anaerobic wastewater treatment of concentrated sewage using a two-stage upflow anaerobic sludge...

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This article was downloaded by: [University of Newcastle (Australia)] On: 26 September 2014, At: 12:29 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20 Anaerobic wastewater treatment of concentrated sewage using a two-stage upflow anaerobic sludge blanket- anaerobic filter system Maha M. Halalsheh a , Zainab M. Abu Rumman a & Jim A. Field b a Water and Environmental Research and Study Centre , University of Jordan , Amman , Jordan b Department of Chemical and Environmental Engineering , University of Arizona , Tucson , Arizona , USA Published online: 28 Jan 2010. To cite this article: Maha M. Halalsheh , Zainab M. Abu Rumman & Jim A. Field (2010) Anaerobic wastewater treatment of concentrated sewage using a two-stage upflow anaerobic sludge blanket- anaerobic filter system, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 45:3, 383-388, DOI: 10.1080/10934520903467824 To link to this article: http://dx.doi.org/10.1080/10934520903467824 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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Page 1: Anaerobic wastewater treatment of concentrated sewage using a two-stage upflow anaerobic sludge blanket- anaerobic filter system

This article was downloaded by: [University of Newcastle (Australia)]On: 26 September 2014, At: 12:29Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and EnvironmentalEngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesa20

Anaerobic wastewater treatment of concentratedsewage using a two-stage upflow anaerobic sludgeblanket- anaerobic filter systemMaha M. Halalsheh a , Zainab M. Abu Rumman a & Jim A. Field ba Water and Environmental Research and Study Centre , University of Jordan , Amman ,Jordanb Department of Chemical and Environmental Engineering , University of Arizona , Tucson ,Arizona , USAPublished online: 28 Jan 2010.

To cite this article: Maha M. Halalsheh , Zainab M. Abu Rumman & Jim A. Field (2010) Anaerobic wastewater treatmentof concentrated sewage using a two-stage upflow anaerobic sludge blanket- anaerobic filter system, Journal ofEnvironmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 45:3, 383-388, DOI:10.1080/10934520903467824

To link to this article: http://dx.doi.org/10.1080/10934520903467824

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Anaerobic wastewater treatment of concentrated sewage using a two-stage upflow anaerobic sludge blanket- anaerobic filter system

Journal of Environmental Science and Health Part A (2010) 45, 383–388Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934520903467824

Anaerobic wastewater treatment of concentratedsewage using a two-stage upflow anaerobic sludgeblanket- anaerobic filter system

MAHA M. HALALSHEH1, ZAINAB M. ABU RUMMAN1 and JIM A. FIELD2

1Water and Environmental Research and Study Centre, University of Jordan, Amman-Jordan2Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona, USA

A two-stage pilot-scale upflow anaerobic sludge blanket – anaerobic filter (UASB-AF) reactors system treating concentrated domesticsewage was operated at 23◦C and at hydraulic retention times (HRT) of 15 and 4 h, respectively. Excess sludge from the downstreamAF stage was returned to the upstream UASB reactor. The aim was to obtain higher sludge retention time (SRT) in the UASB reactorfor better methanization of suspended COD. The UASB-AF system removed 55% and 65% of the total COD (CODtot) and suspendedCOD (CODss), respectively. The calculated SRT in the UASB reactor ranged from 20–35 days. The AF reactor removed the washedout sludge from the first stage reactor with average CODss removal efficiency of 55%. The volatile fatty acids concentration in theeffluent of the AF was 39 mg COD/L compared with 78 mg COD/L measured for the influent. The slightly higher CODtot removalefficiency obtained in this study compared with a single stage UASB reactor was achieved at 17% reduction in the total volume.

Keywords: Two-stage system, anaerobic filter, UASB reactor, HRT, concentrated sewage, domestic wastewater.

Introduction

Anaerobic wastewater treatment using the upflow anaero-bic sludge blanket (UASB) reactor is widely accepted for di-luted sewage treatment in warm climatic regions like Braziland India.[1,2] Wastewater produced in these regions is char-acterized by low to moderate COD concentrations[3] in therange of 215–700 mg/L.[4−7] However, the applicability ofUASB reactor technology is still limited in regions whereconcentrated domestic sewage with high suspended CODfraction is generated. This is the case in many countriesof the Middle East and North Africa (MENA) includingJordan, Palestine, Yemen and rural areas in Egypt. Con-centrated sewage with total COD values exceeding 1000mg/L is produced in such countries partly due to the lowconsumption of fresh water.

Concentrated sewage is characterized by having a highsuspended solids COD (CODss) fraction ranging 50–70%of the total COD (CODtot).[8,9] Limited hydrolysis of sus-pended solids could be a challenge during anaerobic treat-

Address correspondence to Maha Mohammad Halalsheh, Waterand Environmental Research and Study Centre, University ofJordan, Amman-Jordan. E-mail: [email protected] June 2, 2009.

ment especially at low temperatures.[10,11] Relatively longhydraulic retention time (HRT) of 24 h would be needed inorder to guarantee sufficient sludge retention time (SRT)for adequate CODss hydrolysis.[12] Several modificationsto the conventional UASB reactor have been suggested toovercome CODss content and reduce the HRT requiredfor treatment. Such modifications include solids removaland partial hydrolysis in a first step anaerobic reactor;[13]

two-stage UASB system[6,7,12,13] and an integrated UASB-digester system.[14,15]

Two-stage anaerobic systems are simple operating sys-tems that refer to the presence of the same biomassin various environmental conditions (pH, reactor type,concentration).[16] For instance, high food/microorganism(F/M) ratio is introduced in the first stage reactor whilethe second stage is subjected to lower F/M ratio. Lopezet al.[17] estimated that using two reactors operated in serieswould supply higher conversion compared with a single-stage reactor with an equivalent volume. Most studies onstaged anaerobic UASB reactor were considering treatmentof sewage with COD concentration in the range 215–700mg/L.[6,7,12,13] Reported total COD (CODtot) removal effi-ciency for the staged systems range between 40–70%.[4,5,7]

Halalsheh et al.[12] operated a two-stage pilot UASB sys-tem for the treatment of sewage with average CODtot

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concentration of 1505 mg/L at an average water tempera-ture of 26◦C. The two-stage UASB system was operated attotal HRT of 13 to 16 hrs. The achieved CODtot and CODssremoval efficiencies were 55% and 62%, respectively.

Most of the COD removal occurred in the first-stageUASB reactor. The very high organic loading rate of 3.6–5.0 kg COD/m3.d applied in the first-stage reactor resultedin high gas production. Gas production reduced solids re-moval and resulted in sludge wash out from the first-stageto the second-stage reactor. The second-stage UASB reac-tor neither removed the washed out sludge nor achievedCOD conversion. The second-stage removed only 4% ofthe CODtotand almost no removal of CODsswas accom-plished. Halalsheh et al.[12] reported that insufficient sludgeage in the second stage UASB reactor was the main causeof the poor performance. The main aim of this researchwas to improve the performance of the second anaerobicstage by substituting the UASB reactor with an anaero-bic filter (AF) reactor operated at relatively short HRT.The capacity of anaerobic filter consisting of reticulatedpolyurethane foam (RPF) oriented vertically for solids re-moval was investigated earlier by several authors.[18–21] TheAF was mostly operated at 4 h HRT and removed between55–82% of CODss depending on whether the filter was in-troduced for post-treatment of anaerobic reactor or forupstream treatment of raw sewage.

The specific objectives of this research were: (i) To in-vestigate the performance of a two-stage UASB-AF systemoperated at a total HRT of 19 h for treatment of concen-trated sewage, and (ii) to discuss the performance of thetwo-stage UASB-AF system in view of available literatureon previously operated UASB-UASB system.

Materials and methods

The pilot-scale system investigated in this study consistedof a UASB reactor with an effective volume of 1.0 m3 fol-lowed by an AF reactor with an effective volume of 0.25m3 as shown in Figure 1. Dimensions and operating con-ditions for each reactor are shown in Table 1. The HRTof the UASB reactor was calculated based on Equation 1shown. The Filter media in the AF reactor consisted ofreticulated polyurethane foam (RPF) (type Filteren TM10from Recticel, Buren, The Netherlands) with knobs on oneside. The sheets were oriented vertically in the reactor with-out spacing. Each sheet had a thickness of 2.5 cm with aknob thickness of 1.5 cm. RPF are characterized by highspecific surface area that may reach 2400 m2/m3.[22] Theanaerobic filter reactor was operated at hydraulic retentiontime (HRT) of 4 h. Sludge was discharged twice per dayfrom the bottom of the AF reactor. Discharged sludge wasrecycled back to the UASB reactor. The discharged sludgewas first collected in a bucket and then introduced at thebottom of the UASB reactor. Circulation to the UASB re-

Fig. 1. Schematic diagram of the experimental set up. 1. UASB re-actor; 2. AF reactor; 3. Influent; 4. Sludge bed; 5. Sludge blanket;6. Hydraulic seal; 7. Gas flow meter; 8. GLS; 9. Sludge dischargetap; 10. Sludge sampling taps; 11. RPF sheets; 12. Effluent; 13.Excess sludge circulation.

actor was done by stopping the influent flow and pumpingthe sludge at the same rate of influent flow.

The two-stage UASB-AF system was located at Abu-Nusier domestic wastewater treatment plant at the northof Amman; the capital of Jordan. The plant receives do-mestic wastewater from the residential complex of AbuNusier. Influent to the two-stage UASB-AF system wastaken from the main channel carrying wastewater to theplant after screening and grit removal. Wastewater charac-teristics are shown in Table 2. Reactors were started up onAugust 2003. The UASB reactor was seeded with sludge

Table 1. Dimensions and operation conditions of the system.

Diameter HeightDimension Volume (m3) (m) (m)

UASB 1.0 0.6 3.5AF 0.25 0.4 2.0Operational

conditionHRT hs OLR∗

kgCOD/m3.dV+

up(m/hr) Frequencyof sludgedischarge

UASB 15 2.7 0.20 —AF 4 3.7 0.5 Twice/d

* OLR is the organic loading rate.+ Vup is the upflow velocity.

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Anaerobic wastewater treatment of concentrated sewage 385

Table 2. Average wastewater characteristics at Abu-Nusierwastewater treatment plant.

Temp. CODtot CODss CODcol CODdis VFA mg◦C mg/L mg/L mg/L mg/L COD/L

Average 23.5 ± 0.51 1465 ± 60 783 ± 100 221 ± 52 477 ± 42 105 ± 12No. of 79 20 20 9 9 8

samples

brought from 96 m3 UASB reactor operated at KhirbitAs-Samra wastewater treatment plant. The average totalsolids (TS) and volatile solids (VS) of the seed were 76 g/Land 42 g/L, respectively. Reactors were monitored duringthe start-up period of 7 months.[23] The system was thenoperated without monitoring until May 2006.

In this study, UASB reactor was operated at an HRTof 15 h and at an organic loading rate (OLR) of 2.7 kgCOD/m3.d. A gas liquid separator (GLS) was installed atthe top of the reactor. A gas flow meter was connectedto the GLS for gas measurement. However, a considerableamount of gas was noticed at water surface and gas produc-tion was underestimated. The top of the reactor was keptuncovered in order to facilitate scum removal and routinecleaning. Sludge sampling taps were installed at differentheights over the UASB reactor with 0.5 m height intervals.A screw pump was used to pump wastewater to the reac-tor. Both reactors were made up of concrete pipes that areoriginally manufactured for sewer systems. The pipes areavailable in the local market with different diameters.

The water temperature was measured 5 times per week.Composite influent samples were taken over 24 h onceper week, while weekly grab samples from UASB andAF reactors effluents were collected. Samples were thentransferred to laboratory for direct analysis. Wastewatersamples were analysed for CODtot, paper filtered COD(CODpf), soluble COD (CODsol). CODpf tests were per-formed using 8 µm filter papers (Whatman Cat. No. 1440110). Suspended COD (CODss) was considered as the dif-ference between CODtot and CODpf. The colloidal COD(CODcol) was considered as the difference between CODpfand CODsol. The CODsol was considered as the fractionpassing 0.45 µm membrane filters. Volatile fatty acids(VFA) were analyzed 7 times during the experimental pe-riod using a Philips PU 4500 gas chromatograph. Excesssludge from the AF was monitored for total solids (TS) andvolatile solids (VS). Sludge profile was measured twice forthe UASB reactor. All analyses were performed followingAPHA.[24]

Calculations and assumptions

Design HRT for the UASB reactor was calculated accord-ing to the model presented by Zeeman and Lettinga[11]

HRT = C × fSSX

× R × (1 − H) × SRT (1)

where

SRT Sludge retention time (days)R Fraction of the CODss removedH Fraction of removed solids, which is hydrolyzedC COD concentration of the influent (g COD/L)X Sludge concentration in the reactor (g COD/L);

1gVS = 1.6 g COD[12]

fSS CODss/ CODinfluent

Assumptions

50 days SRT was selected.[10] Expected conversion at thisSRT and at a temperature of 25◦C is around 50%.

C was assumed to be 1353 mg COD/L as reported forthe influent to Abu-Nusier wastewater treatmentplant.[21]

SS was assumed to be 61% as reported for the influent ofAbu Nusier wastewater treatment plant.[21]

X was assumed to be 14 gVS/L.[21]

H was assumed to be 45% as reported during primarysludge anaerobic digestion at 25◦C.[10]

R was assumed to be 50%.[21]

The calculated HRT-based on the above mentioned as-sumptions is 13 h. The UASB reactor was operated at 15 hHRT in order to provide additional time and retention forthe sludge returned to the UASB from the AF reactor.

Measured SRT in the UASB reactor is shown as:

SRT = VXQe Xe

(2)

where:

V is the volume to the reactor (m3)X is the CODss concentration of sludge in the reactor

(kg/m3). COD/VS of 1.6 was assumed.[12]

Qe is the effluent flow (m3/d)Xe is the effluent CODss

The maximum SRT was calculated based on sludge con-centration 2 weeks after starting the experiment, while theminimum SRT was calculated based on sludge concentra-tion at the end of the experiment.

Results and discussion

The 95% confidence interval for the mean water temper-ature was 23.5 ± 0.51◦C with minimum and maximumvalues of 19◦C and 27◦C, respectively as shown earlier inTable 2. The table also shows that approximately 53% ofthe influent COD is in the suspended form, which is lowercompared with 61% measured for the same sewage betweenMay and December 2006.[21] However, CODss fraction isstill higher compared with 40% calculated for less concen-trated sewage.[13] The CODtot and CODss of the influent,

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Fig. 2. CODtot (a) and CODss (b) for the influent, effluent of theUASB reactor and effluent of the AF reactor.

effluent of the UASB reactor and effluent of the AF reactorare shown in Figure 2. The average removal efficiencies ofeach reactor and the system compared with values reportedin literature are shown in Table 3. The average removals ofCODtot and CODss in the UASB-AF system were 55% and65%, respectively. The results obtained are comparable withthose reported by Halalsheh et al.[12] as shown in Table 3.Halalsheh et al.[12] operated a two stage UASB-UASB re-actors for sewage treatment at an average temperature of26◦C during the summer and obtained 55% removal effi-ciency for the CODtot. The results are also comparable withthose obtained by Encina et al.[7] who operated a two-stageUASB-UASB reactors for sewage treatment at a tempera-ture range between 9 and 26◦C.

The-first-stage UASB reactor was responsible for the re-moval of 32% and 20% of the CODtot and CODss, respec-tively. The reactor had lower removal efficiency comparedwith the pilot 60 m3 first stage UASB reactor previouslyoperated at 8–10 hrs HRT for concentrated sewage treat-ment at Khirbit As-Samra waste water treatment plant.[12]

The average CODtot and CODss removal efficiencies of thefirst stage UASB reactor at Khirbit As-Samra were 53%and 57%, respectively.[12] It should be noted that CODssof the influent to Khribit As Samra UASB reactor con-

0

5

10

15

20

25

30

35

40

0 50 100 150 200

Tem

pera

ture

(o C

)

Time (d)

Infl. 2006 Effl. 2006 Infl. 2008

Fig. 3. Differences in temperature between the influent and efflu-ent of the UASB reactor in the year 2006 and influent temperaturein the year 2008.

stituted 69–81% of the CODtot,[12] which is much higher

compared with CODss fraction reported in this study.Some investigators reported an increase in total suspendedsolids (TSS) and CODss removal efficiencies with increas-ing sewage concentration.[4,18] The lower removal efficiencyof the UASB reactor in the present study could also be at-tributed to density currents that might be formed due todifferences between influent and the reactor water temper-ature.

Differences in the influent and effluent temperatures ofone to four degrees were noticed during the operation ofthe UASB reactor at Abu Nusier wastewater treatmentplant as shown in Figure 3. It was found that a tem-perature difference of only 1◦C between influent and thesedimentation tank content was enough to induce densitycurrents.[25−27] These currents might also be responsible forbiomass washout at some occasions as shown in Figure2. The figure shows several occasions where CODss in theeffluent of the UASB reactor is higher than the correspond-ing influent values. Biomass profiles of the reactor at thebeginning and end of the experiment are shown in Figure4. The average biomass concentration in the reactor was25 g TS/L at the beginning of the experiment and droppeddown to 19 g TS/L on August 2008. The VS concentra-tion was 19 g VS/L at the beginning of the experiment anddropped down to 11 g VS/L at the end of the experiment.Throughout the experiment there were problems with thesettling of suspended solids in the UASB reactor, which af-fected the performance of the total system. Settling is stillthe limiting factor that determines sludge retention in theUASB reactor even when considering other mechanisms ofsolids removal like adsorption and filtration.[8] The SRT inthe UASB reactor ranged from 20 to 35 d. The estimatedmethanogenesis range between 30–40 % depending on theSRT time as reported during primary sludge anaerobic di-gestion at 25◦C[10] and assuming that the sludge bed in theUASB reactor can be simulated using completely stirredtank reactor (CSTR).

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Anaerobic wastewater treatment of concentrated sewage 387

Table 3. Performance and operational conditions for two stage anaerobic systems.

Operational conditionsCODtot removal

First stage Second stage efficiency (%)

Temp. HRT Vup OLR HRT Vup OLR First Second TotalSystem (◦C) (hs) (m/h) (kgCOD/m3.d) (hs) (m/h) (kgCOD/m3.d) stage stage system Reference

HUSB+-UASB 9–21 3 1.0 5.2 2 37–38 27–48 51–69 Wang[6]

UASB-UASB 9–26 6.1 0.6 1.1–1.5 4.0 0.9 1.0–1.7 40–60 Encina et al.[7]

UASB-UASB 23 6.4 0.62 0.8� 5.6 0.7 0.2� 83 36.1 89.2 Seghezzo[5]

HUSB-UASB 21–22 2.8 2.5� 2.4 6.5 0.94� 0.7 35� 44� 63.5 Alvarez et al.[4]

UASB-UASB 26 8–10 0.5–0.65 3.6–5.0 5–6 0.76–0.94 2.9–4.6 53 4.0 55 Halalsheh et al.[12]

UASB-AF 23 23 0.21 1.4 4 0.5 3.7 50 35 67 Alrajoula et al.[21]

UASB-AF 23 16 0.20 2.6 4 0.5 3.7 32 35 55 This study

+ HUSB: Hydrolytic upflow sludge bed digester.[6]

� Calculated based on data presented by Seghezzo.[12]

� Calculated based on data presented by Alvarez et al.[18]

The AF reactor removed 38% and 55% of the CODtotand CODss, respectively. The results obtained are consid-erably higher than those previously reported for a secondstage UASB reactor operated at an HRT of 6 h that re-moved only 4% of the COD[12]

tot . The AF reactor in this studyproved to have an excellent capacity to remove washed outbiomass from the UASB reactor and maintain a stable ef-fluent quality as shown in Figure 2. The AF reactor wasable to remove CODss with efficiencies as high as 85% de-pending on the influent concentration. The results obtainedare comparable with CODss removal efficiency reported byAlrajoula et al.[21] during the operation of UASB-AF reac-tors at 23◦C and with 23 and 4 h HRT, respectively. Thereactor was also capable of achieving a considerable bio-logical degradation as shown in Table 4. The average valuefor the VFA in the effluent of the AF was 39 mg COD/Lwhile the VFA concentration in the influent of the AF was

Fig. 4. Sludge profile in the UASB reactor at the beginning andend of the experiment.

78 mg COD/L. The average TS concentration of the dailydischarged biomass was 9 g/L with an average VS/TS ratioof 70%. The discharged biomass from the AF, represented16% of the daily COD load to the UASB reactor assum-ing that the COD/VS ratio of the circulated biomass is1.6.[12] It is not clear whether the circulated biomass neg-atively impacted the performance of the UASB reactor asthe reactor was already operated at a relatively high OLR of2.7 kg COD/m3.d. However, some researchers have shownthat the performance of the UASB reactor was not affectedby the recirculated sludge from a trickling filter or sub-merged aerated filters.[28,29] Recirculated sludge in thosestudies contributed up to 4% of the COD to the UASBreactor.[28]

The average removal efficiency of the COD in the UASB-AF system was higher compared with 50% removal effi-ciency obtained when the UASB reactor of the same systemwas operated at an HRT of 23 h.[21] This would correspondto a 17.4% reduction in the volume of the system witha better performance in terms of organic matter removalwhen the UASB-AF is compared with a single-stage UASBreactor. Lopez et al.[17] estimated that for the same totalconversion, a staged anaerobic reactor could have 43% lessvolume compared with a single-stage UASB reactor. Theherein calculated volume reduction was lower comparedwith that estimated by Lopez et al.[17] probably due to dif-

Table 4. Average VFA concentrations in the influent and efflu-ents of the UASB reactor and the AF reactor.

Effluent of the Effluent ofInfluent UASB reactor the AF reactor

Average (mg COD/L) 105 ± 12 78 ± 8 39 ± 9No. of samples 8 8 6

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388 Halalsheh et al.

ficulties faced in solids entrapment in the first stage UASBreactor.

Conclusions

A two-stage UASB-AF system operated at HRT of 15 and4 h, respectively; removed 55% and 65% of the CODtot andCODss from a concentrated sewage. The calculated SRT inthe first stage UASB reactor ranged from 20 to 35 d. Thesecond-stage AF was able to remove washed out suspendedsolids from the UASB reactor with average CODss removalefficiency of 55%. The UASB-AF system would allow fora total volume reduction of 17% compared with a singlestage anaerobic reactor.

Acknowledgment

The authors would like to thank the International AridLand Consortium (IALC) for the grant PO# Y432976 pro-vided to run the research. The authors are also grateful toWater Authority of Jordan (WAJ) and Miyahuna Com-pany for the permissions given for University of Jordan tohave their experimental set up at Abu-Nusier Wastewatertreatment plant.

References

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[2] Sato, N.; Okubo, T.; Onodera, T.; Agrawal, L.K.; Ohashi, A.;Harada, H. Economic evaluation of sewage treatment processesin India. J. Environ. Mgmt. 2007, 48, 447–460.

[3] Metcalf and Eddy Inc. Wastewater engineering treatment disposalreuse, 4rd Ed. McGraw-Hill, New York, pp. 153–213, 1991.

[4] Alvarez, J.A.; Armstrong, E; Gomez, M.; Soto, M. Anaerobic treat-ment of low strength municipal wastewater by a two-stage pilotplant under psychrophilic conditions. Bioresour. Technol. 2008, 99,7051–7062.

[5] Seghezzo, L. Anaerobic treatment of domestic wastewater in sub-tropical regions, PhD thesis, Wageningen University, The Nether-lands, 2004.

[6] Wang, K. Integrated anaerobic and aerobic treatment of sewage,PhD thesis, Wageningen University, The Netherlands, 1994.

[7] Encina, P.G.; Rodriguez, R.; Fernandez, N.; Fdz-Polanco, F. Do-mestic sewage treatment with a two-stage anaerobic reactor. InPreprints of the international congress on options for closed wa-ter systems, Wageningen, The Netherlands, 1998.

[8] Halalsheh, M. Anaerobic pre-treatment of strong sewage. A propersolution for Jordan, PhD thesis, Wageningen University, TheNetherlands, 2002.

[9] Mahmoud, N.; Amarneh, M.N.; Al-Sa’ed, R.; Zeeman, G.; Gijzen,H.; Lettinga, G. Sewage characterization as a toll for the applicationof anaerobic treatment in Palestine. Environ. Pollut. 2003, 126, 115–122.

[10] Halalsheh, M.; Koppes, J.; den Elzen, J.; Zeeman, G.; Fayyad, M.;Lettinga, G. Effect of SRT and temperature on biological conver-

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