Comparison Between a Sequencing Batch Membrane Bioreactor and A

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Process Biochemistry 41 (2006) 87–95

Comparison between a sequencing batch membrane bioreactor and a conventional membrane bioreactor Han-Min Zhang *, Jing-Ni Xiao, Ying-Jun Cheng, Li-Fen Liu, Xing-Wen Zhang, Feng-Lin Yang School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China

Received 1 July 2004; received in revised form 26 January 2005; accepted 19 March 2005

Abstract

An operation of a membrane bioreactor in sequencing batch mode named a sequencing batch membrane bioreactor (SBMBR) was investigated for enhancing nitrogen and phosphorus removal. Its performance was compared with a conventional membrane bioreactor (CMBR) at various influent COD/TN ratios of 3.4–28.2. The operational parameters were optimized to increase the treatment efficiency. COD removal averaged at 94.9 and 97.7%, respectively, for SBMBR and CMBR during the 8 months experimental period. The SBMBR system demonstrated good performance on nitrogen and phosphorus removal at different COD/TN ratios. When COD/TN was 6.3 and the total nitrogen (TN) load was 0.22 kg/(m3 days), the TN and ammonium nitrogen removals of the SBMBR were maintained over 65 and 90%, respectively. Total phosphorus (TP) removal of the SBMBR was approximately 90% during most of the experimental time. In comparison, the CMBR did not perform so well. Its effluent TN concentration was close to that in the influent at COD/TN = 6.3 and TP removal was not stable. The specific nitrification rate test showed that pH value affected the activity of nitrifiers but no irreversible harm was induced. Furthermore, the sequencing batch mode operation of MBR retarded membrane fouling according to the monitoring of trans-membrane pressure (TMP). # 2005 Elsevier Ltd. All rights reserved.

Keywords: Sequencing batch membrane bioreactor (SBMBR); Conventional membrane bioreactor (CMBR); Nitrogen and phosphorus removal; Membrane fouling; Speciﬁc nitriﬁcation rate

1. Introduction However, SBR has a potential deficit in that poor clarification and a turbid effluent are associated with it. The key nutrients causing eutrophication in waterways To overcome these drawbacks and improve system are excess phosphorus and nitrogen concentrations in performance, membrane filtration technology has been effluents from municipal or industrial plants discharged in introduced in biological wastewater treatment. The filtration the environment. Therefore, it is necessary to removal these function of membrane enables a complete solid–liquid from wastewater at their sources. A number of biological separation in membrane bioreactor (MBR), eliminating the nutrient removal (BNR) processes had been developed. need for the formation of flocs or aggregates [2–5]. The Among these processes, the sequencing batch reactor (SBR) combination of MBR have been used for treating various has been applied as one alternative BNR technology since its types of wastewater, which demonstrate good performance process is simple to operate and very flexible for combining for removal of organic matter and biological sludge nitrogen and phosphorus removal. Its cycle format can be separation [6–8]. easily modified at any time to offset changes in process Recent studies have introduced alternating aerobic and conditions, influent characteristics or effluent objectives [1]. anaerobic conditions in a submerged MBR by intermittent aeration for simultaneous removal of carbon and nitrogen. Yeom et al. [9] reported that an intermittently aerated * Corresponding author. membrane bioreactor with a submerged fibre hollow E-mail address: [email protected] (H.-M. Zhang). membrane was capable of achieving 96% COD and 83%

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TN removals at 8–15 h HRT and a very long SRT. Ueda and 2. Materials and methods Hata [10] introduced an intermittent suction method in a membrane bioreactor for treatment of domestic sewage from 2.1. Reactors set-up and operation rural settlements. Ng et al. [11] investigated the effect of MLSS concentration in a sequencing batch membrane Fig. 1 shows a schematic diagram of experimental bioreactor on COD and nitrogen removal. Bae et al. [12] apparatus consisting of two 11 L Plexiglas reactors. evaluated the biological nutrient removal performance of a Temperature was controlled at 25 1 8C using thermostats. membrane separation process coupled to a SBR in the A balance-box with a float-ball valve was used to control treatment of dairy industry wastewater. All these studies water level of the two reactors. The membrane module used demonstrated that MBR operated in an intermittent way can in the two systems was a bundle of U-shaped hollow fibre obtain a high organic substrate and nitrogen removal. The membranes made of polyethylene (DAIKI, Japan) with a removal of phosphorus remains inadequate, however, and pore size of 0.1 mm and a filtration area of 0.15 m2. should be optimized to increase phosphorus removal. SBMBR and CMBR were in continuous operation in The objective of this study was to enhance nitrogen and parallel for about 8 months. The variation of operational phosphorus removal by sequential operation of a membrane parameter is listed in Table 1. The exchange ratio fB is bioreactor in alternating aerobic and anoxic/anaerobic defined as the ratio between the volume discharged per cycle condition. This system was named a sequencing batch and the entire working volume of the reactor. The water fed membrane bioreactor (SBMBR). Besides the advantages of rate of the two systems was the same and the SRT was 60 no sedimentation phase and thus a shorter cycle time, the days. combination may provide a suitable environment for both In the SBMBR system, the water was fed to the reactor nitrobacteria and phosphorus accumulating organisms during the anaerobic phase and the discharging of water (PAO) to survive under the appropriate operational condi- occurred during the aerobic phase. To make it work this way tions. This combination can also make it possible that the a timer-controlled power supply system was used and a Y- solids retention time (SRT) is independent of hydraulic shaped filter was installed before the electromagnetic valve retention time (HRT). Furthermore, the intermittent suction that connected to the influent pipeline. An agitator was used and the introduction of air diffusers near the membrane had for mixing during the anaerobic phase. The alternating been reported as the effective methods to reduce fouling and aerobic and anaerobic conditions in the SBMBR was created prolong membrane operation time and lifespan [13–16].It by the on and off control of the air pump. The air pump and also allowed good growth of strains with different doubling peristaltic pump worked simultaneously during the aerobic times. phase, while the electromagnetic valve placed in the influent The performance of the SBMBR and a conventional pipeline was closed. This resulted in discharge of water and membrane bioreactor (CMBR) on nutrient removal was the descending of water level in the reactor. For the anoxic/ compared at different influent COD/TN ratios from 3.4 to anaerobic phase, the air pump and peristaltic pump were 28.2. The suitable operation parameters under each closed and the agitator started to work to keep a complete condition were selected and membrane fouling monitored mixing of sludge and water. The feed water flowed into at the same time. the SBMBR by gravitational force. Its level was controlled

Fig. 1. Schematic diagram of experimental process. 中国科技论文在线 http://www.paper.edu.cn

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Table 1 Operation parameters in the experiment

Phase Operational days (days) TNin (mg/L) COD/TN HRT (h) fB Cycle time of SBMBR (min) Anaerobic Aerobic I 1–12 16.5 (2.4) 28.2 (3.1) 22 9.1 60 60 II 13–30 22.9 (1.8) 19.4 (1.5) 22 9.1 60 60 III 31–72 47.1 (6.7) 9.4 (2.7) 22 9.1 60 60 IV 73–110 75.6 (8.1) 6.3 (1.4) 22 9.1 60 60 V 111–168 75.6 (8.1) 6.3 (1.4) 22 12.2 40 120 VI 169–175 75.6 (8.1) 6.3 (1.4) 11 24.2 40 120 VII 176–209 122.8 (7.2) 3.4 (1.4) 11 24.2 40 120 VIII 210–236 122.8 (7.2) 3.4 (1.4) 7.3 36.4 40 120

Standard deviation is given between parentheses; TNin, inﬂuent TN concentration.

by the balance-box during this time. When this anaerobic removal of nitrogen and phosphorus. For the SBMBR, the phase was over the next cycle began with the aerobic phase. effluent of one cycle was collected in a beaker and the The CMBR was operated continuously in aerobic mixture used for analysis, while the instantaneous effluent environment through the whole experimental period. The was used for CMBR. influent and the effluent were not interrupted. Other conditions were the same as the SBMBR. 2.5.2. Cyclic studies This test was carried out when the performance of 2.2. Feed medium SBMBR was stable. It was applied to the SBMBR for optimizing nutrient removal, which includes a series of Synthetic wastewater fed to the reactors consisted of sampling and analysis during one cycle. t = 0 min was set for sucrose, NH4Cl, KH2PO4 and mineral solution containing the starting of the anaerobic phase. The sampling was done MgSO47H2O, CaCl22H2O, FeSO42H2O and NaCl. The by manual adjustment peristaltic pump during the anaerobic initial complete influent contained 400 mg COD/L, 20 mg + + phase. Cyclic studies could clarify the transformation of NH4 -N/L and 4 mg PO4 -P/L. Holding the COD and TP various pollutions during the cycle time and provide influent concentration, a gradual increase of TN concentra- information for adjusting the operation parameters. tion was adopted during the experiment (Table 1). 2.5.3. Specific nitrification rate test 2.3. Inocula + The specific nitrification rate (mmol NH4 -N/g VSS) was Two reactors were seeded with sludge from the feedback determined using two 0.5 L reactors in duplicate for the sludge tank of the municipal wastewater treatment plant sludge-nitrification performance in SBMBR and CMBR. located in Chun-liu, Dalian, pre-incubated before being Fifty millilitres mixed liquor was centrifuged at 3500 rpm equally divided into the above two reactors. The sludge for 10 min and the liquid phase was decanted. The concentration in both reactors was 2 g/L MLSS to ensure the concentrated sludge was resuspended in 400 mL synthetic same behavior of the sludge. wastewater with 50 mL pH 7.0 phosphoric acid buffer solution added. The temperature was constant at 25 8C. The air was injected. At intervals of 30 min samples were taken 2.4. Analyses + and immediately filtered. The content of NH4 -N was + + measured. According to the NH4 -N decreasing rate, Ammonia nitrogen (NH4 -N), nitrate nitrogen (NO3 -N), specific nitrification rate could be calculated. nitrite nitrogen (NO2 -N), total phosphorus (TP) and COD were analyzed according to the standard methods for the examination of water and wastewater (APHA) [17].The total organic carbon (TOC) and total nitrogen (TN) contents 3. Results were determined with a Total Organic Carbon analyzer 3.1. Performance of the SBMBR and CMBR (TOC-VCPH, SHIMADZU). Extracellular polymeric substances (EPS) were determined using the method reported by Zhang et al. [18]. 3.1.1. Organic substance removal Fig. 2 presents the varieties of COD concentration and its 2.5. Experiments carried out removal efficiency of the two systems during the whole operation period. Both SBMBR or CMBR showed good 2.5.1. Comparison experiments performance in organic carbon removal. The COD removals These experiments were conducted for the comparison averaged at 97.7 and 94.9%, respectively. These results between the capacities of SBMBR and CMBR in the indicated that change of COD/TN ratio in influent did not 中国科技论文在线 http://www.paper.edu.cn

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Fig. 2. Comparison of COD concentrations and removal efficiencies in the two systems. Symbols:(*) CMBR effluent; (~) SBMBR effluent; (*) CMBR COD removal; (~) SBMBR COD removal.

affect COD removal efficiency significantly for both efficiency climbed to 93.1%, was not effect by load increase reactors. (phase VI). Since the influent COD/TN ratio decreased further to 3.4 (TN load was 0.33 – 0.36 kg TN/(m3 days)) on + 3.1.2. Nitrogen and phosphorus removal the 176th day, an even lower NH4 -N and TN removal + Fig. 3 illustrates the profile of NH4 -N, TN and TP efficiency was obtained, averaged at 75.2 and 20.7%, concentrations as well as their removal efficiencies in the respectively (phase VII). On the 210th day, the fB was raised two systems during the whole experiment. It can be seen that again to enhance TN removal (TN load was 0.50 – in SBMBR, when COD/TN >19.4 (phases I and II) a high 0.60 kg TN/(m3 days)). TN removal was enhanced to + + NH4 -N removal efficiency (>90%) has been achieved. TN 36.4%, while a low removal efficiency of NH4 -N and TP removal in this period averaged at 91.5 and 93.3%, (<50%) occurred, and the effluent average concentration + respectively. NH4 -N removal efficiency decreased to about reached 70.1 mg/L (phase VIII). This result indicates that a 80% when the influent COD/TN ratio reduced to 9.4 (phase favorable nitrogen removal could not be reached under this III). On increasing the air flow rate from 100 to 150 L/h, the low COD/TN ratio. + NH4 -N removal increased rapidly to over 90% and the After lengthening the aerobic time on the 111th day, the + effluent NH4 -N concentration decreased to below 1 mg/L. SBMBR phosphorus removal worsened and the average This indicates that the higher TN load was the more drastic removalefficiencywas49.9%.SincethehigherfB wasadopted competition for dissolve oxygen between heterotrophic on the 169th day and organic carbon supply increased and bacteria with nitrifying bacteria. The increased intensity of the phosphorus removal recovered. Except phase V, the injected air enhanced the penetrability of oxygen into SBMBRphosphorusremovalefficiencywaskeptatabout90% biological flocs, with benefits for both types of organism. TN and the average removal efficiency was 91.4%. removal efficiency declined temporarily but soon recovered. As far as the CMBR was concerned, the TN and TP The removal of TP did not make significant change during removal efficiency was 30.5 and 35.3%, respectively, when this period. the influent COD/TN ratio was 19.4 and 28.2. During this + It was after 73 days when the influent COD/TN ratio time the NH4 -N average removal efficiency was 93.3%. + tuned down to 6.3 that the NH4 -N removal efficiency of This result indicated that the limiting step of nitrogen SBMBR started a sharp decrease and the maximum effluent removal in the CMBR was denitrification. TN concentration + NH4 -N concentration had reached to 76.1 mg/L (phase IV). in effluent was close to the level in the influent at COD/ On the 111th day, the SBMBR cycle time was adjusted to TN = 6.3 and TP removal efficiency fluctuated from 14 to 40 min anaerobic and 120 min aerobic (total 160 min) from 95% under influent experiment conditions. 60 min anaerobic and 60 min aerobic (total 120 min) (Table 1). This step was to enhance nitrification. Nitrifica- 3.2. Cyclic studies of SBMBR + tion improved and the NH4 -N removal efficiency increased + to 95% (Fig. 3(a)). But the average TN removal efficiency During phases I–III a higher removal efficiency of NH4 - was only 31.9% in this period as shown in Fig. 3(b) (phase N, TP and TN persisted in the SBMBR. The DO cyclic test V). In order to enhance denitrification and increase the of SBMBR on the 28th day (Fig. 4(a)) showed that after the organic carbon supply, a higher fB (24.2%) was adopted on aerobic period the massive high volume influent can the 169th day. The TN load was 0.22 kg TN/(m3 days). decrease the DO concentration quickly in reactor, thus Denitrification was enhanced and TN removal efficiency sufficient anaerobic time was ensured, which contributed to + was improved to 67.6%. As to NH4 -N, the average removal higher nitrogen and phosphorus removal in the SBMBR. 中国科技论文在线 http://www.paper.edu.cn

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+ Fig. 3. The comparison of nitrogen and phosphorus concentrations and removal efficiencies in the two systems: (a) NH4 -N, (b) TN and (c) TP. Symbols:(— *—) CMBR effluent; (—~—) SBMBR effluent; (—*—) CMBR removal efficiency; (—~—) SBMBR removal efficiency.

The cyclic test on the 109th day (Fig. 4(b)) showed that anaerobic and 120 min aerobic from 60 min anaerobic and + the time was not long enough for NH4 -N removal when 60 min aerobic. COD/TN ratio was 6.3 with the aerobic time-controlled at This and the cyclic test on the 167th day (Fig. 4(c)) 1 h (i.e. nitrification under this condition was not complete). indicates that nitrification is complete within 120 min The anaerobic time 60 min has a margin for denitrifying and aerobic time but denitrification was restrained. In order to phosphorus release. A shorter anaerobic time was therefore enhance denitrification without changing anaerobic time, a adopted in the following days to avoid the secondary higher fB (24.2%) was adopted to increase the organic phosphorus release. The cycle time was adjusted to 40 min carbon supply. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 5. Ammonia nitrogen degradation rate.

decrease pH value. The existence of an anoxic phase in the SBMBR provides a condition for denitrification, which counteracted the alkalinity consumption of nitrification. The effluent pH value of this reactor remained 7.0–8.0 during the whole experiment, which was just suitable for the nitrification reactions. The case in the CMBR was different. It suited the nitrification reaction but hardly any denitrification occurred. The higher influent TN load the more alkalinity was consumed. An obvious decline of effluent pH value was observed and a low pH value (5.0–6.0) appeared even at the COD/TN ratio of 9.4. Therefore, the nitrification + activity was inhibited which led to a lower NH4 -N removal. The specific nitrification rate test demonstrated that pH value could affect the activity of nitrobacteria but not be the destructive factor.

3.4. Sludge characterization

Sludge in the two reactors appeared different because of the different operation mode. The colour of the sludge was khaki in the CMBR and filemot in the SBMBR. Microscopic examination revealed a different community structure between the two systems. Categories of microbes were plentiful in the CMBR. Filamentous fungi and protozoa organisms, including Ciliates (mainly Aspidisca and Fig. 4. DO profile on the 28th (a) and nutrient profiles on the 109th (b) and Lionotus), Amoebae (mainly Amoeba radiosa) and Rotifers 167th (c) day, respectively, during cyclic tests. Symbols:(—*—) TP; (— were observed abundantly. Metazoans including Aeoloosma ~—) NH +-N; (—5—) NO -N. 4 3 hemprichii also appeared in this system. In the SBMBR, the dominant community of bacteria existed as Epistylis and 3.3. Specific nitrification rate test Vorticella. A small quantity of Aspidisca and Rotifers was found in this ecosystem. Fig. 6 displays the microscopy + In Fig. 5, the NH4 -N concentration versus time profile photographs of sludge in both reactors. from SNR batch test is shown. For CMBR and SBMBR, the + specific nitrification rate were 0.56 and 0.40 mmol NH4 -N/ 3.5. Membrane fouling (g VSS days), respectively. This indicates that in comparison with SBMBR, the sludge in the CMBR possesses a Membrane fouling is one of the main problems of MBR higher nitrification activity. systems. The avoidance of fouling or reduction of its severity As the pH value was considered, the optimal pH range for has been pursued for a long time. Researchers have reported the nitrobacteria is 7.0–8.0 and the result of nitrification is to that intermittent suction and low trans-membrane pressure 中国科技论文在线 http://www.paper.edu.cn

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Fig. 7. Trans-membrane pressure (TMP) proﬁles.

(TMP) helped to prevent irreversible damage to the membranes [16,19]. It has also been reported that extracellular polymeric substances (EPS) in MBR can increase the viscosity of the mixed liquid and the membrane filtration resistance, which would accelerate membrane fouling [20–23]. In this work, two parameters, TMP and EPS, were introduced to reflect the membrane fouling of the two experimental systems. As slight difference of TMP (TMP < 1 kPa) was observed between CMBR and SBMBR until a higher fB (Table 1) was conducted in phase VIII. It can be seen from Fig. 7 that a rapid increase of TMP occurred in the CMBR from less than 10 to about 60 kPa. Hydrocleaning was carried out on the 225th day and then the membrane filtration performance recovered. There is no significant increase of TMP value observed in the SBMBR before the 225th day. The sharp increase of TMP 17–54 kPa between the 226th and 227th day was caused by the malfunction of the air-injection system. When the malfunction was overcame and the membrane was cleaned with tap water, the membrane filtration performance recovered. This demonstrated that the membrane-fouling rate in the CMBR is greater than in the SBMBR, although the membrane

Fig. 6. Microscopically photographs of sludge in both reactors.

Fig. 8. The comparison of EPS concentration in mixed liquid. 中国科技论文在线 http://www.paper.edu.cn

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flux of the SBMBR was more than 1.33–2 times of the is ensured. Third, fB and cycles of the SBMBR can be CMBR. adjusted easily according to the nitrogen and phosphorus At the beginning of experiment, the MLSS was about concentration in the influent. 2 g/L in both systems. The activated sludge was discharged Attached Ciliates were always the dominant community according to the same SBR during all experiments. At the in the SBMBR, which indicated that the sequencing batch end of the experiment, the MLSS was almost the same in the operation is suitable for growth of attached Ciliates. The SBMBR and the CMBR, which reached to around 5.6 g/L. sludge floc would be bigger and tighter when the attached The EPS concentrations in both sludge systems varied Ciliates are dominant. The liquid filter capacity is better and between 340 and 500 mg/L as shown in Fig. 8. A higher this is one reason why the membrane-fouling rate of the concentration persisted in the SBMBR than in the CMBR SBMBR is lower than that of the CMBR. Another reason during the whole experimental examination. These results might be attributed to the sequencing batch operation mode. did not agree with the literature that there is a direct Agitation was introduced into the anaerobic phase when the correlation between EPS concentration and membrane suction was discontinued. As a result, the aggregation of fouling. substance onto the membrane surface was reduced but the diffusion of adsorbents from the membrane surface back to the liquid was enhanced. Therefore, EPS in the SBMBR was 4. Discussion always higher than that in the CMBR, but the membrane- fouling rate of the SBMBR was lower than that in the The CMBR and SBMBR have higher organics removal, CMBR. the average COD removal were 94.9 and 97.7%, respectively. COD removal decreased to about 80% as the experiment was close to the end (Fig. 2). The influent of the 5. Conclusions SBMBR is high and of high flow at the beginning of anaerobic time and fresh nutrients was not put into the Nutrient removal from synthetic wastewater was studied reactor during one cycle. The microorganism in the SBMBR using membrane bioreactors in both sequencing batch and experienced following the log phase, stationary phase and continuous operations. The main findings from this study are death phase. The influent of the CMBR was continuous. as follows: Along with the MLSS increasing and F:M decreasing, the microorganism of CMBR would get into death phase for (1) Both SBMBR and CMBR showed good performance on long time for lack of nutrients, need to rely on themselves for organic substance removal during the whole experi- obtaining energy, the products of metabolism were mental period. The average COD removals were 97.7 accumulated and led to worsening of the quality of the and 94.9%, respectively. Change of COD/TN ratio in final effluent in CMBR. influent did not affect COD removal efficiency After lengthening the aerobic time on the 111th day, significantly in either reactor. enhanced nitrification resulted in a high nitrate concentra- (2) The SBMBR demonstrated a good performance on tion in the anaerobic phase. Consequently, there is a nitrogen and phosphorus removal at different influent competition for the organic carbon between denitrifying COD/TN ratios. Even running at low COD/TN ratio bacteria and PAOs. The denitrifying bacteria using nitrate as (COD/TN = 6.3) with 0.22 kg TN/(m3 days), in sui- + electron acceptor can obtain more energy and were more table operation conditions, TN and NH4 -N removal competitive, thus organic carbon was not converted into efficiencies were maintained above 65 and 90%, VFAs with inhibition of phosphorus release of PAOs and respectively. As to TP, the removal efficiency was formation of PHB. During phase V, the phosphorus removal maintained at approximately 90% all through the worsened and the average removal efficiency decreased to whole experiment except for the starved organic 49.9%. The sequencing batch operation of the SBMBR in substrate feeding. this experiment can increase organic carbon supply of one (3) The variation of influent COD/TN ratio affected the cycle by increase fB and, does not require an exogenous CMBR more extensively than the SBMBR. TP removal supply of organic carbon. After adjusting fB, on 169th day, efficiency fluctuated between 14 and 95% under influent the TP average removal efficiency increased to about 90% experiment conditions and the TN concentration in the and TN removal improved. effluent was close to that in the influent at the feed ratio The CMBR showed some nitrogen and phosphorus COD/TN = 6.3. removal by the existing anaerobic microenvironment, but (4) Specific nitrification rate tests demonstrated that the pH the capacity was lower than that of the SBMBR. The reasons value could affect the activity of nitrifying bacteria with are: first, the sequencing batch operation of the SBMBR can no irreversible harm. offer alternative anaerobic/aerobic environments; second, (5) The sequencing batch operation mode can reduce the influent was high and of high volume at the beginning of membrane fouling although a higher EPS existed in the anaerobic time in the SBMBR and sufficient anaerobic time SBMBR system. 中国科技论文在线 http://www.paper.edu.cn

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