ARTICLE IN PRESS

0964-8305/$ - se

doi:10.1016/j.ib

�CorrespondE-mail addr

International Biodeterioration & Biodegradation 55 (2005) 279–284

www.elsevier.com/locate/ibiod

Removal of organic pollutants and analysis of MLSS–COD removalrelationship at different HRTs in a submerged membrane bioreactor

Nanqi Rena, Zhaobo Chena,�, Aijie Wanga, Dongxue Hub

aSchool of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, ChinabSchool of Mathematics and Computer Science, Harbin Normal University, Harbin 150080, China

Abstract

In order to investigate the influence of hydraulic retention time (HRT) on organic pollutant removal in a submerged membrane

bioreactor (SMBR), a laboratory-scale experiment was conducted using domestic sewage as influent. The dissolved oxygen (DO)

concentration was controlled at 2.0–3:0mgL�1 during the experimental period. The experiments demonstrated that when HRT was

3, 2 and 1 h, the reduction of chemical oxygen demand (COD) was 89.3–97.2, 88.5–97.3 and 80–91.1%, and the effluent COD was

38.9–11.2, 41.6–10.8 and 63.4–35:8mgL�1, respectively. It is suggested that an HRT of 1 h could meet the normal standard of

discharged domestic sewage, and an HRT of 2 h could meet that of water reclamation. In addition, we use mathematical software

MATLAB to analyse the relation of mixed liquor suspended solids (MLSS) and COD removal. The results showed that the

optimum MLSS concentration should be maintained at around 6000mgL�1 in the SMBR. The results also showed that the COD

removal was related to HRT ðtÞ, influent concentration ðS0Þ and sludge loading rate for COD removal (NS). Moreover, the high

COD removal could be achieved through adjusting t, S0 and NS.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Submerged membrane bioreactor (SMBR); Domestic sewage; HRT; MLSS; COD removal

1. Introduction

It is well known that submerged membrane bioreac-tors (SMBRs) have the following advantages for waste-water treatment: high sludge concentration (HalilHasara et al., 2002), high quality of effluent, longcontact time between activated sludge and organicpollutants (Brindle and Stephenson, 1996), and completeseparation of the hydraulic retention time (HRT) andsludge retention time (SRT) (Ueda et al., 1996; Devieset al., 1998; Guender and Krauth, 1998). Moreover,highly treated water in an SMBR is free from bacteriaand has potential for municipal and industrial reuse(Xing et al., 1998). Although there are shortcomings ofhigh-cost and high-energy consumption, SMBR tech-nology has been applied to wastewater treatment and

e front matter r 2005 Elsevier Ltd. All rights reserved.

iod.2005.03.003

ing author. Fax: +86451 8628 2009.

ess: czbhdx@163.com (Z. Chen).

reclamation previously (Chiemchaisri et al., 1993; Kno-block et al., 1994; Trouve et al., 1994). In Europe,America and Japan, SMBRs are used to rebuild sewagetreatment plants and to reclaim wastewater.

It is accepted that HRT is the key to furtherimproving the capacity of an SMBR. At present, whenan SMBR is used for domestic sewage treatment, theHRT is set at 1.5–7.5 h in laboratory-scale tests and at2.7–34.2 h in the pilot-scale tests (Makoto et al., 1998;Urbain et al., 1998; Defrance and Jaffrin, 1999; Huanget al., 2000; Gu and He, 2002; Shim et al., 2002). WhenRosenberger et al. (2002) used a membrane bioreactorto treat municipal wastewater at HRTs varying from10.4 and 15.6 h, the concentration of mixed liquorsuspended solids (MLSS) gradually increased and thechemical oxygen demand (COD) was reduced by 95%.Until now, the lowest HRT, 1.5 h, has been designed byStefan and Walter (2001) to treat synthetic wastewater.The organic loading rate (OLR) in their study was in the

ARTICLE IN PRESSN. Ren et al. / International Biodeterioration & Biodegradation 55 (2005) 279–284280

range 6.0–13:0 kgm�3 day�1, and COD reduction was495%. However, little has been reported on theoptimum HRT needed to meet reused water qualitystandard, and how to control the operational conditionsof SMBRs in order to reach different water qualitystandards.

The purpose of this study was to investigate theshortest HRT needed in SMBRs in order to reachdifferent water quality standards and the effect of HRTon COD removal. A laboratory-scale experiment wasconducted using artificial domestic sewage as influent inan SMBR. The dissolved oxygen (DO) concentrationwas controlled at 2.0–3:0mgL�1 for the duration of theexperimental period, and the HRT at 3, 2 and 1 h. Theeffect of MLSS on COD removal at different HRTs isexamined.

2. Materials and methods

2.1. SMBR

The plexiglass SMBR (Fig. 1) had a working volumeof 7.0 L. The hollow polypropylene fibre membranemodule employed in this study was 0.5m long and had apore size of 0:1mm and surface area of 2:0m2. Theprocess was maintained at 20–25 �C. The height wasfrom liquid surface in the bioreactor to effluent port.The mixed liquor in the bioreactor was driven under theheight by gravitation and passed through the hollow

Pump

Gas flow mAir pump

T

Ballcock

Water balance tank

Feed tank

Fig. 1. Schematic diagr

fibre membrane module. Aeration was employed tomaintain an aerobic environment for the normal growthof activated sludge. The amount of air was adjustedusing a gas flow meter and controlled at 0.3–0:5m3 h�1.The water level in the SMBR was controlled by aballcock in the water balance tank, which balanced theflux of effluent and influent.

2.2. Substrate

The artificial domestic sewage containing ðmgL�1Þ

glucose (300–400), beef grease (20–40), peptone (20–40),NH4Cl (5–10), Na2HPO4 (5–10) and NaH2PO4 (5–10)was used as influent. The influent was maintained at pH7.0 by adding NaOH. Influent COD concentration was350–500mgL�1.

2.3. Inoculation and acclimation of activated sludge

Activated sludge was taken from the aeration pool inHarbin Refinery and was incubated in batch culture.After 7 days, MLSS reached 1858mgL�1. An oldmembrane module (operated for 1 year in the samebioreactor) was added to the bioreactor to operate theSMBR continuously. Seven days later, MLSS reached2820mgL�1. The old membrane module was takenfrom the bioreactor and a new membrane module wasadded to the bioreactor to operate the SMBR. Withincubation and acclimation for 14 days, the colour ofthe flocs changed to a brown colour. The amount of

eter SMBR Air diffuser

Sample port

Height

Hollow fibre membrane

hermometer

Backwashing device

Effluent

am of the SMBR.

ARTICLE IN PRESSN. Ren et al. / International Biodeterioration & Biodegradation 55 (2005) 279–284 281

protozoa, such as rotifer species was observed through amicroscope. Effluent COD was around 80mgL�1.Based on the above, it was judged that the process ofincubation and acclimation was completed.

2.4. SMBR operational conditions

To maintain a stable HRT, an air-driven valve linkedto an electromagnetic flow meter automatically con-trolled the flow rate of effluent. Dissolved oxygen wasmaintained at 2.0–3:0mgL�1 by adjustment of a rotaryflow meter. Three membrane bioreactors operatedsynchronously in this study. The flux in the threebioreactors was 38.8, 58.3 and 116:7mlmin�1 when theHRT was 3, 2 and 1 h, respectively.

2.5. Analytical methods

Standard methods (APHA, 1995) were used todetermine COD, MLSS, DO and pH values. Therelation between MLSS and COD removal was analysedusing the mathematical software program MATLAB,which from the literature does not appear to have beenused previously for analysis of this relationship in theSMBRs.

3. Results and discussion

3.1. Effect of HRT on COD removal

The changes in COD removal and effluent CODvalues with time for each of the three HRT settings areillustrated in Fig. 2. When the HRT was 3 h, the highestCOD removal was 97.2%, after 50 days operation. It is

70

75

80

85

90

95

100

30 32 34 36 38 40 42 44 46 48 50 52

time (day)

CO

D r

emov

al (

%)

0

20

40

60

80

100

120

140

Eff

luen

t CO

D(m

g L

−1)

COD removal(HRT3h) COD removal(HRT2h) COD removal(HRT1h)

Effluent COD(HRT3h) Effluent COD(HRT2h) Effluent COD(HRT1h)

Fig. 2. COD removal at different HRTs.

viewed that the removal of organic pollutants was a co-function of microbial metabolism and membrane filtra-tion. The effluent COD concentration reached a level ofo30mgL�1, and even fell to 10mgL�1, although theinfluent COD fluctuated from 350 to 500mgL�1. Whenthe HRT was 2 h, the COD removal fluctuated between88.5% and 97.3% and the effluent COD between 41.6and 10:8mgL�1. COD removal was therefore very highwhen the HRT was either 2 or 3 h, and the effluent CODwas less than 30mgL�1. This is sufficient to meet thereused water quality standard in China. When the HRTwas 1 h, COD removal increased with the increase inMLSS. The COD removal fluctuated from 80.0% to91.1% and effluent COD from 63.4 to 35:8mgL�1. Thismeets the water quality standard for discharge in China.It can therefore be concluded that the removal oforganic pollutants was high and stable when SMBRtechnology was applied to treat domestic sewage.

3.2. Effect of MLSS on the COD removal at different

HRTs

The effect of MLSS on COD removal (Fig. 3) showedthe same trend at different HRTs. The mathematicalanalysis of the relations between MLSS and CODremoval was developed based on the experimental datato describe this trend using software of MATLAB.

A fourth-order polynomial function (1) was consid-ered to simulate the data in Fig. 3

y ¼ a4x4 þ a3x3 þ a2x

2 þ a1x þ a0. (1)

Unknown coefficients of a0, a1, a2, a3 and a4 needed tobe calculated by the equation group (2), where a matrixwith ten rows and five columns showed the coefficients.

70

75

80

85

90

95

100

500 2500 4500 6500 8500 10500

MLSS (mg L-1)

CO

D r

emov

al (

%)

HRT3h

HRT2h

HRT1h

Fig. 3. Effect of MLSS on COD removal at different HRTs.

ARTICLE IN PRESSN. Ren et al. / International Biodeterioration & Biodegradation 55 (2005) 279–284282

y1

y2

y3

y4

y5

y6

y7

y8

y9

y10

266666666666666666664

377777777777777777775

¼

1 x1 x21 x3

1 x41

1 x2 x22 x3

2 x42

1 x3 x23 x3

3 x43

1 x4 x24 x3

4 x44

1 x5 x25 x3

5 x45

1 x6 x26 x3

6 x46

1 x7 x27 x3

7 x47

1 x8 x28 x3

8 x48

1 x9 x29 x3

9 x49

1 x10 x210 x3

10 x410

266666666666666666664

377777777777777777775

�

a0

a1

a2

a3

a4

26666664

37777775. (2)

MATLAB would output a row vector of the polynomial(Gander, 1999; Ren et al., 2001; Su, 2002). The calculationof coefficients showed as follows: When the HRT was 1h,a0 ¼ 74:000, a1 ¼ 0:0035, a2 ¼ 3:7e� 007, a3 ¼ �1:1e�010 and a4 ¼ 5:9e� 015; when the HRT was 2h,a0 ¼ 77:000, a1 ¼ �0:0023, a2 ¼ 3:3e� 006, a3 ¼

�6:2e� 010 and a4 ¼ 3:4e� 014; and when the HRTwas 3h, a0 ¼ 85:000, a1 ¼ �0:0082, a2 ¼ 7:6e� 006,a3 ¼ �1:7e� 009 and a4 ¼ 1:1e� 013. Consequently,the following three mathematical models could beobtained with regard to the effect of MLSS on CODremoval at different HRT:

y1 ¼ 5:9e� 015x4 � 1:1e� 010x3 þ 3:7e

� 007x2 � 0:0035x þ 74:000,

y2 ¼ 3:4e� 014x4 � 6:2e� 010x3 þ 3:3e

� 006x2 � 0:0023x þ 77:000,

y3 ¼ 1:1e� 013x4 � 1:7e� 009x3 þ 7:6e

� 006x2 � 0:0082x þ 85:000.

The comparison of simulated results and the original data(Fig. 4) demonstrated that the fourth-order polynomialwas very stable.

In order to analyse the relation of COD removal andMLSS more directly, Z and M respectively replaced y

and x, and the following mathematical model wasobtained.

Z ¼ k4M4 � k3M3 þ k2M2 � k1M þ DZ. (3)

Fig. 4. Curve simulation results at HR

Here Z is COD removal, M is MLSS, k1, k2, k3, k4 areconstants, and DZ is a constant item, which is defined byCOD removal when M is equal to zero.

Table 1 shows the mathematical model and correla-tive mathematical parameters of the effect of MLSS onCOD removal at different HRTs in this experiment,from which it is concluded that:

(i) Fig. 4 and Eq. (3) show that Z increased with theincrease of M. When M increased to 6000mgL�1,the trend of Z gradually became stable. At the start, themembrane bioreactor MLSS was very low, and conse-quently, the microbial metabolism and COD removalwere quite low. After that, the sludge continued toincrease and COD removal also increased. However,when the MLSS arrived at a particular value ðMLSS ¼

6000mgL�1Þ COD removal and effluent COD werestable, as a result of a stable gelatin layer at the surfaceof the membrane. Therefore, the simulation results forthis mathematical model showed that the optimumMLSS should be maintained at around 6000mgL�1 inthe SMBR.

(ii) Because of k45k35k2, the fourth degree equationcould be replaced by the second degree equation

Z ¼ k2M2 � k1M þ DZ. (4)

(iii) Supposing Ns stands for sludge removal loadingrate, S0 for influent COD, Se for effluent COD and tstands for hydraulic retention time.

According to the equation for active sludge kinetics,NS ¼ ðS0 � SeÞ=ðM � tÞ, it can be deduced that

M ¼S0 � Se

NS � t. (5)

Accordingly, the mathematical model for COD removalwith the change of sludge loading rate:

Z ¼ k2ðS0 � SeÞ

2

N2S � t2

� k1S0 � Se

NS � tþ DZ. (6)

Formula (6) shows that COD removal ðZÞ was related toHRT ðtÞ, the COD of influent ðS0Þ and sludge loadingrate ðNsÞ. Perfect COD removal could be gained byadjusting parameters such as t, S0 and Ns.

In addition, the coefficients of simulation and norm ofresiduals at different HRTs were used to validate the

T of (a) 1 h (b), 2 h and (c) 3 h.

ARTICLE IN PRESS

Table 1

Correlative mathematical parameters and models of COD and MLSS removal at different HRTs

HRT (h) DZ Mathematical model k=k1, k2, k3, k4

1 74 y1 ¼ 5:9e� 015x4 � 1:1e� 010x3 þ 3:7e� 007x2 � 0:0035x þ 74:000 �0:0035 3:7e� 007 � 1:1e� 010 5:9e� 015

2 77 y2 ¼ 3:4e� 014x4 � 6:2e� 010x3 þ 3:3e� 006x2 � 0:0023x þ 77:000 �0:0023 3:3e� 006 � 6:2e� 010 3:4e� 014

3 85 y3 ¼ 1:1e� 013x4 � 1:7e� 009x3 þ 7:6e� 006x2 � 0:0082x þ 85:000 �0:0082 7:6e� 006 � 1:7e� 009 1:1e� 013

Fig. 5. Residuals of curve simulation at HRT of (a) 1 h (b), 2 h and (c)

3 h.

N. Ren et al. / International Biodeterioration & Biodegradation 55 (2005) 279–284 283

correctness of our mathematical models. Fig. 5 indicatesthat when the HRT was 3, 2 or 1 h all the norms ofresiduals were o3:0 (Wen et al., 2000; Su, 2002),proving that the model could simulate the experimentaldata correctly.

4. Conclusions

(1) When the SMBR was used to treat domesticsewage, the experimental results showed that an HRT of1 h could meet the normal standard for dischargeddomestic sewage, and an HRT of 2 h could meet that forwater reclamation.

(2) Mathematical analysis of the experimental datashowed that COD removal increased with increase ofMLSS. When MLSS increased to 6000mgL�1, COD

removal gradually stabilised. It was demonstrated thatthe optimum MLSS in an SMBR should be maintainedat around 6000mgL�1.

(3) Formula,

Z ¼ k2ðS0 � SeÞ

2

N2S � t2

� k1S0 � Se

NS � tþ DZ,

showed that COD removal ðZÞ was related to HRT ðtÞ,influent COD ðS0Þ and sludge removal loading rate ðNsÞ.Perfect removal of COD could be obtained by adjustingthese three parameters.

Acknowledgements

The present publication has been made possiblethrough the financial, technical and administrativeassistance of China National ‘‘863’’ Hi-Tech R & DProgram.

References

APHA,1995. Standard Methods for the Examination of Water and

Wastewater, 20th ed. American Public Health Association,

Washington, DC.

Brindle, K., Stephenson, T., 1996. Application of membrane biological

reactors for the treatment of wastewaters. Biotechnology and

Bioengineering 49, 601–610.

Chiemchaisri, C., Yamamoto, K., Vigneswaran, S., 1993. Household

membrane bioreactor in domestic wastewater treatment. Water

Science and Technology 27, 171–178.

Defrance, L., Jaffrin, M.Y., 1999. Comparison between filtrations at

fixed transmembrane pressure and fixed permeate flux: application

to a membrane bioreactor used for wastewater treatment. Journal

of Membrane Science 152, 203–210.

Devies, W.J., Le, M.S., Heath, C.R., 1998. Intensified activated sludge

process with submerged membrane microfiltration. Water Science

and Technology 38, 421–428.

Gander, W., 1999. Solving Problems in Scientific Computing Using

Maple and MATLAB. China Higher Education Press Beijing/

Springer, Berlin.

Gu, G.W., He, Y.L., 2002. Study and Application of the Membrane

Bioreactor in Wastewater Treatment. China Chemistry Industry

Press, Beijing.

Guender, B., Krauth, Kh., 1998. Replacement of secondary clarifica-

tion by membrane separation: results with plate and hollow fibre

modules. Water Science and Technology 38, 383–393.

Halil, H., Cumali, K., Ayhan, U., 2002. Viability of microbial mass in

a submerged membrane bioreactor. Desalination 150, 263–268.

ARTICLE IN PRESSN. Ren et al. / International Biodeterioration & Biodegradation 55 (2005) 279–284284

Huang, X., Gui, P., Qian, Y., 2000. Performance of submerged

membrane bioreactor for domestic wastewater treatment. Tsinghua

Science and Technology 5, 237–240.

Knoblock, M.D., Sutton, P.M., Mishra, P.N., Gupta, K.,

Janson, A., 1994. Membrane biological reactor system for

treatment of oily wastewaters. Water Environment Research 66,

133–139.

Makoto, O., Yutaka, O., Naoki, O., 1998. Decoloration system using

rotation membrane UF module. Water Science and Technology 38,

35–43.

Ren, N.Q., Zhang, Y., Chen, Z.B., 2001. The mathematical

model analysis of membrane flux by using submerged bioreactor

with different membrane forms. High Technology Letters 11,

100–103.

Rosenberger, S., Kruger, U., Witzig, R., Manz, W., Szewzyk, U.,

Kraume, M., 2002. Performance of a bioreactor with submerged

membranes for aerobic treatment of municipal wastewater. Water

Research 36, 413–420.

Shim, J.K., Yoo, I.K., Lee, Y.M., 2002. Design and operation

considerations for wastewater treatment using a flat submerged

membrane bioreactor. Process Biochemistry 38, 279–285.

Stefan, H., Walter, T., 2001. Treatment of urban wastewater in a

membrane bioreactor at high organic loading rates. Journal of

Biotechnology 92, 95–101.

Su, X.S., 2002. Engineering Application of MATLAB 6.0. China

Science Press, Beijing.

Trouve, E., Urbain, V., Manem, J., 1994. Treatment of municipal

wastewater by a membrane bioreactor: results of a semi-industrial

pilot-scale study. Water Science and Technology 30, 151–157.

Ueda, T., Hata, K., Kikuoka, Y., 1996. Treatment of domestic sewage

from rural settlements by a membrane bioreactor. Water Science

and Technology 34, 189–196.

Urbain, V., Mobarry, B., Silva, V., Stahl, D.A., Rittmann, B.E.,

Manem, J., 1998. Integration of performance, molecular biology

and modeling to describe the activated sludge process. Water

Science and Technology 37, 223–229.

Wen, X., Zhou, L., Wang, D.L., Xiong, X.Y., 2000. Application

Design of MATLAB NN. China Science Press, Beijing.

Xing, C.H. Qian, Y., Wen, X.H., Meng, Y.B., 1998. Performance of

microfiltration membrane bioreactor for domestic wastewater

reclamation. In: Proceedings of WEFTEC Asia ’98, January

1998, Singapore.