cm2)was not enough to meet the standard with feed II,even if the total coliform value achieved (5 CFU/100 ml) was very close to the target. Furtherdetails on pilot plant, feed characteristics and mainresults of the previous part of the UV investigationmay be found elsewhere [12].

Fig. 1. Pilot plant configuration during UV disinfection experiments (MF, multilayer pressure filter; RV, 5 m3 fibre-glassvessel; UVA ,non contact Teflon UV apparatus; UVCP,UV control panel; P, pumps; FM, flow meter; V, valve).

P1

P2

UVA

UVCP

II CL

MF RV

FMP3

V

2

1

3

4

For both feeds F and CL, about thirty cycles(i.e. a given feed submitted to a given dose) werereplicated in the same conditions in order to checkfor selected pathogenic parasites (Nematodeseggs, Giardia lamblia cysts and Cryptosporidiumparvum oocysts) before and after disinfection aswell as for potential DBP formation (i.e., nitro-phenols and N-nitroso-amines).

Analytical procedures were according to Stan-dard Methods [13] except as specified below:• Giardia lamblia cysts and Cryptosporidium

parvum oocysts: the method (Standard MethodNo. 9711 B as modified by Portincasa [14]) in-volves pressure (4 atm) tangential ultrafiltrationof a 10-l sample through 142 mm diameter(1.2 µm porosity) cellulose acetate membranes.The membranes were eluted with 0.1% Tween 80solution using magnetic stirring and the eluatewas centrifuged at 1500 rpm using plastic tubes.The centrate was purified by Percoll-sucrosegradient and identified by microscopy usingimmunofluorescent monoclonal antibodies.

• Nematodes eggs: the method (not standardizedyet) involves the filtration of a 2-l sample,membrane elution and centrifugation as for

318 L. Liberti et al. / Desalination 152 (2002) 315–324

Giardia and Cryptosporidium. The centratewas identified by phase- and differential-inter-ference-contrast optic microscopy.

• Nitro-phenols: liquid-liquid extraction GC/MSmethod (Standard Method No. 6410 B) modifiedfor extraction and concentration procedures:a 2-l sample, previously filtered on 0.45-mmcellulose nitrate membrane, was acidified atpH 2 with sulphuric acid (1:1) and extractedby vacuum filtration on a previously con-ditionated polystyrene-divinil-benzene (SDVB)membrane. The membrane was eluted withmethylene-chloride, the eluate was concen-trated (up to 1 ml) under vacuum and analysedby GC/MS. Control of extraction and concen-tration procedures was obtained with blanksand standard solutions.

N-nitroso-amines: liquid–liquid extraction GC/MSmethod (Standard Method No. 6410 B) modifiedfor extraction and concentration procedures:a 2-l sample was alkalinised to pH 11 with 10NNaOH, filtered on 0.45-µm cellulose nitratemembrane and extracted by vacuum filtrationon a previously conditionated C18 membrane.The membrane was eluted with methylene-chloride, the eluate was concentrated (up to1 ml) under vacuum and analyzed by GC/MS.Control of extraction and concentration pro-cedures was obtained with blanks and standardsolutions.

The following analytical instruments were used:• tangential ultrafiltration apparatus mod. Sarto-

con 2 and mod. Sartocon Mini by Sartorius;• optic microscope (direct light, phase- and dif-

ferential-interference-contrast) mod. AxioskopMC 80 by Zeiss;

• fluorescence microscope mod. BH2 by Olym-pus;

• gas chromatograph/mass spectrometer (GC/MS) mod. Saturn 3 by Varian with purge andtrap autosampler mod. 3000 by Tekmar;

• vacuum concentration apparatus mod. AES1000 by Savant;

• liquid chromatograph/mass spectrometer (LC/MS) mod. API 300 by Perkin Elmer.

3. Results and discussion

3.1. Parasite removal performance

As said, several cycles were run at UV dose of100 and 160 mWs/cm2 for F and CL feed res-pectively to evaluate UV effectiveness towardsselected pathogenic parasites (Giardia lambliacysts, Cryptosporidium parvum ocysts andNematodes eggs) likely to occur in local waste-water and are reportedly resistant to chemicaldisinfectants.

The occurrence of these pathogens in CL andF feed before and after UV disinfection is reportedin Table 1 (average values).

Nematodes eggs were never found in the feedsadmitted to disinfection, confirming the effective-ness of clarification and filtration steps in removingconsistently such large and heavy parasites [15],but not towards smaller ones like Giardia cystsand Cryptosporidium oocysts detected in appreci-able number in feed CL and removed only in partby filtration, as expected [16]. In these conditionsUV radiation was rather effective in both feedstowards both protozoan parasites with an averageremoval around 60 and 65% for Giardia andCryptosporidium respectively.

Table 1Selected parasites before and after UV disinfection ofclarified (CL) and clarified-filtered (F) feeds (UV dose: 160and 100 mWs/cm2 respectively)

Parasite Feed In Out % removal

Nematodes

eggs (N/l)

CL

F

0

0

0

0

—

—

Giardia lamblia

cysts (N/l)

CL

F

345

114

156

44

55

62

Cryptosporidium

Parvum oocysts (N/l)

CL

F

23

6

8

2

65

67

L. Liberti et al. / Desalination 152 (2002) 315–324 319

These results came not unexpected as protozoancysts are resistant species, requiring higher UVdoses than bacteria and viruses [17]. Accordingto literature data, UV doses lower or similar tothose investigated (around 60, 80 and 120–180 mWs/cm2) are claimed to produce 80, 90 and99% inactivation of Giardia cysts [18–20] andCryptosporidium oocysts [21,22] in drinkingwater. The multiple barrier concept involvingclarification and filtration (plus eventually GACadsorption) prior to UV disinfection undoubtedlyappears the most effective approach for completeparasite removal in water and wastewater treat-ment [20].

It must be pointed out that the proved occurrenceof some protozoa in the disinfected effluents doesnot cause restriction for their reuse in agriculture.In fact, according to the WHO guidelines [23], theonly parasites of concern are intestinal nematodes(MAC ≤ 1 egg/l), never found during this investi-gation. However, these latter are intended to serveas indicator organisms for all parasitic pathogensand it is implied that all helminth eggs and protozoancysts should be removed to the same extent toavoid the risk of waterborne disease outbreaks.

3.2. Disinfection by-products formation

As for eventual formation of UV disinfectionby-products during drinking water production, itis generally assumed that moderate ultravioletirradiation (<50 mWs/cm2) affects the structureof organic compounds to a much lesser extent thanchemical reagents like chlorine or ozone [17]. How-ever, some results dealing with degradation of pesti-cides and other specific compounds in industrialwastewater under high UV doses (>1000 mWs/cm2) indicated the need for further informationabout UV promoted transformation of organiccompounds in municipal wastewaters [24,25].

On the basis of photochemistry fundamentals,in fact, it cannot be excluded that UV irradiationof wastewater could affect the identity of the organicsubstances through either direct or indirect

interaction forming potentially toxic by-products[26, 27]. In the former case, a molecule known asa chromophore may be chemically modified as aresult of direct radiation absorption. Indirectphotolysis may occur when UV radiation acts ona species known as a photosensitiser whichstrongly absorbs the radiation energy and theresulting highly energetic species interacts withanother molecule producing a chemical trans-formation [28,29].

Considering that amino- and phenolic-deriva-tives (chromophores) as well as nitrate/nitrite ionsand humic materials (photosensitisers) are speciescommonly occurring in municipal wastewater andpotentially capable of reacting [30], nitro-phenolsand N-nitroso-amines were specifically searchedfor in this investigation as possible DBPs followingUV irradiation.

Figs. 2–5 show the corresponding GC/MSchromatograms of F and CL feeds before and afterUV disinfection (doses of 100 and 160 mWs/cm2

respectively).The nearly total overlapping of all spectra

seems to exclude the formation of both harmfulN-derivatives above the instrumental detection limit(0.01 ppb) in the conditions investigated.

A possible explanation could be the formationof non-volatile DBPs (NVDBP) which are not de-tected by GC/MS but only by LC/MS (or MS/MS)analytical technique. A special experiment wascarefully planned to check this hypothesis at labscale. In particular, in order to maximize the pro-bability of detecting the eventual NVDBP formation,a 500-ml sample of CL feed was irradiated in a glass-batch-UV reactor using purposely an extremelyhigh UV dose (25,000 mWs/cm2). Samples beforeand after the irradiation were concentrated 25times by lyophilization and post-column injectedinto a LC-MS spectrometer. The obtained massspectra reported in Fig. 6, once again, do not showany significant difference between irradiated andnon-irradiated samples, excluding the eventualformation of NVDBPs even under the extremeirradiation adopted during the experiment.

320 L. Liberti et al. / Desalination 152 (2002) 315–324

Fig. 2. Nitro-phenols search: GC/MS spectra of F feed before and after UV disinfection (100 mWs/cm2).

Fig. 3. N-nitroso-amines search: GC/MS spectra of F feed before and after UV disinfection (100 mWs/cm2).

Fig. 4. Nitro-phenols search: GC/MS spectra of CL feed before and after UV disinfection (160 mWs/cm2).

L. Liberti et al. / Desalination 152 (2002) 315–324 321

Fig. 5. N-nitroso-amines search: GC/MS spectra of CL feed before and after UV disinfection (160 mWs/cm2).

The above evidences are in good agreementwith the conclusions reported for similar investi-gation stating that UV-promoted transformationof chemical compounds, at least in clean waters,requires much higher doses than necessary formunicipal wastewater disinfection [31–35].

Fig. 6. Post injection LC/MS spectra of lyophilizated samples of CL feed before and after UV irradiation (25,000 mWs/cm2).

3.3. Cost estimates

To assess the economic feasibility of UV disin-fection treatment for wastewater reuse in agriculture,operation and maintenance (O&M) costs werepreliminarily estimated on the basis of the experi-

322 L. Liberti et al. / Desalination 152 (2002) 315–324

mental results obtained by reference to the optimalUV dose for each feed that permitted the achieve-ment of the target total coliforms standard of2 CFU/100ml, i.e., 100 and 160 mWs/cm2 for F andCL feeds respectively.

The following assumptions were made:• O&M costs account essentially for electric

power consumption and lamp replacement,including also maintenance requirements andmiscellaneous equipment repair costs

• power consumption of UV equipments is3.1 kWh

• average electricity cost is € 0.065/kWh• UV lamp (€ 45 each) replacement is based on

8760 h of use.

As shown in Table 2, O&M costs of UV ad-vanced disinfection of the intermediate (clarified,CL) and the full tertiary treated feed (clarified-filtered, F) amounted to € 35 and € 17.5/1000 m3

respectively. The above estimates do not includecapital costs and can be influenced by variablessuch as feed quality, plant configuration, plant size(scale factor) and market situation.

Cost effectiveness of UV advanced disinfection,in particular of F feed, is evident considering thatchlorination of municipal wastewater just for seadischarge in compliance with Italian regulations(20,000 CFU/100 ml for total coliforms) at WestBari treatment plant costs approximately € 5/1000 m3.

4. Conclusions

The experimental results of a 9-month pilot-scale (100 m3/h) investigation, carried out at theWest Bari (S. Italy) municipal wastewater treat-

Table 2Cost estimates for UV disinfection of CL and F feeds at West Bari pilot plant

O&M costs, €/1000 m3 Feed UV dose,

mWs/cm2

Total coliforms target achieved,

CFU/100 ml Electric power Lamp replacement Total

F 100 1 6.7 10.6 17.3

CL 160 1 13.5 21.3 34.8

ment plant, focused on parasite removal anddisinfection by-product (DBP) formation duringthe UV disinfection of clarified (CL) and clarified-filtered (F) secondary effluents at doses (160 and100 mWs/cm2 respectively) necessary for achievingthe total coliforms standard of 2 CFU/100 ml forunrestricted reuse of wastewater in agriculture,provided the following indications:

1. At the above doses, UV radiation was rathereffective in both feeds towards protozoan parasiteslike Giardia lamblia cysts and Cryptosporidiumparvum oocysts (approx. 60 and 65% removalrespectively); Nematodes eggs were never foundin the feeds admitted to disinfection because alreadyremoved by clarification and sedimentation;

2. The multiple barrier concept involvingclarification and filtration prior to UV disinfectionwas confirmed to be the most effective approachfor complete parasite removal in wastewater treat-ment;

3. None of the N-derivatives (i.e. nitro-phenolsand N-nitroso-amines) searched for after UV dis-infection of both feeds was detected by GC/MSanalytical technique whilst LC/MS analysesexcluded also the formation of non-volatile DBPs,suggesting the absence of detectable photo-chemical reactions at UV doses usually used inwastewater disinfection;

4. O&M costs of UV disinfection averaged€ 17.5 and € 35/1000 m3 for F and CL feed res-pectively.

Further investigation is planned to assess UVeffectiveness towards viruses, to exclude UVpromotion of other DBPs, to prevent effluentrecontamination (bacteria photoreactivation bycell repair and regrowth) and to evaluate possible

L. Liberti et al. / Desalination 152 (2002) 315–324 323

synergy and/or catalytic effects of UV radiation withlong-acting chemical disinfectants such as H2O2.

References[1] State of California. Wastewater Reclamation Criteria.

California Administrative Code, Title 22, Division 4,California Department of Health Services, SanitaryEngineering Section, Berkeley, CA, 1978.

[2] T. Asano, Wastewater Reclamation and Reuse. Tech-nomic, Lancaster, 1998.

[3] A. Prost, Health risks stemming from wastewaterreutilization, Wat. Quality Bull., 12 (1987) 73.

[4] H.V. Smith, L.G. Robertson and J.E. Ongherth,Cryptosporidiosis and Giardiasis: the impact of water-borne transmission, J. Water SRT-AQUA, 44(4)(1995) 258.

[5] J.J. Rook, Formation of haloforms during chlorinationof natural waters, Wat. Treat. Examin., 23 (1974) 234.

[6] R.A. Minear and G.L. Amy, Disinfection By-Productsin Water Treatment, Lewis Publishers, New York,1996.

[7] V. Lazarova, P. Savoye, M.L. Janex, E.R. BlatchleyIII and M. Pommepuy, Advanced wastewater dis-infection technologies: state of the art and per-spectives, Wat. Sci. Technol., 40(4–5) (1999) 203.

[8] L. Liberti and M. Notarnicola, Advanced treatmentand disinfection for municipal wastewater reuse inagriculture, Wat. Sci. Technol., 40(4–5) (1999) 235.

[9] L. Liberti, A. Lopez and M. Notarnicola, Disinfectionwith peracetic acid for domestic sewage re-use in agri-culture, J. Ch. Instn. Wat. Envir. Mangt., 13(4) (1999)262.

[10] L. Liberti, M. Notarnicola and A. Lopez, Advancedtreatment for municipal wastewater reuse in agri-culture. III – Ozone disinfection, Ozone Sci. Eng.,22(2) (2000) 151.

[11] L. Liberti, A. Lopez, M. Notarnicola, N. Barnea, R.Pedahzur and B. Fattal, Comparison of advanced dis-infecting methods for municipal wastewater reuse inagriculture, Wat. Sci. Technol., 42(1–2) (2000) 215.

[12] L. Liberti, M. Notarnicola, A. Lopez and G. Boghetich,Advanced treatment for municipal wastewater reusein agriculture. UV disinfection: bacteria inactivation,J. Water SRT-AQUA, 50(5) (2001) 275.

[13] Standard Methods for the Examination of Water andWastewater. 19th ed., American Public Health Asso-ciation/American Water Works Association/WaterEnvironment Federation, Washington, DC, 1995.

[14] F. Portincasa, G. Torchietti, F. Donadio, D. Carnimeo,M.A. Panaro, S. Lisi, F. Maddalena and O. Brando-nisio, Method for determination of Giardia cysts andCryptosporidium oocysts in water, Igiene moderna,107 (1997) 543 (in Italian).

[15] D.C. Watson, M. Satchwell and C.E. Jones, A studyof the prevalence of parasitic helminths eggs and cystsin sewage sludge disposed of to agricultural land, Wat.Poll. Control, 82 (1983) 285.

[16] Z. Bukhari, H.V. Smith, N. Sykes, S.W. Humphreys,C.A. Paton, R.W.A. Girdwood and C.R. Fricker,Occurrence of Cryptosporidium spp oocysts andGiardia spp cysts in sewage influents and effluentsfrom treatment plants in England, Wat. Sci. Technol.,35 (11–12) (1997) 385.

[17] R.L. Wolfe, Ultraviolet disinfection of potable water,Environ. Sci. Technol., 24(6) (1990) 768.

[18] E.W. Rice and J.C. Hoff, Inactivation of Giardialamblia cysts by ultraviolet irradiation, Appl. Environ.Microbiol., 42 (1981) 546.

[19] D.A. Carlson, R.W. Seabloom, F.B. DeWalle, T.F.Wetzler, J. Engeset, R. Butler, S. Wangsuphachart andS. Wang, Ultraviolet disinfection of water for smallwater supplies. Project summary. EPA/600/S2-85/092,Office of Research and Development, US Environ-mental Protection Agency, Cincinnati, OH, 1985.

[20] P. Karanis, W.A. Maier, H.M. Seitz and D. Schoenen,UV sensitivity of protozoan parasites, J. Water SRT-AQUA, 41(2) (1992) 95.

[21] M.E. Ransome, T.N. Whitmore and E.G. Carrington,Effect of disinfectants on the viability of Crypto-sporidium parvum, Wat. Suppl., 11 (1993) 75.

[22] A.T. Campbell, L.J. Robertson, M.R. Snowball andH.V. Smith, Inactivation of oocysts of Crypto-sporidium parvum by ultraviolet irradiation, Wat. Res.,29(11) (1995) 2583.

[23] WHO. Health guidelines for the use of wastewater inagriculture and aquaculture. Technical Report Series778, World Health Organization, Geneva, Switzerland,1989.

[24] K. Nick, H.F. Scholer, G. Mark, T. Soylemez, M.S.Akhlaq, H.-P. Schuchmann and von C. Sonntag,Degradation of some triazine herbicides by UV radia-tion such as used in the UV disinfection of drinkingwater, J. Water SRT-AQUA, 41(2) (1992) 82.

[25] M. Otaki and S. Ohgaki, Photochemical decom-position of organo-chlorine compounds by a mediumpressure UV lamp. Proc. Water Quality Interna-tional’94, IAWQ 17th Biennal International Con-ference, Budapest, Hungary, 1994, p. 110.

324 L. Liberti et al. / Desalination 152 (2002) 315–324

[26] B. Zoeteman, J. Hrubec, E. Greef and H.J. Kool,Mutagenic activity associated with by-products ofdrinking water disinfection by chlorine, chlorinedioxide, ozone and UV irradiation, Environ. HealthPerspectives, 46 (1982) 197.

[27] C. von Sonntag and H.-P. Schuchmann, UV disin-fection of drinking water and by-product formation— some basic considerations, J. Water SRT-AQUA,41(2) (1992) 67.

[28] R.W. Matthews, Environment: photochemical andphotocatalytic processes. Degradation of organiccompounds. In: Photochemical Conversion andStorage of Solar Energy. E. Pelizzetti and M. Schia-vello, Eds., Kluwer Academic Publishers, The Nether-lands, 1991.

[29] J. Kopecky, Organic Photochemistry: A VisualApproach. VCH Publishers, New York, 1992.

[30] R.P. Schwarzenbach, P.M. Gschwend and D.M.Imboden, Environmental Organic Chemistry. JohnWiley & Sons, New York, 1993.

[31] E. Lee, R.L. Jolley, M.S. Denton and J.E. Thompson,Ultraviolet irradiation of municipal wastewater:evaluation of effects on organic constituents, Environ-ment International, 7 (1982) 403.

[32] J. Maarschalkerweerd, R. Murphy and G. Sakamoto,Ultraviolet disinfection in municipal wastewatertreatment plants, Wat. Sci. Technol., 22(7–8) (1990)145.

[33] J.A. Oppenheimer, J.E. Hoagland, J.-M. Laîné, G.Jacangelo and A. Bhamrah, Microbial inactivation andcharacterization of toxicity and by-products occurringin reclaimed wastewater disinfected with UV radia-tion, WEF Specialty Conference Series: Planning,Design and Operation of Effluent Disinfection Sys-tems, Water Environment Federation, Alexandria, VA.,1993, p. 13.

[34] Elsinore Valley Municipal Water District and NationalWater Research Institute. A Comparative Study of UVand Chlorine Disinfection for Wastewater Reclama-tion. Executive Summary. Montgomery Watson,Pasadena, CA, 1994.

[35] K.G. Linden, G.S. Soriano and J.L. Darby, Investi-gation of disinfection by-product formation followinglow and medium pressure UV radiation of wastewater.Proc. Disinfection’98 Specialty Conference, WaterEnvironment Federation, Baltimore, Maryland, 1998,p. 137.