Desalination 227 (2008) 167–177

Treatment of a combined dairy and domestic wastewater in anup-flow anaerobic sludge blanket (UASB) reactor followed by

activated sludge (AS system)

A. Tawfika*, M. Sobheyb, M. Badawya

aWater Pollution Control Department, National Research Center, El-tahrir St., Dokki, PO Box 12622, Giza, EgyptTel/Fax.: +20 (2) 335-1573; email: Tawfik8@yahoo.com

bDepartment of Sanitary Engineering, Ain Shams University, Cairo, Egypt

Received 27 December 2006; Accepted 19 June 2007

Abstract

The feasibility of using an up-flow anaerobic sludge blanket (UASB) reactor followed by activated sludge (ASsystem) for the treatment of wastewater discharged from dairy factory was explored. The UASB reactor was operatedat a hydraulic retention time (HRT) of 24 h and organic loading rates (OLRs) ranging from 1.9 to 4.4 kgCOD/m3.d.The average total chemical oxygen demand (COD total) and total biological oxygen demand (BOD5total) concentrationsof the UASB reactor effluent were 1385 and 576 mg/l, corresponding to percentage removal of 69% and 79%,respectively. Total suspended solids (TSS) and volatile suspended solids (VSS) removal averaged above 72% and75%, respectively. Residual phosphorous and oil and grease concentrations of the UASB reactor effluent were 8.2and 44 mg/l, corresponding to percentage removal values of 63% and 83%, respectively. This good performancecould be attributed to the relatively long sludge residence time (SRT =76 d) imposed to the reactor. Total and faecalcoliform counts were reduced in the treated effluent by a value of 1.07 and 0.9 log10, respectively. The net sludgeyield coefficient was found to be 0.2 g VSS per g CODtotal removed per day, corresponding to 20% of the totalinfluent COD imposed to the UASB reactor. The volatile solids / total solids (VS/TS) ratio of 0.66 of excess sludgerevealed its good quality. Preliminary batch experiments of the AS system treating UASB reactor effluent indicatedfirst-order removal kinetics between total organic carbon (TOC) and contact time. The TOC removal reached 80%,resulting in only 47 mg/l in the final effluent at a HRT of 2.0 h. Accordingly, the AS system was operated at a HRTof 2.0 h. The system achieved a substantial reduction of CODtotal, BOD5 total, TSS and oil and grease resulting effluentquality with residual values of only 35.0, 7.0, 14.0 and 2.8 mg/l, respectively. The geometric mean of total and faecalcoliform counts was reduced by a value of 1.28 and 1.64 log10, respectively. Based on these results, it isrecommended to use of an integrated system consisting of a UASB reactor followed by the AS system for thetreatment of a combined dairy and domestic wastewater to produce a good effluent quality complying with thestandards for discharge into agricultural drains.

Keywords: Dairy wastewater; Domestic wastewater; UASB; Biodegradability; Sludge; Activated sludge; Batchexperiments

*Corresponding author.

0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.doi:10.1016/j.desal.2007.06.023

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1. Introduction

The dairy industry is one of the major sourcesof water pollution in Egypt, which generatesstrong wastewaters characterized by high BOD5and COD values representing their high organiccontent [1]. These effluents may cause seriousproblems in terms of organic load on the localmunicipal wastewater treatment plant [2]. There-fore, appropriate treatment is required prior todisposal into sewer network or receiving waterbodies.

Most of the wastewater produced in the dairyindustry results from cleaning of transport linesand equipment between production cycles, clean-ing of tank trucks, washing of milk silos andequipment malfunctions or operational errors[3]. Dairy wastewater has been extensivelytreated using coagulation/flocculation and a sedi-mentation process. The main disadvantages ofthis method are due to the high coagulant cost,high sludge production, and the poor removal ofCODdissolved. Therefore, biological treatment isusually recommended [4]. Dairy wastewater isgenerally characterized by its relatively hightemperature and variation in organic content.Therefore, anaerobic treatment is considered theappropriate choice [5–7].

Recently, an up-flow anaerobic sludge blanket(UASB) reactor has been successfully employedfor treatment of dairy wastewater in full-scaleapplications [8]. The use of a laboratory-scalehybrid UASB reactor for treatment of dairywastewater at an operational temperature of 30ECwas previously investigated [9]. It was found thatCODtotal removal varied between 85% and 88% ata HRT of 5.0 h and organic loading rates (OLRs)of 8.5 g CODtotal/l.d. Another laboratory-scaleinvestigation pointed out that more than 97%CODtotal reduction could be achieved using anUASB reactor treating cheese whey wastewater[10]. At a high OLR of 31 g CODtotal/l.d, theUASB reactor treating cheese production waste-water provided percentage removal of 90% for

CODtotal [11]. However, the CODtotal and BOD5 totalconcentrations in the effluent of the anaerobictreatment step generally do not comply withstandards for discharge into receiving waterbodies. Therefore, post-treatment is required.

A laboratory-scale sequencing batch reactor(SBR) treating the effluent of anaerobic hybridreactor treating cheese whey wastewater has beenwas investigated [12]. The integrated systemachieved more than 90% removal for bothCODtotal and nutrients (N&P). Full-scale anaero-bic/aerobic treatment of cheese wastewater by asystem consisting of grease trap, UASB reactor,aerated pond and an effluent polishing pond wasalso investigated [13]. The removal values ofBOD5total, CODtotal, TSS, and oil and grease up to98%, 96%, 98% and 99.8% were respectivelyachieved.

The aim of the present study is toC evaluate the effectiveness of a system con-

sisting of a UASB–AS system for treatment ofcombined dairy and domestic wastewater(DDWW). Emphasis will be accorded to theremoval efficiency of the CODtotal, BOD5 total,F. coliform and for TSS. An important aspectin this study is to assess the ability of thecombined system to produce wastewater suit-able for safe discharge into agricultural drain;

C determine the anaerobic biodegradability ofthe DDWW.

C estimate excess sludge production in a UASBreactor and determine its characteristics.

2. Materials and methods

2.1. Characterization of wastewater

The end of pipe effluent of the Arab DairyFactory (Banha, Kalubia, Egypt) consists ofmixtures of 350 m3/d (dairy wastewater) and150 m3/d (domestic wastewater). The main char-acteristics of the DDWW, i.e. the feed of theintegrated system on these experiments, are givenin Table 1.

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Table 1Mean characteristics of the end of pipe effluent of theArab Dairy Factory

pHTOC, mg/l: Total SolubleTotal COD, mg/lBOD5, mg/l: Total SolubleTSS, mg/lVSS, mg/lTKN, mg/lTP, mg/lOil and grease, mg/lTotal coliform/100 ml

7.9 ± 1.2

1379 ± 134961 ± 983383 ± 1345

1941± 864517 ± 123831 ± 392746 ± 21451 ± 5.722 ± 4.8263 ± 235.5×107 ± 2.3×104

2.2. Laboratory-scale UASB reactor and ope-rational conditions

The experiments were carried out in alaboratory-scale UASB reactor (Fig. 1). TheUASB reactor (5.0 l) consists of a cylindricalcolumn with a conical shaped bottom and gassolid separator (GSS). The UASB reactor has aheight of 70 cm, and an internal diameter of10 cm. Ports for obtaining sludge samples arearranged along the reactor height, the first one at5.0 cm above the base of the reactor and theothers at 15, 25, 40 and 55 cm. The UASBreactor was inoculated with primary digestedsludge from the wastewater treatment plant ofZeneen, Cairo (Table 2). The total amount ofdigested sludge added to the reactor wasapproximately 3.0 l, which represents 60% of thetotal reactor volume. Inoculum pH was 6.8.

Based on the results of previous studies[14,15], the UASB reactor in the present studywas operated at a constant HRT of 24 h, flow rateof 5.0 l/d, temperature of 20EC and OLR of3.4 kg COD/m3.d.

The sludge bed of the UASB reactor was keptbelow tap 5.0, ca. 55 cm from the bottom of thereactor, by once weekly opening this tap for

Table 2Characteristics of the inoculum sludge used for UASBreactor treating DDWW

Parameters Unit Values

Sludge volume (SV) ml/l 980Total solids at 105EC (TS) g/l 22.6Volatile solids at 550EC (VS) g/l 13.1VS/TS 0.57Sludge volume index (SVI) ml/gTS 43.4

Fig. 1. Schematic diagram of the UASB reactor treatingDDWW.

discharging the sludge accumulated above. Thewasted sludge from the upper part of the UASBreactor is better stabilized [16]. The excess sludgewithdrawn from tap 5.0 was analyzed for totalsolids, volatile solids and sludge volume index(SVI) [17].

For calculating the SRT of the UASB reactor,it is assumed that the effluent VSS had the sameSRT as the excess sludge. The SRT of the UASB

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reactor was calculated according to Eq. (1).

(1)*

* *w w e

V XSRTQ X Q X

⎛ ⎞= ⎜ ⎟+⎝ ⎠

where V is the reactor volume (l), X the averagesludge concentration in the reactor (mgVSS/l),Qw the excess sludge (l/d), Xw the concentration ofthe excess sludge (mgVSS/l), Q the wastewaterflow rate (l/d), Xe the effluent concentration(mgVSS/l) and, according to de Zeeuw [18], Xe isthe CODss/1.4.

Anaerobic biodegradability test: Batch anaero-bic digestion experiments using two serum bottleswith a capacity of 500 ml were used to assess theanaerobic biodegradability of the DDWW. Thefirst bottle was supplied with 450 ml DDWW(BOD5 total = 812 mg/l and CODtotal = 1400 mg/l).The second bottle was supplied with 450 ml ofsterile membrane fil-tered wastewater (0.45 µm,Whatman, England) (BOD5dissolved = 656 mg/l andCODdissolved = 787 mg/l). The bottles wereincubated at 30EC for 60 days. The headspaces ofthe bottles were flushed with nitrogen gas and thebottles sealed with butyl rubber stoppers andaluminum crimps. The volume of biogas in theheadspace of each bottle was regularly measuredusing a Marriott displacement set-up filled with a5.0% NaOH solution. The biodegradability wasmeasured once in triplicates. The biodegradabilityof the DDWW was calculated according to Eqs.(2) and (3).

Biodegradability (%) = COD-CH4/Influent CODtotal *100 (2)

Biodegradability (%) = COD-CH4/Influent CODdissolved *100 (3)

2.3. Batch aerobic experiments

Two batch aerobic experimental runs wereconducted. The first one was carried out to deter-

mine the optimum HRT of the AS system. Theobjective of the second run was to study theperformance of the AS system treating UASBreactor effluent at optimum HRT. Both experi-mental runs were carried out using a laboratorycolumn of 2 l capacity. Oxygen was supplied tothe reactor by using spargers to promote forma-tion of small air bubbles. Dissolved oxygen waskept constant at 2.0 mg/l. The reactor was ope-rated at room temperature (17–22EC) and withoutpH control, which varied between 7.0 and 7.5.The used biomass (3.6 gVSS/l and SVI of62.5 ml/gTSS) was taken from a nearby full-scaleAS plant treating domestic wastewater (Zeneen,Cairo).

2.3.1. Strategy of operationThe system was initially inoculated with 1.0 l

biomass. From the first to the fifth day, thereactor was fed only with domestic wastewater.On day 6 the domestic wastewater was replacedstepwise with a fraction of the effluent of UASBreactor treating DDWW. After day 12 it was fedexclusively with the effluent from the UASBreactor.

2.3.2. First experimental runThe activated sludge column was fed at t = 0

with 1.0 l of UASB effluent. One hundred ml ofthe mixed liquor were taken from the column att = 0, 0.5, 1, 2, 3, and 4 h and put into a graduatedcylinder. The mixed liquor was allowed to settlefor 1.0 h, and then the supernatant was withdrawnand analyzed for determination of TOC concen-tration. The experiment was repeated four times.

2.3.3. Second experimental runThese experiments were carried out at a con-

stant reaction time of 2.0 h. CODtotal, BOD5 total,TSS, oil and grease, total and faecal coliformconcentrations were measured in the effluent ofthe AS system. The experiment was repeated sixtimes.

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2.4. Sampling and analytical methods

Grab samples of the influent and the effluentsof the UASB reactor and activated sludge systemwere analyzed for pH, 5-day biological oxygendemand (BOD5), COD, TSS, VSS, and totalKjeldahl nitrogen (TKN) according to the APHA[17]. The filtrate of the 0.45 µm sterile membranefilter (Whatman, England) was used to determinethe dissolved BOD5, COD and TOC. Total andfaecal coliform count was determined accordingto multiple tube fermentation technique [17]. Dueto a lack of facilities, volatile fatty acids and gascomposition were not measured.

3. Results3.1. Efficiency of UASB reactor

The UASB reactor was operated at a constantHRT of 24 h throughout the study, while OLRsvaried from 1.9 to 4.4 kgCODtotal/m3.d due tochanges in influent composition. Despite varia-tion in OLR, the reactor provided a more or lessconstant effluent quality around 1385 mg/l forCODtotal, and 576 mg/l for BOD5total, corres-ponding to the percentage removal values of 69%and 79% respectively (Fig. 2). This relativelygood performance could be attributed to the longSRT (76 d) which would effectively increase the

efficiency of hydrolysis and subsequent digestionof organic matter., The results clearly show thatthe UASB reactor achieved a substantial reduc-tion of TSS and VSS, resulting in an averagepercentage removal of 72% and 75% respectively(Fig. 3). This indicates that the high efficiency ofthe UASB reactor for removing of SS at a rela-tively high suspended solids loading rate of0.83 kgTSS/m3.d.

The results presented in Table 3 show that theresidual phosphorous and oil an grease in thetreated effluent were 8.2 and 44 mg/l, corre-sponding to average percentage removal of 63%and 83%, respectively. Apparently, the removalof phosphorous was due to precipitation [19],while the removal of oil and grease was due toentrapment/ adsorption to the sludge bed of theUASB reactor.

Bacteriological examination showed that theUASB reactor achieved 1.07 and 0.9 log10 reduc-tion for total and faecal coliform, correspondingto percentage removal values of 91.6 and 87.5%respectively (Table 3). Residual counts of totaland faecal coliform were 4.6×106 and 0.4×106

MPN/100ml, respectively. The partial removal oftotal and faecal coliform in the UASB reactor canbe attributed to physical process of settling totalcoliform attached to suspended solids especiallyat a relatively long SRT imposed to the reactor.

Fig. 2. CODtotal and BOD5 total concentrations in the effluent of the UASB reactor treating DDWW.

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Fig. 3. TSS and VSS concentrations in the effluent of the UASB reactor treating DDWW.

Table 3Phosphorous, oil and grease, total and faecal coliform removal in an UASB reactor treating DDWW

Parameters/Samples Unit Influent UASB effluent %R

Phosphorous mg/l 22 ± 11 8.2 ± 2.3 63.0 ± 12Oil and Grease mg/l 263 ± 43 44 ± 6.9 83.0 ± 8.7Total coliform (Log10) MPN/100ml 7.74 ± 4.36 6.66 ± 4.1 1.07a ± 0.5Faecal coliform (Log10) MPN/100ml 6.5 ± 4.3 5.6 ± 3.8 0.9a ± 0.4

aLog10 reduction.

3.2. Retained and excess sludge

The characteristics of the retained and excesssludge from the UASB reactor are presented inTable 4. The VS/TS ratio of 0.66 indicates thatthe wasted sludge was almost stabilized [20]. Themean value of the net sludge yield coefficient wasfound to be 0.2 g VSS per g CODtotal removed perday, corresponding to 20% of the total influentCODtotal. This is a very important feature of theUASB reactor since it is significantly smallerthan that normally found in conventional aerobicsystems [21]. Sludge production in the UASBreactor may be attributed to (1) flocculation ofnon-biodegradable particulate matter, forming the

inert sludge mass fraction and (2) the biologicalsludge mass that is generated as a result of anae-robic conversion in the UASB reactor [22].

3.3. BiodegradabilityThe biodegradability of DDWW at a tempera-

ture of 30EC was 66±12%, while that of themembrane filtered fraction was relatively lower(58±11%). This can be attributed to the appliedshort test period for achieving a stabilization oforganic matter. [23] found that the biodegrada-bility of domestic wastewater was 21% after43 days and increased to 74% after 135 days at30EC.

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Table 4Characteristics of the retained and excess sludge from the UASB reactor treating DDWW

Parameters/samples Unit Retained sludge Excess sludge

Sludge volume (SV) ml/l 980 ± 142 55 ± 23Total solids at 105EC (TS) g/l 28 ± 12 0.6 ± 0.2Volatile solids at 550EC (VS) g/l 18 ± 9 0.4 ± 0.15VS/TS 0.64 ± 0.18 0.66 ± 0.2Sludge volume index (SVI) ml/gTS 35 ± 12 91.6 ± 23Sludge production gVSS/d — 2.0 ± 0.8Sludge yield coefficient gVSS/gCOD removed. d 0.2Sludge residence time (SRT) d — 76

Table 5Performance of the activated sludge system treating UASB reactor effluent at a HRT of 2.0 h

Parameters/samples Unit UASB effluent AS effluent %R

CODtotal mg/l 1385 ± 367 35.0 ± 7.0 97.4 ± 1.3BOD5 total mg/l 576 ± 212 7.0 ± 3.0 98.8 ± 0.9TSS mg/l 300 ± 123 14 ± 5.0 95.3 ± 2.3Oil and grease mg/l 263 ± 43 2.8 ± 1.1 93.6 ± 0.9Total coliform (Log10) /100 ml 6.66 ± 4.1 5.38 ± 4.1 1.28a ± 0.4Faecal coliform (Log10) /100 ml 5.6 ± 3.8 3.95 ± 3.2 1.64a ± 0.44

aLog10 reduction.

3.2. Batch aerobic experiments (post-treatment)The effluent quality of the anaerobic step does

not meet discharge standards into agriculturaldrain. Therefore, the AS system has been inves-tigated as a post-treatment for UASB effluent.Preliminary experiments indicated first-orderremoval kinetics between TOC and contact time.Results of the residual TOC as a function ofretention time in the activated sludge column arepresented (one of the experiments) in Fig. 4. Theremoval of TOC can be described by anexponential equation:

TOCin = TOCout e!kt (4)

where TOCin is the influent TOC (mg/l), TOCoutthe effluent TOC (mg/l), k the first-order removal

constant (h), and t is the time or detentiontime (h).

The measured values for TOC are presented inFig. 4. As expected, the TOC dropped rapidly inthe activated sludge column fed with the effluentof UASB reactor. The TOC removal efficiencyreached to 80% within 2.0 h, resulting only47 mg/l in the treated effluent.

The CODtotal, BOD5 total and TSS removal datafound in the AS system operated at a HRT of2.0 h are given in Table 5. The results clearlyshow that the AS process achieved a substantialreduction of COD total, resulting in an averageeffluent concentration of only 35 mg/l. This indi-cates the high efficiency of the AS system treat-ing UASB reactor effluent for removing CODtotalat a relatively short HRT.

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Fig. 4. Effect of retention time on the removal of TOC in batch aerobic experiments (T = 19EC, pH = 7.6, DO = 2.0 mg/land VSS = 3.6 g/l).

The results presented in Table 5 furthermoreshow that the system achieved an almost com-plete removal of BOD5total, i.e. only 7.0 mg/lremained in the final effluent. This excellentperformance towards the removal of organicmatter can be attributed to the high active bio-mass present in the reactor. Moreover, the resultsalso clearly demonstrate that the activated sludgesystem can produce an effluent quality containinglow concentration of TSS as shown in Table 5.The suspended matter is adsorbed on and/orenmeshed into the biomass and then hydrolysedby extracellular enzymes.

The results obtained in Table 5 indicate thatthe AS system clearly is quite effective in remov-ing oil and grease because a very low concen-tration of oil and grease was obtained, i.e.,2.8 mg/l.

The available data in Table 5 show that the ASsystem treating UASB reactor effluent achieveda substantial reduction of total and faecal coli-form, with average removal values of 94.7% and97.75% respectively. Total and faecal coliformcount was reduced in the treated effluent by avalue of 1.28 and 1.64 log10, corresponding to aresidual count of 2.4×105 and 0.9×104 MPN/100 ml, respectively, which means that accordingto Egyptian law (48/1982), the treated effluentcan be safely discharged into agricultural drains.

Apparently, the removal of total and faecalcoliform in activated sludge system is a two stepprocess. First a rapid sorption of bacteria to thesludge flocs takes place, followed by a slowerelimination of bacteria, which is presumably dueto predation by ciliated protozoa.

4. Discussion

The UASB reactor has become the mostwidely applied reactor technology for treatmentof dairy wastewater. The main reason for thesuccess of the UASB reactor is its relatively hightreatment capacity compared to other systems[24]. This is due to its compact size and costeffectiveness.

The results obtained in this investigationrevealed that the UASB reactor was efficientlyfeasible for treatment of DDWW at an averagewastewater temperature of 20ºC. The reactorachieved removal values of 69% for CODtotal,79% for BOD5 total and 72% for TSS. This is dueto a combination of different processes such asphysical, chemical (entrapment/adsorption) andbiodegradation of organic matter [25]. Moreover,high anaerobic biodegradability of DDWW(66%), and high BOD5 total removal, in the UASBreactor was apparently due to compounds origi-nating from milk residues, while some of the

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residual CODtotal may have included biologicallyinert compounds from detergents and solublemicrobial products [26].

Present CODtotal removal was lower than thatpreviously reported for UASB reactor treatingcheese production wastewater at lower OLR (1.5–1.9 g CODtotal/l.d) and substantially longer HRT(30–40 days) [27] and also considerably lowerthan those obtained from the UASB reactortreating dairy wastewater at higher OLR (2.4–13.5 kgCODtotal/m3.d) and shorter HRT of 3.0 h(61–95.6% CODtotal) [28]. Moreover, the CODtotalremoval was comparable with those reported atOLR (2.3–4.5 g CODtotal/l.d) and longer HRT(10–20 days) for a UASB reactor treating cheeseproduction wastewater [27] and at 5.5 kgCODtotal/m3.d for an anaerobic filter treating icecream wastewater [29].

Taking into consideration the importance ofthe adsorption phenomena in the CODtotal andTSS removal which occurs in anaerobic treatmentof complex fat containing effluents, it is accept-able to assume that in a UASB reactor, the sludgebed acts as a filter retaining the organic matterwhich leads to the growth of sludge [30]. Oncethe storage capacity is exhausted, unintentionalwashout of sludge together with the effluent willtake place. This indicates that the sludge bed ofthe UASB reactor had lost its adsorption orretention capacity originating a break-throughphenomenon similar to that common in anadsorption column. In this investigation, washoutof biomass did not occur; this was due to theregular intentional sludge discharge from theUASB reactor. The excess sludge fraction of theUASB reactor amounted to 20% of the totalinfluent COD. Omil et al. [31] found that thewasted sludge generated from full scale anaerobicfilter treating complex dairy wastewater was 2–3 kgVSS/d.

The effluent of the UASB reactor can betreated in a second anaerobic treatment step forremoval of most of the remaining CODtotal,BOD5total, TSS and oil and grease. Two phase

anaerobic treatment of dilute milk wastes waspreviously investigated [32] using a completestirred reactor for acidogenesis and an up-flowanaerobic filter for methanogenesis. The totalsystem achieved a removal efficiency of 92% forCODtotal at a total HRT of 4.4 days. Two-stageanaerobic fluidized beds were also used in ano-ther investigation for dairy wastewater treatmentwith an influent CODtotal concentration of5000 mg/l [33]. CODtotal removal values variedbetween 76% and 92% at an OLR of 9.4 kgCODtotal/m3.d.

The results of this study have shown that theuse of an integrated treatment train consisting ofa UASB reactor, followed by an AS treatmentstep, can produce a good effluent quality com-plying with the standards given by Egyptian law(CODtotal = 80 mg/l, BOD5 total = 60 mg/l, TSS =60 mg/l and faecal coliform = 105/100ml). Theintegrated system provided 35 mg/l for CODtotal,7.0 mg/l for BOD5total, 14.0 mg/l for TSS, 2.8 mg/lfor oil and grease and 0.9×104 MPN/100ml forfaecal coliform. These results are comparable tothe results obtained by Malaspina et al. [12] thatused a sequencing batch reactor for treatment ofthe anaerobic down flow-up flow hybrid reactoreffluent and activated sludge system treatinganaerobic reactor effluent [34]. Over 90% ofCODtotal was achieved at a sludge age of 20 days.The excess sludge from the activated sludge unitwas returned to the anaerobic reactor for furtherdigestion.

5. Conclusions

The results obtained in this investigationrevealed that the UASB reactor is an efficienttechnique for pre-treatment of DDWW at anaverage wastewater temperature of 20EC and aHRT of 24 h. The reactor achieved percentageremoval values of 69% for CODtotal, 79% forBOD5total and 72% for TSS. The regular inten-tional sludge discharge from the UASB reactor

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overcome washout out of biomass. Moreover, thewasted sludge from the UASB reactor was almoststabilized (VS/TS ratio = 0.66). The sludge yieldcoefficient was around 0.2 g VSS per g CODtotalremoved per day, corresponding to 20% of thetotal influent COD. Considering these results werecommended to apply an UASB reactor for pre-treatment of DDWW at a HRT of 24 h and OLRof 3.4 kgCODtotal/m3.d.

The combination of a UASB and AS systemrepresent a very promising option for the treat-ment of DDWW at a total HRT of 26 h (24 h forUASB and 2.0 h for the AS system). This com-bined system achieved an overall removal effi-ciency of 98.9% for CODtotal, 99.6% for BOD5 totaland 98.9% for oil and grease. The effluent qualityof the combined system satisfies standardsrequired for discharge into agricultural drains.

Acknowledgments

The authors would like to express their grati-tude to Prof. Dr. Fatma El-Gohary for valuablediscussion and the manager of Arab Dairy factoryfor helping us. The first author would like tothank Tarek Abdel-Shafi, Amer El-keleny and thetechnical staff of the Water Pollution ControlDepartment for technical support.

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