xing c.h, terdien e, et al. ultrafiltration membrane bioreactor

Journal of Membrane Science 177 (2000) 73–82

Ultrafiltration membrane bioreactorfor urban wastewater reclamation

C.-H. Xinga, E. Tardieub, Y. Qiana, X.-H. Wena,∗a State Key Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science and Engineering,

Tsinghua University, Beijing 100084, PR Chinab Direction Dêpartementale de l’Agriculture et de la Forêt, Centre Administratif Condé, 18013 Bourges Cedex, France

Received 26 November 1999; received in revised form 1 May 2000; accepted 1 May 2000

Abstract

A 162-day pilot-scale operation for reclamation of urban wastewater was studied by using an ultrafiltration membranebioreactor (UMBR). Performance of the UMBR was investigated with a sludge retention time (SRT) of 5, 15, and 30 days, ahydraulic retention time (HRT) of 5 h, and membrane flux between 75 and 150 l m−2 h−1, respectively. It was observed that thehighest sludge concentration in the reactor viz. a suspended solids (SS’s) concentration of 23.1 g l−1 and a volatile suspendedsolids (VSS’s) concentration of 13.5 g l−1, respectively, could be reached. The ratio of sludge VSS to sludge chemical oxygendemand (COD) was 1.428 in the study, which approximated to the theoretical value of 1.415. Mass loading rates of the UMBRwere close to those of conventional activated sludge processes (CASP’s), while the volumetric loading rates were two to fivetimes those of CASP. Averaged 97% of COD, 96.2% of ammonia nitrogen (NH3-N), and 100% of SS’s were removed. Itwas found that the bioreactor was responsible for 85% of COD removal, while 12% was due to separation of the membranemodule. The reclaimed water could be reused directly for municipal purposes or indirectly for industrial uses after additionaltreatment. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Activated sludge; Bioreactor; Ceramic membrane; Reclamation; Ultrafiltration; Urban wastewater

1. Introduction

Urban wastewater is usually treated by conven-tional activated sludge processes (CASP’s), whichinvolve the natural biodegradation of pollutants byheterotrophic bacteria (i.e. activated sludge) in aer-ated bioreactors. Activated sludge could be separatedby gravitational setting [1]. The treatment efficiencyis usually limited by the difficulties in separatingsuspended solids (SS’s). The optimal sludge concen-

∗ Corresponding author. Fax:+86-10-6277-1742.E-mail address:[email protected] (X.-H. Wen)

tration is generally up to 5 g l−1, which imposes largesize of aerated bioreactor [2].

Membrane bioreactor (MBR) is an improvementof the 100-year old CASP, where the traditionalsecondary clarifier is replaced by a membrane unitfor the separation of treated water from the mixedsolution in the bioreactor [3]. Originated from theuse of membrane separation, MBR technology hasvarious advantages [4]. The absolute retention of allmicro-organisms insures an increase in sludge con-centration and complete disinfection of treated water.It allows a complete separation of the hydraulic re-tention time (HRT) and sludge retention time (SRT).As a result, a high sludge concentration can be main-

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74 C.-H. Xing et al. / Journal of Membrane Science 177 (2000) 73–82

tained in the bioreactor and high-strength wastewatercan be treated effectively [5,6].

Due to the absence of secondary clarifier and thepresence of a high sludge concentration, the overallsize of the treatment plant can be reduced significantly[4]. Furthermore, the contact time between activatedsludge and organic pollutants can be enhanced, whichfacilitates an effective removal of low biodegradablepollutants. A highly treated water (by the MBR) isfree from bacteria and has a potential in municipal andindustrial reuse [8].

About 200 MBR’s are currently in operation forvarious wastewaters, and 90% of them are employedin municipal treatment [3]. In the application of MBRthat is based on polymeric materials, the leadingcountry is Japan, where most MBR systems are usedfor water recycling in buildings [9,10]. As these or-ganic membranes are normally sensitive to causticcleaning reagents, the difficulty met with in cleaningis often encountered especially when the membranemodule is seriously fouled during industrial oper-ation [7]. In order to overcome this difficulty, anMBR system equipped with ceramic membranes wasfirst developed in France [11]. It makes the cleaningof membrane easy and convenient in situ becausethese inorganic membranes possess a high degree ofresistance to chemical abrasion and biological degra-dation. The membrane has a great chemical stabilityin a wide range of pH and temperature [12].

Fig. 1. Schematic of the pilot UMBR.

Table 1Characteristics of urban wastewater

Items Typical Range

COD (mg l−1) 200–800 50–2234SS (mg l−1) 100–600 80–1327NH3-N (mg l−1) 10–30 10–40Coliform (number l−1) 105–106 105–106

Turbidity (NTU) 50–70 50–80pH value 7.5–8.5 7.5–8.5Temperature (◦C) 15–25 15–25

The objective of this study was to investigate thelong-term performance of a pilot-scale cross-flow ul-trafiltration membrane bioreactor (UMBR) for urbanwastewater reclamation. The impact of operationalparameters, such as SRT, HRT, and membrane flux,on effluent quality was evaluated. Contributions ofthe bioreactor and membrane module to the removalefficiency were examined. Moreover, the reuse poten-tial of treated water was discussed by comparing withcurrent water quality standards.

2. Experimental

The urban wastewater used in the study was pumpedfrom a local sewage station. As shown in Table 1, thewastewater can represent the medium-strength urbanwastewater seen in most cities around the world. Aschematic of the UMBR is shown in Fig. 1. The influ-

C.-H. Xing et al. / Journal of Membrane Science 177 (2000) 73–82 75

ent was taken from the feed tank to the bioreactor byperistaltic pump 1. The bioreactor that was filled withactivated sludge had a working volume of 30 l. An aer-ator was employed to maintain an aerobic environmentfor the normal growth of activated sludge. To keep anoptimal temperature, a heat exchanger was installed inthe bioreactor. A stirrer was used to ensure completemixing of the influent and the activated sludge. A levelcontroller together with pump 1 was used in order tomaintain a constant working volume. The bioreactor,centrifugal pump 2, and membrane module constituteda loop, where the activated sludge was circulated ata high speed. In addition, centrifugal pump 3 was in-stalled to enhance circulation of the activated sludge.

To maintain a stable HRT, the flowrate of effluentwas automatically controlled by an air-driven valvelinked up to an electromagnetic flowmeter. The vari-ation of SRT (from 5 to 15 and then 30 days) couldbe brought about by altering the flowrate of excessivesludge discharge. To prevent membrane fouling and si-multaneously reduce energy consumption, cross-flowvelocity inside membrane channels was set at 4 m s−1.To explore the difference of effluent quality under dif-ferent fluxes, the membrane flux was varied from 75to 150 l m−2 h−1 by altering the flowrate of effluent orthe number of membrane modules in operation. In thecleaning mode, the cleaning tank that was filled withcleaning reagents was substituted for the bioreactor toform a circulation loop with the membrane module.

The ultrafiltration membrane employed in the studywas ceramic tubular KerasepTM X3 type (Tech-Sep,

Fig. 2. Evolution of COD concentration as a function of time.

France). The membrane skin layer and support ma-terials were made of zirconia (ZrO2) and alumina(g-Al2O3). Each membrane had seven channels thathad a diameter of 4.5 mm. The membrane was 40 cmin length, while the surface area of each membranewas 0.04 m2. The membrane pore size was about0.02mm and the molecular weight cut-off was about300,000 Da. Initial permeability of the new membranewas about 4–5 l m−2 h−1 kPa−1 on the basis of tapwater test at 25◦C.

The UMBR system was monitored by daily mea-surement of redox potential by Monec 8935 andtemperature. Turbidity was measured by a turbiditymeter (model 965-10, Orbeco analytical System Inc.,USA). Samples for supernatant chemical oxygen de-mand (COD) of activated sludge were taken aftercentrifugation at 4500 rpm for 15 min. COD, SS’s,volatile suspended solids (VSS’s), and ammonia ni-trogen (NH3-N), and other items were measured asper the Standard Methods for Examination of Waterand Wastewater [13].

3. Results and discussion

3.1. Removal of COD, NH3-N, and SS

The UMBR was operated for 162 days with the in-fluent and effluent COD values illustrated in Fig. 2.The influent COD was fluctuated from 200 to 800mg l−1; however, the effluent COD was maintained


Table 2Operation parameters in the UMBR

Items Duration (day) HRT (h) SRT (day) Flux (l m−2 h−1)

Run 1 12 5 5 150a

11 5 5 75b

Run 2 78 5 15 75a

Run 3 22 5 30 75a

39 5 30 150b

a One tubular membrane was in operation.b Two tubular membranes were in operation.

at a low level. Taking the 162 days of experiment asa whole, 94% of the effluent COD data were lowerthan 12 mg l−1, while the averaged COD of effluentwas only 9.4 mg l−1. An averaged COD removal effi-ciency of 97% was achieved. It can be concluded thatthe removal of organic pollutants by the UMBR wasvery high in terms of COD and a good-quality efflu-ent can be achieved during the long-term operation.Note that the influent COD in Run 3 was once as highas 2234 mg l−1, yet the effluent COD remained lowerthan 10 mg l−1. This indicated that the UMBR had apotential in treating high-strength urban wastewater.

As listed in Table 2, the SRT was sequentiallyincreased from 5 to 15 and then 30 days and themembrane flux was increased from 75 (two tubularmembranes were in operation) to 150 l m−2 h−1 (onetubular membrane was in operation). However, there

Fig. 3. Evolution of NH3-N concentration as a function of time.

were no remarkable changes in effluent COD duringRuns 1–3 (see Fig. 2). This may be attributed to the‘forced separation’ of ultrafiltration membrane be-cause its pore sizes were fixed and its mechanicalinterception to macromolecules from sludge-mixedliquor was no longer affected by the variation of SRTand membrane flux. Thus, the effluent COD was notinfluenced by change in the SRT and membrane flux.

The highest value of effluent COD was observed at30 mg l−1 on Day 24 because the pilot had been haltedfor 10 h on Day 23. No nutrient was available for themaintenance and growth of aerobic bacteria, while theaerator still worked normally. As a result, some of thebacteria died of starvation and partial self-hydrolysisof activated sludge occurred in the bioreactor. Thesludge activity was therefore dramatically diminished.Though the influent COD on the following day wasonly 133 mg l−1, the effluent COD reached the highestrecord in the study. However, the effluent COD wentdown gradually to the normal level of about 10 mg l−1

soon after the UMBR system was restored.The concentration of NH3-N was measured weekly

during the pilot-scale experiment. Fig. 3 illustrates theevolution of NH3-N concentration with time. Whenthe influent NH3-N concentration was changed from10 to 40 mg l−1, the effluent concentration was re-duced to the low level of 0.2–1.3 mg l−1. The averagedremoval of NH3-N was 96.2%, indicating that theNH3-N in the influent had been deeply nitrified in theUMBR system. This was mainly due to two reasons.


First, as the nitrifying population was completely con-fined within the bioreactor, these autotrophic nitrifiershaving long generation times were ‘forced’ to prolif-erate speedily without any loss. However, they wereunavoidably washed out in the CASP when the SRTwas kept too low [14]. Secondly, as the sludge pro-duction was low in MBR processes, nitrifiers in thebioreactor faced less menace from those heterotrophicbacteria which were better competitors for the NH3-N[15,16]. Consequently, a high nitrification could beachieved in the UMBR system even at an HRT of 5 hand an SRT of 5 days during Run 1 of this study.

Similar to the COD removal discussed above, theNH3-N removal in Runs 1–3 was stable when the SRTwas varied from 5 to 15 and then 30 days (see Fig. 3)and the membrane flux was changed from 75 (Days13–24 and 102–123) to 150 l m−2 h−1 (Days 1–12 and124–162) (see Table 2), respectively. However, therewas no observable difference in the NH3-N concentra-tion of UMBR effluent. In other words, the efficiencyof the removal of NH3-N was irrelevant to the alter-ation in the SRT and flux.

As illustrated in Fig. 4, there was no SS detectedin the UMBR effluent during the 162-day experimenteven though a great fluctuation was observed in the in-fluent SS concentration. The efficiency of the removalof SS remained as high as 100%, which demonstratedthe better separation effect of the ultrafiltration mem-brane module than that of the secondary clarifier inCASP [4,10].

Fig. 4. Evolution of influent SS as a function of time.

3.2. Evolution of sludge concentration

The microbial population in the bioreactor waspractically measured by means of easily determinedparameters such as the SS’s and the VSS’s of acti-vated sludge [17]. The information on variation ofsludge concentration with time was necessary to un-derstand the actual performance of the bioreactor. Asdepicted in Fig. 5, the highest sludge concentration interms of SS and VSS had reached values of 23.1 and13.5 g l−1 on Day 155 in Run 3. The VSS to SS ratiowas vacillated around 0.6 during Runs 1–3 in spite ofthe fact that the absolute concentration of sludge in theUMBR system showed significant changes with thesequential extension of SRT. The VSS/SS ratio of theUMBR corresponded to the range of CASP known tobe 0.5–0.8 in most cases. Furthermore, the relativelystable ratio of VSS to SS implicated that the amount ofactive biomass and inorganic remainders constituted adynamic balance. There was no accumulation of inertfractions in the bioreactor during the pilot-scaleexperiment [14].

From Figs. 2–4, it can be seen that the variationof sludge SS and VSS had no effect on the removalefficiencies of COD, NH3-N, and SS. High treatmentefficiency could always be maintained regardlessof the absolute level of sludge concentration in thebioreactor. The evolution of sludge concentrationwith time was just an inevitable result of the pas-sive adaptation of micro-organisms to the change


Fig. 5. Evolution of sludge concentration as a function of time.

in the influent concentration and operation parame-ters such as HRT and SRT. The higher the influentconcentration, the shorter the HRT and the longerthe SRT, the higher the sludge concentration (SS,VSS), and vice versa. This may be due to the factthat the quality of UMBR effluent was no longerrestricted by the setting of activated sludge and there-fore irrelevant to the sludge concentration in thebioreactor.

The relationship between sludge COD and VSSduring the long-term experiment is illustrated inFig. 6. The correlation coefficient (R2) was 0.9629,

Fig. 6. Relationship between VSS and COD of sludge.

suggesting the favorably linear relationship betweensludge COD and VSS. The ratio of COD to VSS was1.428, which was close to the theoretical value of1.415 derived from the bacteria formula (C5H7NO2)when completely oxidized. The difference betweenthe experimental and theoretical data was only 0.9%.Furthermore, this evinced a new method for quickdetermination of sludge concentration by measuringits COD concentration. When the CO D/VSS andVSS/SS ratios were taken into account, the sludgeSS could be computed [7]. The precision was soundenough to meet the requirements of scientific research


Fig. 7. Distribution of COD removal efficiency in the UMBR system.

and practical operation of a wastewater treatmentplant.

3.3. Distribution of removal efficiency

In the UMBR system, COD removal was accom-plished by the two functional units viz. the bioreactorand the membrane module. Their respective contri-butions to removal efficiency are presented in Fig. 7.The total removal of COD was 97% on an averageduring the whole experiment, out of which, 85% wasremoved by the bioreactor. Only the residual 12% re-sulted from membrane separation. It was demonstratedthat the removal of organic pollutant in terms of CODwas mainly due to the bioreactor unit. The role ofmembrane module was to confine the biomass withinthe bioreactor, and thus, maintain a good separationof activated sludge and treated water.

From Fig. 7, it can also be seen that COD removalby the bioreactor was as low as 61.9% on an averagewhen the influent concentration had a sudden decreaseon Days 27, 58, 87, 98, and 154. However, COD re-moval by the membrane module remarkably went upto 32%, which was about one-third that of the biore-actor. On the other hand, if the influent concentrationhad a sudden increase, COD removal by the biore-actor exceeded 90% on an average. Removal by themembrane module was generally lower than 10% onDays 11, 35, 66, 80, 98, 111, 136, and 154. Thesedemonstrated that the membrane module played a mi-nor role in COD removal if the influent concentrationwas higher, and vice versa.

3.4. Mass loading rate and volumetric loading rate

The evolution of sludge mass loading rate over timeis shown in Fig. 8. Taking Runs 1–3 as a whole, theaveraged mass loading rate was about 0.54 kg CODkg VSS−1 per day, which was in the range of CASP.Due to the fluctuation of influent concentration, themass loading rates had proportional response duringthe long-term experiment on the UMBR. The massloading rate on Day 65 was only 0.49 kg COD kgVSS−1 per day. Because of the sudden increase in in-fluent COD, the mass loading rate went up to the max-imum value of 1.99 kg COD kg VSS−1 per day onthe following day, which was about 5–20 times that ofCASP (it generally ranged from 0.1 to 0.4 kg COD kgVSS−1 per day). However, the effluent COD still re-mained as low as 10 mg l−1. It can be concluded thatthe UMBR had a strong ability to resist a shock load-ing. We noticed that a very low or high mass loadingrate would inevitably lead to poor effluent quality inCASP. However, the fluctuation of mass loading rateswould not impose a negative effect on the quality ofUMBR effluent. The mass loading rate, one of the mostimportant parameters in CASP design, was no longercrucial to the full-scale design of the UMBR systemwhen applied to urban wastewater reclamation [7].

With an extension of the SRT, the averaged massloading rates were sequentially diminished from 0.76to 0.6 and then 0.4 kg COD kg VSS−1 per day (seeTable 3). This was because a longer SRT induced moreof sludge VSS when the HRT and influent concentra-tion were unchanged. As a result, the mass loading


Fig. 8. Evolution of mass loading rates as a function of time.

Table 3Mass loading rates in various operation periods

Items Minimum (kg CODkg VSS−1 per day)

Average (kg CODkg VSS−1 per day)

Maximum (kg CODkg VSS−1 per day)

Run 1 0.52 0.76 1.09Run 2 0.11 0.6 1.99Run 3 0.08 0.4 1.5Overall 0.08 0.54 1.99

rates were decreased thereafter. The lower the sludgemass loading rates, the better the purification effect ofthe UMBR. Therefore, longer SRT was recommendedin the UMBR operation.

The evolution of volumetric loading rates is illus-trated in Fig. 9. The maximum volumetric loading

Fig. 9. Evolution of volumetric loading rates as a function of time.

rate of 10.72 kg COD m−3 per day occurred on Day112, which was 13–27 times that of CASP (0.4–0.8 kgCOD m−3 per day). However, the corresponding efflu-ent COD on that day (see Fig. 2) was only 21 mg l−1.It was proven once again that the UMBR system hadthe potential to tolerate a shock loading.


Table 4Volumetric loading rates in various operation periods

Items Minimum (kgCOD m−3 per day)

Average (kgCOD m−3 per day)

Maximum (kgCOD m−3 per day)

Run 1 0.83 1.38 2.84Run 2 0.45 1.63 6.85Run 3 0.99 3.15 10.72Overall 0.99 2.1 10.7

As shown in the figure, the minimum volumetricloading rate was obtained at 0.45 kg COD m−3 perday on Day 28, which approached the lower limit ofCASP. The average volumetric loading rate was 2.1 kgCOD m−3 per day that was about two to five timesthat of CASP. This indicated that, treating the samewastewater, use of UMBR could not only eliminatethe secondary clarifier but also reduce the size of thebioreactor by two to five times in comparison with theCASP. Therefore, the UMBR was treated as one ofthe cost-effective solutions [7].

As shown in Table 4, the average volumetric load-ing rates were increased steadily from 1.38 to 1.63and then 3.15 kg COD m−3 per day during Runs 1–3,respectively. This was caused by increase in the influ-ent concentration, while the HRT was kept constant inthe study.

3.5. Reuse potential of UMBR effluent

The effluent quality of the UMBR is listed inTable 5. It shows that the UMBR system can providea good-quality effluent that is completely acceptablefor reuse. The reclaimed water in this study can bedirectly reused for municipal watering, toilet flushing,and car washing. After the softening treatment, thereclaimed water could be used as cooling supply andprocessing water. Therefore, lots of urban wastewa-ter can be effectively harnessed, and moreover, largequantities of water could be saved. The developmentof water industry would be more sustainable [18].

4. Conclusions

From the technical point of view, reclamation of ur-ban wastewater by the UMBR was basically applica-ble. The reclaimed water could be reused directly for

municipal purposes and indirectly for industrial watersupply.

The removal efficiency of COD was on the averageas high as 97%, in which 85% was attributed to thebioreactor and the residual 12% resulted from mem-brane separation. The averaged removal of NH3-Nand SS could reach 96.2 and 100%, respectively. Thehighest sludge concentration in terms of SS and VSSwas 23.1 and 13.5 g l−1, respectively. The VSS to SSratio of activated sludge was vacillated around 0.6,

Table 5Comparison between effluent quality and drinking water standards

Items Units UMBReffluent

Waterreusea

Color TCUb <2.5 30Turbidity NTUb <2 10pH value – 8.2 6.0–9.0Chloride as Cl− mg l−1 45.4 350Fluoride as F− mg l−1 0.3 c

Nitrate as N mg l−1 19.0 c

Nitrite as N mg l−1 0.1 c

Hardness as CaCO3 mg l−1 325 450Phenols mg l−1 <0.002 c

Cyanide as CN− mg l−1 <0.002 c

Sulfate as SO42− mg l−1 23.0 c

Arsenic mg l−1 <0.001 c

Mercury mg l−1 <0.2 c

Chromium as Cr6+ mg l−1 <0.004 c

Manganese mg l−1 <0.05 0.1Lead mg l−1 <0.01 c

Iron mg l−1 <0.05 0.4Total coliforms – –d 3 number

100 ml−1

a Water reuse standard for car washing, land watering etc., PRChina (CJ25.1-89).

b TCU: true color units; NTU: nephelometric turbidity units.c No requirement.d The ultrafiltration membrane could effectively retain the bac-

teria (size from 0.5 to 5mm) and viruses (size from 0.01 to0.3mm). As a result, there were no coliforms and MS-2 virusdetected [4,19].


which corresponded to the range of CASP known tobe 0.5–0.8. The relationship between VSS and CODof activated sludge was linear and the VSS to CODratio was about 1.428, which was close to the theoret-ical value of 1.415. The UMBR system had a strongability for resisting shock loading. Its mass loadingrates corresponded to those of the CASP, but the vol-umetric loading rates were two to five times those ofCASP. This demonstrated that the volume of the biore-actor by UMBR could be greatly reduced in compar-ison with that by CASP if the same wastewater wastreated. Therefore, a large amount of space and invest-ment could be saved.

Acknowledgements

The authors wish to acknowledge the equipment andfinancial support from CIRSEELyonnaise des Eaux,France and the State Key Laboratory of Environmen-tal Simulation and Pollution Control at Tsinghua Uni-versity, China.

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xing c.h, terdien e, et al. ultrafiltration membrane bioreactor

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