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Effect of temperature on the treatment of domestic wastewater with a staged anaerobic fluidized membrane bioreactor R. H. Yoo, J. H. Kim, P. L. McCarty and J. H. Bae
ABSTRACT A laboratory staged anaerobic fluidized membrane bioreactor (SAF-MBR) system was applied to the treatment of primary clarifier effluent from a domestic wastewater treatment plant with W
temperature decreasing from 25 to 10 C. At all temperatures and with a total hydraulic retention time of 2.3 h, overall chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) removals were 89% and 94% or higher, with permeate COD and BOD5 of 30 and 7 mg/L or lower, respectively. No noticeable negative effects of low temperature on organic removal were found, although a slight increase to 3 mg/L in volatile fatty acids concentrations in the effluent was observed. Biosolids production was 0.01–0.03 kg volatile suspended solids/kg COD, which is far less than that with aerobic processes. Although the rate of trans-membrane pressure at the membrane flux of 9 L/m2/h increased as temperature decreased, the SAF-MBR was operated for longer than 200
R. H. Yoo J. H. Kim P. L. McCarty J. H. Bae (corresponding author) Department of Environmental Engineering, Inha University, Namgu, Inharo 100, Incheon, Republic of Korea E-mail: [email protected] P. L. McCarty Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
d before chemical cleaning was needed. Electrical energy potential from combustion of the total methane production (gaseous and dissolved) was more than that required for system operation. Key words
| anaerobic fluidized bed bioreactor, membrane, sewage treatment, temperature
INTRODUCTION Anaerobic treatment of domestic wastewater is receiving increased attention because of the recognized potential for net energy recovery and low sludge production when compared with traditional aerobic processes (Skouteris et al. ; Smith et al. ; Stuckey ; Lin et al. ). Anaerobic membrane bioreactors (MBRs) have recently been demonstrated to be capable of achieving high effluent quality, even at low temperatures, thus demonstrating their potential for meeting more rigorous effluent quality requirements. Chemical oxygen demand (COD) removal efficiencies at the upper 90% level have been demonstrated for dilute synthetic wastewaters, even at low temperatures, but meeting similar efficiencies with more complex domestic wastewaters has proven to be more difficult, largely because of the slow hydrolysis rate for suspended organic matter and the higher content of refractory dissolved organic matter (Smith et al. ; Lin et al. ). Most studies have emphasized COD rather than biochemical oxygen demand (BOD) removal so that efficiency comparisons between domestic wastewater and highly biodegradable synthetic wastewaters are not very meaningful. doi: 10.2166/wst.2013.793
To determine the practical usefulness of anaerobic membrane bioreactors for treatment of domestic wastewater, studies with actual, rather than synthetic, wastewaters are necessary. An increasing number of laboratory and a few pilot plant studies are meeting this requirement (Lin et al. ). Most studies have been with the more traditional membrane bioreactors that use either internal or external membranes and also use either gas sparging or hydraulic shear to reduce membrane fouling. Hydraulic retention times (HRTs) have generally been 8 h or longer and with membrane fluxes lower than 15 L/m2/h (LMH). Most studies have been at temperatures of 20 C or higher. Studies at lower temperatures have generally resulted in reduced COD removal efficiencies and higher effluent COD. Better performance in terms of flux, COD removal, and lower HRTs are desirable to increase the likelihood of adaption of anaerobic MBRs to meet more stringent effluent requirements under the range of usual temperature conditions for municipal wastewaters. One possibility for improved performance at lower temperatures is a novel staged anaerobic fluidized W
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membrane bioreactor (SAF-MBR) system consisting of a granular activated carbon (GAC) anaerobic fluidized bed reactor (AFBR) followed by an anaerobic fluidized membrane bioreactor (AFMBR) (Kim et al. ). Control of membrane fouling is achieved in this system by a unique energy-efficient approach, the scouring of membranes by the movement of GAC particles across the membrane surfaces. The SAF-MBR has been applied for the treatment of primary-settled domestic wastewater pre-treated by 2 mm screening. At a total HRT of 2.3 h and 25 C, COD and BOD5 removals were 84–91% and 92–94%, with average effluent concentrations of 25 and 7 mg/L, respectively, for 310 d of operation (Bae et al. ). Besides energy economy, other advantages of the fluidized bed reactor are the high efficiency obtained at very short HRTs, and the addition of GAC sorption, which can provide trace organic removal as well as quicker reactor startup. Although the operation of the SAF-MBR was successful at 25 C, as previously reported by Bae et al. (), its applicability at lower temperatures needs to be addressed. This is because reduced bacterial activity at low temperatures may decrease suspended solids degradation as well as increase the soluble COD concentration, both of which might adversely affect the performance of membrane operation significantly (Lin et al. ; Trzcinski & Stuckey ). The objective of this study was to evaluate the effect of temperature from 25 C down to 10 C on the performance of the SAF-MBR system while treating settled domestic wastewater. Resulting effluent quality, sludge production, trans-membrane pressure, and net energy production at each temperature were compared.
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MATERIALS AND METHODS The laboratory-scale SAF-MBR system illustrated in Figure 1 has been described in detail by Yoo et al. (). In essence the 0.245 L AFBR (50 cm long and 25 mm diameter acryl tube) contained 30 g of 10 × 30 mesh fresh GAC as support medium for bacterial growth. The same sized AFMBR contained 54 g of fresh GAC and a submerged membrane module consisting of eight 0.45 m long, polyvinylidene fluoride hollow fiber microfiltration membranes (pore size 0.1 µm), with total surface area of 0.0215 m2. The SAF-MBR system was operated for over 14 months through four different temperature modes (25, 20, 15 and 10 C) with respective periods of operation of 99, 123, 180, and 36 d. Before Mode I, the SAF-MBR had been operating W
Figure 1
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Schematic diagram of the SAF-MBR system (after Yoo et al. 2012).
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for 210 d at 25 C under different operating conditions (Bae et al. ). The feed was primary effluent from a domestic wastewater treatment plant in Bucheon, Korea, which was taken at least once a week and stored under 4 C refrigeration. No changes in wastewater quality during the storage period were observed. Just prior to use, the wastewater was passed through a 2 mm screen as the only pre-treatment. Even though total suspended solids (TSS) rejection through the screen was negligible, concern was removed about potential harm from occasional large particles that might otherwise be present. The HRT of the AFBR and the AFMBR were 1.0 and 1.3 h, respectively, yielding a total HRT of 2.3 h. The membrane flux of the AFMBR was maintained at the previously determined (Yoo et al. ) sustainable flux of 9 L/m2/h (LMH). Fluidization of GAC was maintained with magnetic pumps to maintain the desired recirculation flow rates, which were 510 and 1,080 L/d for the AFBR and AFMBR, respectively. About 420 mL of AFMBR recycle fluid was withdrawn each week from the settling tank located at the top of the reactor to remove excess suspended biosolids production for disposal. AFBR effluent samples were obtained from the overflow line, and AFMBR bulk liquid samples from the settling tank located at the top of the reactor. Gas samples came from the gas bags attached to the reactor head space. COD, BOD5, TSS and volatile suspended solids (VSS) were determined according to procedures in Standard Methods (APHA ). Details of chemical analyses of sulfate, volatile fatty acids (VFAs), gas composition and dissolved methane are described by Yoo et al. (). W
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RESULTS AND DISCUSSION Organic removal and effluent quality Table 1 contains a summary of the steady-state performance of each reactor as well as of the overall SAF-MBR system. The AFBR achieved average COD removals of 58–72%, and with additional treatment by the AFMBR, the overall removal for the two-stage system averaged 89–93%. Effluent BOD5 averaged no more than 7 mg/L, and overall BOD5 removal efficiency was 94% or higher, even at 10 C. A major portion of the influent COD was removed by the AFBR, but this portion decreased from 66% at 25 C to 58% at 10 C. This resulted in a higher loading to the AFMBR, where the removal of the remaining COD increased from 67% at 25 C to 83% at 10 C. COD removal in the AFMBR was not only through biodegradation or sorption, but also through membrane filtration. Thus, even though VSS destruction was reduced at lower temperatures, as found with the AFBR, VSS were effectively removed by filtration in the AFMBR. This partly accounts for the increased percentage COD removal obtained in the AFMBR at lower temperatures. As a result, the treatment W
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Table 1
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efficiency in the AFMBR was relatively constant and unaffected by temperature. The system thus displayed resistance to changes in temperature as well as to organic loading rate (OLR). For example, the average OLR of the AFMBR increased from 1.2 kg COD/m3-d at 25 C to 2.3 kg COD/m3-d at 10 C; COD removal was not affected by either parameter. Although organic removal was excellent at low temperatures, a small increase in effluent total VFA concentration up to 3 mg/L was found especially at 15 C, which then might have been related to decreased microbial activity. The lack of further increase in VFA concentration at 10 C might be correlated to less food for VFA transforming organisms due to the reduced hydrolysis rate of VSS occurring then. Effluent VSS concentration was always near zero as a result of membrane filtration. The percentage of influent VSS destroyed in the AFBR was about the same at 25 and 20 C (74–76%), but decreased to 69% and 61% as temperature decreased to 15 and 10 C, respectively. Although much of the remaining VSS in the AFBR effluent was destroyed in the AFMBR, VSS in the bulk liquid of the AFMBR increased from 124 to 257 mg/L as temperature decreased from 15 to 10 C due to decreased hydrolysis rate. The removal of the W
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Steady-state performance of SAF-MBR system at different temperatures
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Mode I (25 C)
AFBR
AFMBR Overall W
Mode II (20 C)
AFBR
AFMBR Overall W
Mode III (15 C)
AFBR
AFMBR Overall W
Mode IV (10 C)
AFBR
AFMBR Overall
TSS
VSS
COD
BOD5
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
235 ± 44 66 ± 12 72 ± 7 22 ± 8 67 ± 14 91 ± 4
105 ± 28 30 ± 10 71 ± 12 6±2 80 ± 9 94 ± 2
93 ± 27 21 ± 7 77 ± 10 1±0 95 ± 2 99 ± 0
78 ± 23 19 ± 7 76 ± 11 1±0 95 ± 2 99 ± 0
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
273 ± 47 93 ± 25 66 ± 11 29 ± 8 69 ± 12 89 ± 3
133 ± 21 27 ± 9 80 ± 7 6±2 78 ± 10 95 ± 2
107 ± 34 27 ± 7 75 ± 10 0±1 100 ± 0 100 ± 0
88 ± 26 23 ± 6 74 ± 10 1±0 96 ± 1 99 ± 0
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
299 ± 47 111 ± 48 63 ± 17 26 ± 13 77 ± 15 91 ± 5
175 ± 44 49 ± 22 72 ± 14 7±5 86 ± 12 96 ± 3
126 ± 35 49 ± 12 61 ± 14 1±1 98 ± 2 99 ± 1
103 ± 29 32 ± 9 69 ± 12 1±0 97 ± 1 99 ± 0
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
300 ± 42 126 ± 24 58 ± 10 21 ± 7 83 ± 6 93 ± 3
198 ± 49 45 ± 27 77 ± 15 5±2 89 ± 8 97 ± 1
135 ± 32 55 ± 17 59 ± 16 1±1 98 ± 2 99 ± 1
110 ± 25 43 ± 12 61 ± 14 1±0 98 ± 1 99 ± 0
VFA
1±1 1±1
2±2 1±2
7±6 3±3
3±3 3±1
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remaining equivalent of 2.8–9.7 mg/L VSS was accomplished by withdrawing 420 mL/week of AFMBR bulk liquid from the recycle line, as well as by cleaning of the recycle line and sampling. This corresponds to 3.5–9.4% of the influent VSS, representing sludge production of only 0.012, 0.018, 0.032, and 0.038 g VSS/g COD for Mode I, II, III, and IV, respectively, which is far less than that from a typical aerobic treatment system of 0.42 g VSS/g COD (Rittmann & McCarty ). Although not great, increased sludge production at low temperature is another indication of the reduced hydrolysis rate at lower temperatures. Variations in other operational characteristics were as follows. The average influent sulfate concentration was similar in each mode with average and standard deviation of 64 ± 30 mg/L. Of the influent sulfate, about 50% was removed in the AFBR through sulfate reduction, and the remainder was removed in the AFMBR. Additionally, no significant difference between modes was observed in pH and alkalinity over the study period. Average influent pH and alkalinity were 7.01 ± 0.24 and 260 ± 12 mg/L, respectively. Effluent pH averaged 7.0 ± 0.1 in the AFBR and 7.2 ± 0.2 in the AFMBR, and effluent alkalinity averaged 237 ± 10 mg/L in the AFBR and 242 ± 14 mg/L in the AFMBR. The above pH values resulted without the need for chemical addition because of low reactor VFA and carbon dioxide concentrations. Gaseous carbon dioxide percentages resulting from treatment of very dilute wastewaters are generally much lower than found when treating higher influent COD concentrations because of the lower concentration produced and the high CO2 solubility, Also, a rather high nitrogen concentration is generally observed when treating dilute wastewaters, and results from evolution into the gas phase of N2 that is dissolved in influent wastewaters, where it is in equilibrium with atmospheric N2 (Kobayashi et al. ; Lettinga et al. ). In accordance with these observations, the gas phase carbon dioxide composition from the AFBR varied from 4.9 ± 1.9% (Mode I) to 11.6 ± 2.3% (Mode III), while that from the AFMBR varied from 6.3 ± 1.6% (Mode I) to 12.5 ± 1.5% (Mode III). Gas phase methane composition was slightly higher from the AFBR (37 ± 9% in Mode I and 63 ± 4% in Mode III) than from the AFMBR (29 ± 11% in Mode I and 54 ± 4% in Mode III). The gaseous nitrogen concentration, resulting from expulsion of dissolved N2 contained in the influent wastewater, decreased in the AFBR from 59 ± 7% (Mode I) to 25 ± 5% (Mode III), while similarly for the AFMBR, it decreased from 61 ± 12% (Mode I) to 34 ± 5% (Mode III). The
Figure 2
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COD mass balances at each operational mode.
changes found with temperature decrease are perhaps more a result of the significant increased influent BOD at decreased temperature, rather than with temperature itself. Calculated COD mass balances at each mode are illustrated in Figure 2, except for Mode IV for which sufficient data on dissolved methane concentrations had not been obtained. From a COD mass balance for Mode III, methane production, sulfate reduction, biosolids production, and effluent COD accounted for 54%, 15%, 5% and 9%, respectively, of the influent total COD. About 17% of the influent COD was unaccounted for, this could have resulted from some removal by sorption on the GAC (Weber et al. ) or simply from analytical errors normally associated with such large influent COD variations. The COD equivalent of dissolved methane was relatively constant, 58 and 55 mg COD/L in Mode I and III, respectively, despite the fact that methane solubility increases about 20% as temperature decreases from 25 to 15 C. On the other hand, gaseous methane production during Mode III was three times that during Mode I, mainly due to the elevated influent COD and BOD for Mode III. High influent COD is favourable in terms of methane production and recovery. Effluent dissolved methane should be removed and destroyed, preferably through use in energy production, as it is a strong greenhouse gas that should not be allowed to escape to the atmosphere. W
Membrane fouling control The AFMBR was operated continuously for 650 d through this and the previous study (Yoo et al. ) with chemical washings only at 396 d and 613 d. Additionally, a 2 h membrane relaxation period was applied occasionally when the trans-membrane pressure (TMP) jumped rapidly, a problem mainly caused by incomplete GAC fluidization following a pumping problem. TMP usually returned to its previous level after a relaxation period. The normally long-term
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Effect of temperature on the treatment of domestic wastewater with SAF-MBR
fouling-free operation observed resulted from the scouring effect of fluidized GAC against the membranes (Kim et al. ). During Mode I, the TMP increased from 0.095 to 0.122 bar over a period of 100 d, as illustrated in Figure 3. During Mode II, TMP increased rather rapidly, and membrane relaxation was effective for only a limited period. This suggested irreversible fouling had become significant, and so chemical cleaning was performed at 396 d by soaking the membranes sequentially in 1% solutions of NaOCl, citric acid, and NaOH for 3 h for each treatment. TMP then decreased below 0.1 bar for the rest of Mode II. During Mode III at 15 C, the rate of TMP increase was higher than during Modes I and II, and TMP reached 0.2 bar after 200 d of operation. Then, membrane relaxation was no longer effective, the TMP rose to over 0.3 bar within 20 h after a relaxation period. Another chemical cleaning was conducted at the end of Mode III, after which Mode IV operation at 10 C was begun. To quantify the effect of temperature on membrane fouling, the rate of TMP increase for each temperature was determined from regression analysis. For such analysis, all TMP data except those just before and just after a 2 h relaxation period were used. The TMP rate increases in Modes I and II were the same or 0.0004 bar/d, while in Modes III and IV, they were higher at 0.0008 and 0.0510 bar/d, respectively. At the Mode IV temperature of 10 C, TMP reached 0.3 bar within 36 d of operation, indicating that GAC scouring alone had limited fouling control capacity at this temperature and flux. Jiang et al. () also reported that a submerged membrane bioreactor suffered greater fouling at lower temperatures, which they hypothesized resulted from a decreased biodegradation rate, reduced back-transport of fouling materials from the membrane surface, increased viscosity, and a higher deflocculation rate. This problem could perhaps be reduced through operation at a somewhat lower membrane flux rate. W
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A question has been raised in a review (Smith et al. ) about the long-term effect on membrane material when GAC fluidization is used to control membrane fouling. However, no adverse impact has yet been noted through about 2 years of operation with this reactor in the combination of this and a previous study (Yoo et al. ), as well as during similar periods in others of our studies. Even so, this is a significant issue that will need continued evaluation. Energy balance The major energy requirement for the SAF-MBR system is for recycling of reactor liquid to fluidize the GAC, which is proportional to the product of the total reactor flow rate and the hydraulic head loss being pumped against (Kim et al. ). Using the procedure they outlined, the electrical energy requirements for GAC fluidization and for permeate flow in this laboratory-scale system were calculated to be 0.012 and 0.037 kWh/m3 for the AFBR and AFMBR, respectively, resulting in a total electrical energy requirement of 0.049 kWh/m3. More energy for the AFMBR is needed to fully fluidize the GAC over the membrane surfaces, while only 40% GAC expansion was needed in the AFBR to achieve adequate mass transfer efficiency. Electrical energy can be produced from combustion of the produced methane. Gaseous methane production from both the AFBR and AFMBR during Mode III was sufficient to generate 0.119 kWh/m3 of electrical energy, assuming an efficiency of converting methane to electricity in a co-generating system of about 33%. This is more than the 0.049 kWh/m3 required to operate the SAF-MBR system. Furthermore, an additional 0.063 kWh/m3 of electricity could be generated from the effluent dissolved methane and more still from anaerobic treatment of primary sludge, which together could provide excess electrical energy. Capturing dissolved methane is essential, not only because it is a potential energy source, but also because it is a strong greenhouse gas that should not be allowed to escape to the atmosphere. However, some energy might be required for management of dissolved methane and sulfide from the effluent, a requirement yet needing to be addressed.
CONCLUSIONS The SAF-MBR system was applied for the treatment of a primary-settled domestic wastewater at a total HRT of 2.3 h with temperature decreasing from 25 to 10 C. Overall COD and BOD5 removals were little affected by low W
Figure 3
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Variations in trans-membrane pressure with time and operational mode.
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temperature, and averaged higher than 89% and 94% with average effluent concentrations equal to or lower than 30 and 7 mg/L, respectively. Biosolids production was estimated to be 0.01–0.03 g VSS/g COD; far less than that for a comparable aerobic system. Although the rate of TMP increase doubled with temperature decrease from 20 to 15 C, the SAF-MBR was operated for longer than 200 d before chemical cleaning was required because of the efficient scouring effect of the GAC particles on the membrane surfaces. However, a sixfold TMP increase rate was found as temperature decreased from 15 to 10 C, indicating that the increased membrane fouling at such temperatures needs to be considered in the design. For such temperatures a reduced membrane flux may be desirable. For the influent COD of 235–300 mg/L, electrical energy potential from combustion of all methane produced (gaseous and dissolved) was 3.7 times that required for total system operation. Therefore, the SAF-MBR system has good potential as a low-energy, low-biosolids-producing, high-efficiency domestic wastewater treatment system for wastewater temperatures of 10 C and above. W
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ACKNOWLEDGEMENTS This research was supported by Korean Ministry of the Environment as ‘Global Top Project’ (Project No: GT–12– B–01–012–0) and the World Class University Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology (grant number R33-2008-000-10043-0).
REFERENCES APHA Standard Methods for the Examination of Water and Wastewater, 20th edn. American Public Health Association, Washington, DC. Bae, J., Yoo, R., Lee, E. & McCarty, P. L. Two-staged anaerobic fluidized-bed membrane bioreactor treatment of settled domestic wastewater. Water Science & Technology 68 (2), 394–399.
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Jiang, T., Kennedy, M. D., Guinzbourg, B. F., Vanrolleghem, P. A. & Schippers, J. C. Optimising the operation of a MBR pilot plant by quantitative analysis of the membrane fouling mechanism. Water Science and Technology 51 (6–7), 19–25. Kim, J., Kim, K., Ye, H., Lee, E., Shin, C., McCarty, P. L. & Bae, J. Anaerobic fluidized bed membrane bioreactor for wastewater treatment. Environmental Science and Technology 45 (2), 576–581. Kobayashi, H. A., Stenstrom, M. K. & Mah, R. A. Treatment of low strength domestic wastewater using the anaerobic filter. Water Research 17 (8), 903–909. Lettinga, G., Roersma, R. & Grin, P. Anaerobic treatment of raw domestic sewage at ambient-temperatures using a granular bed UASB reactor. Biotechnology and Bioengineering 25 (7), 1701–1723. Lin, H. J., Xie, K., Mahendran, B., Bagley, D. M., Leung, K. T., Liss, S. N. & Liao, B. Q. Sludge properties and their effects on membrane fouling in submerged anaerobic membrane bioreactors (SAnMBRs). Water Research 43 (15), 3827–3837. Lin, H., Peng, W., Zhang, M., Chen, J., Hong, H. & Zhang, Y. A review on anaerobic membrane bioreactors: applications, membrane fouling and future perspectives. Desalination 314, 169–188. Rittmann, B. E. & McCarty, P. L. Environmental Biotechnology, Principles and Applications. McGraw-Hill, New York, 754 pp. Skouteris, G., Hermosilla, D., Lopez, P., Negro, C. & Blanco, A. Anaerobic membrane bioreactors for wastewater treatment: a review. Chemical Engineering Journal 198, 138–148. Smith, A. L., Stadler, L. B., Love, N. G., Skerlos, S. J. & Raskin, L. Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: a critical review. Bioresource Technology 122, 149–159. Stuckey, D. C. Recent developments in anaerobic membrane reactors. Bioresource Technology 122, 137–148. Trzcinski, A. P. & Stuckey, D. C. Treatment of municipal solid waste leachate using a submerged anaerobic membrane bioreactor at mesophilic and psychrophilic temperatures: analysis of recalcitrants in the permeate using GC–MS. Water Research 44 (3), 671–680. Weber Jr., W. J., Hopkins, C. B. & Bloom Jr., R. Physicochemical treatment of wastewater. Journal of Water Pollution Control Federation 42, 83–99. Yoo, R., Kim, J., McCarty, P. L. & Bae, J. Anaerobic treatment of municipal wastewater with a staged anaerobic fluidized membrane bioreactor (SAF-MBR) system. Bioresource Technology 120, 133–139.
First received 13 July 2013; accepted in revised form 4 December 2013. Available online 18 December 2013
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© IWA Publishing 2014 Water Science & Technology
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Effect of temperature on the treatment of domestic wastewater with a staged anaerobic fluidized membrane bioreactor R. H. Yoo, J. H. Kim, P. L. McCarty and J. H. Bae
ABSTRACT A laboratory staged anaerobic fluidized membrane bioreactor (SAF-MBR) system was applied to the treatment of primary clarifier effluent from a domestic wastewater treatment plant with W
temperature decreasing from 25 to 10 C. At all temperatures and with a total hydraulic retention time of 2.3 h, overall chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) removals were 89% and 94% or higher, with permeate COD and BOD5 of 30 and 7 mg/L or lower, respectively. No noticeable negative effects of low temperature on organic removal were found, although a slight increase to 3 mg/L in volatile fatty acids concentrations in the effluent was observed. Biosolids production was 0.01–0.03 kg volatile suspended solids/kg COD, which is far less than that with aerobic processes. Although the rate of trans-membrane pressure at the membrane flux of 9 L/m2/h increased as temperature decreased, the SAF-MBR was operated for longer than 200
R. H. Yoo J. H. Kim P. L. McCarty J. H. Bae (corresponding author) Department of Environmental Engineering, Inha University, Namgu, Inharo 100, Incheon, Republic of Korea E-mail: [email protected] P. L. McCarty Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
d before chemical cleaning was needed. Electrical energy potential from combustion of the total methane production (gaseous and dissolved) was more than that required for system operation. Key words
| anaerobic fluidized bed bioreactor, membrane, sewage treatment, temperature
INTRODUCTION Anaerobic treatment of domestic wastewater is receiving increased attention because of the recognized potential for net energy recovery and low sludge production when compared with traditional aerobic processes (Skouteris et al. ; Smith et al. ; Stuckey ; Lin et al. ). Anaerobic membrane bioreactors (MBRs) have recently been demonstrated to be capable of achieving high effluent quality, even at low temperatures, thus demonstrating their potential for meeting more rigorous effluent quality requirements. Chemical oxygen demand (COD) removal efficiencies at the upper 90% level have been demonstrated for dilute synthetic wastewaters, even at low temperatures, but meeting similar efficiencies with more complex domestic wastewaters has proven to be more difficult, largely because of the slow hydrolysis rate for suspended organic matter and the higher content of refractory dissolved organic matter (Smith et al. ; Lin et al. ). Most studies have emphasized COD rather than biochemical oxygen demand (BOD) removal so that efficiency comparisons between domestic wastewater and highly biodegradable synthetic wastewaters are not very meaningful. doi: 10.2166/wst.2013.793
To determine the practical usefulness of anaerobic membrane bioreactors for treatment of domestic wastewater, studies with actual, rather than synthetic, wastewaters are necessary. An increasing number of laboratory and a few pilot plant studies are meeting this requirement (Lin et al. ). Most studies have been with the more traditional membrane bioreactors that use either internal or external membranes and also use either gas sparging or hydraulic shear to reduce membrane fouling. Hydraulic retention times (HRTs) have generally been 8 h or longer and with membrane fluxes lower than 15 L/m2/h (LMH). Most studies have been at temperatures of 20 C or higher. Studies at lower temperatures have generally resulted in reduced COD removal efficiencies and higher effluent COD. Better performance in terms of flux, COD removal, and lower HRTs are desirable to increase the likelihood of adaption of anaerobic MBRs to meet more stringent effluent requirements under the range of usual temperature conditions for municipal wastewaters. One possibility for improved performance at lower temperatures is a novel staged anaerobic fluidized W
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membrane bioreactor (SAF-MBR) system consisting of a granular activated carbon (GAC) anaerobic fluidized bed reactor (AFBR) followed by an anaerobic fluidized membrane bioreactor (AFMBR) (Kim et al. ). Control of membrane fouling is achieved in this system by a unique energy-efficient approach, the scouring of membranes by the movement of GAC particles across the membrane surfaces. The SAF-MBR has been applied for the treatment of primary-settled domestic wastewater pre-treated by 2 mm screening. At a total HRT of 2.3 h and 25 C, COD and BOD5 removals were 84–91% and 92–94%, with average effluent concentrations of 25 and 7 mg/L, respectively, for 310 d of operation (Bae et al. ). Besides energy economy, other advantages of the fluidized bed reactor are the high efficiency obtained at very short HRTs, and the addition of GAC sorption, which can provide trace organic removal as well as quicker reactor startup. Although the operation of the SAF-MBR was successful at 25 C, as previously reported by Bae et al. (), its applicability at lower temperatures needs to be addressed. This is because reduced bacterial activity at low temperatures may decrease suspended solids degradation as well as increase the soluble COD concentration, both of which might adversely affect the performance of membrane operation significantly (Lin et al. ; Trzcinski & Stuckey ). The objective of this study was to evaluate the effect of temperature from 25 C down to 10 C on the performance of the SAF-MBR system while treating settled domestic wastewater. Resulting effluent quality, sludge production, trans-membrane pressure, and net energy production at each temperature were compared.
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MATERIALS AND METHODS The laboratory-scale SAF-MBR system illustrated in Figure 1 has been described in detail by Yoo et al. (). In essence the 0.245 L AFBR (50 cm long and 25 mm diameter acryl tube) contained 30 g of 10 × 30 mesh fresh GAC as support medium for bacterial growth. The same sized AFMBR contained 54 g of fresh GAC and a submerged membrane module consisting of eight 0.45 m long, polyvinylidene fluoride hollow fiber microfiltration membranes (pore size 0.1 µm), with total surface area of 0.0215 m2. The SAF-MBR system was operated for over 14 months through four different temperature modes (25, 20, 15 and 10 C) with respective periods of operation of 99, 123, 180, and 36 d. Before Mode I, the SAF-MBR had been operating W
Figure 1
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Schematic diagram of the SAF-MBR system (after Yoo et al. 2012).
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for 210 d at 25 C under different operating conditions (Bae et al. ). The feed was primary effluent from a domestic wastewater treatment plant in Bucheon, Korea, which was taken at least once a week and stored under 4 C refrigeration. No changes in wastewater quality during the storage period were observed. Just prior to use, the wastewater was passed through a 2 mm screen as the only pre-treatment. Even though total suspended solids (TSS) rejection through the screen was negligible, concern was removed about potential harm from occasional large particles that might otherwise be present. The HRT of the AFBR and the AFMBR were 1.0 and 1.3 h, respectively, yielding a total HRT of 2.3 h. The membrane flux of the AFMBR was maintained at the previously determined (Yoo et al. ) sustainable flux of 9 L/m2/h (LMH). Fluidization of GAC was maintained with magnetic pumps to maintain the desired recirculation flow rates, which were 510 and 1,080 L/d for the AFBR and AFMBR, respectively. About 420 mL of AFMBR recycle fluid was withdrawn each week from the settling tank located at the top of the reactor to remove excess suspended biosolids production for disposal. AFBR effluent samples were obtained from the overflow line, and AFMBR bulk liquid samples from the settling tank located at the top of the reactor. Gas samples came from the gas bags attached to the reactor head space. COD, BOD5, TSS and volatile suspended solids (VSS) were determined according to procedures in Standard Methods (APHA ). Details of chemical analyses of sulfate, volatile fatty acids (VFAs), gas composition and dissolved methane are described by Yoo et al. (). W
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RESULTS AND DISCUSSION Organic removal and effluent quality Table 1 contains a summary of the steady-state performance of each reactor as well as of the overall SAF-MBR system. The AFBR achieved average COD removals of 58–72%, and with additional treatment by the AFMBR, the overall removal for the two-stage system averaged 89–93%. Effluent BOD5 averaged no more than 7 mg/L, and overall BOD5 removal efficiency was 94% or higher, even at 10 C. A major portion of the influent COD was removed by the AFBR, but this portion decreased from 66% at 25 C to 58% at 10 C. This resulted in a higher loading to the AFMBR, where the removal of the remaining COD increased from 67% at 25 C to 83% at 10 C. COD removal in the AFMBR was not only through biodegradation or sorption, but also through membrane filtration. Thus, even though VSS destruction was reduced at lower temperatures, as found with the AFBR, VSS were effectively removed by filtration in the AFMBR. This partly accounts for the increased percentage COD removal obtained in the AFMBR at lower temperatures. As a result, the treatment W
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Table 1
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efficiency in the AFMBR was relatively constant and unaffected by temperature. The system thus displayed resistance to changes in temperature as well as to organic loading rate (OLR). For example, the average OLR of the AFMBR increased from 1.2 kg COD/m3-d at 25 C to 2.3 kg COD/m3-d at 10 C; COD removal was not affected by either parameter. Although organic removal was excellent at low temperatures, a small increase in effluent total VFA concentration up to 3 mg/L was found especially at 15 C, which then might have been related to decreased microbial activity. The lack of further increase in VFA concentration at 10 C might be correlated to less food for VFA transforming organisms due to the reduced hydrolysis rate of VSS occurring then. Effluent VSS concentration was always near zero as a result of membrane filtration. The percentage of influent VSS destroyed in the AFBR was about the same at 25 and 20 C (74–76%), but decreased to 69% and 61% as temperature decreased to 15 and 10 C, respectively. Although much of the remaining VSS in the AFBR effluent was destroyed in the AFMBR, VSS in the bulk liquid of the AFMBR increased from 124 to 257 mg/L as temperature decreased from 15 to 10 C due to decreased hydrolysis rate. The removal of the W
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Steady-state performance of SAF-MBR system at different temperatures
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Mode I (25 C)
AFBR
AFMBR Overall W
Mode II (20 C)
AFBR
AFMBR Overall W
Mode III (15 C)
AFBR
AFMBR Overall W
Mode IV (10 C)
AFBR
AFMBR Overall
TSS
VSS
COD
BOD5
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
235 ± 44 66 ± 12 72 ± 7 22 ± 8 67 ± 14 91 ± 4
105 ± 28 30 ± 10 71 ± 12 6±2 80 ± 9 94 ± 2
93 ± 27 21 ± 7 77 ± 10 1±0 95 ± 2 99 ± 0
78 ± 23 19 ± 7 76 ± 11 1±0 95 ± 2 99 ± 0
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
273 ± 47 93 ± 25 66 ± 11 29 ± 8 69 ± 12 89 ± 3
133 ± 21 27 ± 9 80 ± 7 6±2 78 ± 10 95 ± 2
107 ± 34 27 ± 7 75 ± 10 0±1 100 ± 0 100 ± 0
88 ± 26 23 ± 6 74 ± 10 1±0 96 ± 1 99 ± 0
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
299 ± 47 111 ± 48 63 ± 17 26 ± 13 77 ± 15 91 ± 5
175 ± 44 49 ± 22 72 ± 14 7±5 86 ± 12 96 ± 3
126 ± 35 49 ± 12 61 ± 14 1±1 98 ± 2 99 ± 1
103 ± 29 32 ± 9 69 ± 12 1±0 97 ± 1 99 ± 0
Inf. (mg/L) Eff. (mg/L) Rem. (%) Eff. (mg/L) Rem. (%) Rem. (%)
300 ± 42 126 ± 24 58 ± 10 21 ± 7 83 ± 6 93 ± 3
198 ± 49 45 ± 27 77 ± 15 5±2 89 ± 8 97 ± 1
135 ± 32 55 ± 17 59 ± 16 1±1 98 ± 2 99 ± 1
110 ± 25 43 ± 12 61 ± 14 1±0 98 ± 1 99 ± 0
VFA
1±1 1±1
2±2 1±2
7±6 3±3
3±3 3±1
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remaining equivalent of 2.8–9.7 mg/L VSS was accomplished by withdrawing 420 mL/week of AFMBR bulk liquid from the recycle line, as well as by cleaning of the recycle line and sampling. This corresponds to 3.5–9.4% of the influent VSS, representing sludge production of only 0.012, 0.018, 0.032, and 0.038 g VSS/g COD for Mode I, II, III, and IV, respectively, which is far less than that from a typical aerobic treatment system of 0.42 g VSS/g COD (Rittmann & McCarty ). Although not great, increased sludge production at low temperature is another indication of the reduced hydrolysis rate at lower temperatures. Variations in other operational characteristics were as follows. The average influent sulfate concentration was similar in each mode with average and standard deviation of 64 ± 30 mg/L. Of the influent sulfate, about 50% was removed in the AFBR through sulfate reduction, and the remainder was removed in the AFMBR. Additionally, no significant difference between modes was observed in pH and alkalinity over the study period. Average influent pH and alkalinity were 7.01 ± 0.24 and 260 ± 12 mg/L, respectively. Effluent pH averaged 7.0 ± 0.1 in the AFBR and 7.2 ± 0.2 in the AFMBR, and effluent alkalinity averaged 237 ± 10 mg/L in the AFBR and 242 ± 14 mg/L in the AFMBR. The above pH values resulted without the need for chemical addition because of low reactor VFA and carbon dioxide concentrations. Gaseous carbon dioxide percentages resulting from treatment of very dilute wastewaters are generally much lower than found when treating higher influent COD concentrations because of the lower concentration produced and the high CO2 solubility, Also, a rather high nitrogen concentration is generally observed when treating dilute wastewaters, and results from evolution into the gas phase of N2 that is dissolved in influent wastewaters, where it is in equilibrium with atmospheric N2 (Kobayashi et al. ; Lettinga et al. ). In accordance with these observations, the gas phase carbon dioxide composition from the AFBR varied from 4.9 ± 1.9% (Mode I) to 11.6 ± 2.3% (Mode III), while that from the AFMBR varied from 6.3 ± 1.6% (Mode I) to 12.5 ± 1.5% (Mode III). Gas phase methane composition was slightly higher from the AFBR (37 ± 9% in Mode I and 63 ± 4% in Mode III) than from the AFMBR (29 ± 11% in Mode I and 54 ± 4% in Mode III). The gaseous nitrogen concentration, resulting from expulsion of dissolved N2 contained in the influent wastewater, decreased in the AFBR from 59 ± 7% (Mode I) to 25 ± 5% (Mode III), while similarly for the AFMBR, it decreased from 61 ± 12% (Mode I) to 34 ± 5% (Mode III). The
Figure 2
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COD mass balances at each operational mode.
changes found with temperature decrease are perhaps more a result of the significant increased influent BOD at decreased temperature, rather than with temperature itself. Calculated COD mass balances at each mode are illustrated in Figure 2, except for Mode IV for which sufficient data on dissolved methane concentrations had not been obtained. From a COD mass balance for Mode III, methane production, sulfate reduction, biosolids production, and effluent COD accounted for 54%, 15%, 5% and 9%, respectively, of the influent total COD. About 17% of the influent COD was unaccounted for, this could have resulted from some removal by sorption on the GAC (Weber et al. ) or simply from analytical errors normally associated with such large influent COD variations. The COD equivalent of dissolved methane was relatively constant, 58 and 55 mg COD/L in Mode I and III, respectively, despite the fact that methane solubility increases about 20% as temperature decreases from 25 to 15 C. On the other hand, gaseous methane production during Mode III was three times that during Mode I, mainly due to the elevated influent COD and BOD for Mode III. High influent COD is favourable in terms of methane production and recovery. Effluent dissolved methane should be removed and destroyed, preferably through use in energy production, as it is a strong greenhouse gas that should not be allowed to escape to the atmosphere. W
Membrane fouling control The AFMBR was operated continuously for 650 d through this and the previous study (Yoo et al. ) with chemical washings only at 396 d and 613 d. Additionally, a 2 h membrane relaxation period was applied occasionally when the trans-membrane pressure (TMP) jumped rapidly, a problem mainly caused by incomplete GAC fluidization following a pumping problem. TMP usually returned to its previous level after a relaxation period. The normally long-term
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fouling-free operation observed resulted from the scouring effect of fluidized GAC against the membranes (Kim et al. ). During Mode I, the TMP increased from 0.095 to 0.122 bar over a period of 100 d, as illustrated in Figure 3. During Mode II, TMP increased rather rapidly, and membrane relaxation was effective for only a limited period. This suggested irreversible fouling had become significant, and so chemical cleaning was performed at 396 d by soaking the membranes sequentially in 1% solutions of NaOCl, citric acid, and NaOH for 3 h for each treatment. TMP then decreased below 0.1 bar for the rest of Mode II. During Mode III at 15 C, the rate of TMP increase was higher than during Modes I and II, and TMP reached 0.2 bar after 200 d of operation. Then, membrane relaxation was no longer effective, the TMP rose to over 0.3 bar within 20 h after a relaxation period. Another chemical cleaning was conducted at the end of Mode III, after which Mode IV operation at 10 C was begun. To quantify the effect of temperature on membrane fouling, the rate of TMP increase for each temperature was determined from regression analysis. For such analysis, all TMP data except those just before and just after a 2 h relaxation period were used. The TMP rate increases in Modes I and II were the same or 0.0004 bar/d, while in Modes III and IV, they were higher at 0.0008 and 0.0510 bar/d, respectively. At the Mode IV temperature of 10 C, TMP reached 0.3 bar within 36 d of operation, indicating that GAC scouring alone had limited fouling control capacity at this temperature and flux. Jiang et al. () also reported that a submerged membrane bioreactor suffered greater fouling at lower temperatures, which they hypothesized resulted from a decreased biodegradation rate, reduced back-transport of fouling materials from the membrane surface, increased viscosity, and a higher deflocculation rate. This problem could perhaps be reduced through operation at a somewhat lower membrane flux rate. W
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A question has been raised in a review (Smith et al. ) about the long-term effect on membrane material when GAC fluidization is used to control membrane fouling. However, no adverse impact has yet been noted through about 2 years of operation with this reactor in the combination of this and a previous study (Yoo et al. ), as well as during similar periods in others of our studies. Even so, this is a significant issue that will need continued evaluation. Energy balance The major energy requirement for the SAF-MBR system is for recycling of reactor liquid to fluidize the GAC, which is proportional to the product of the total reactor flow rate and the hydraulic head loss being pumped against (Kim et al. ). Using the procedure they outlined, the electrical energy requirements for GAC fluidization and for permeate flow in this laboratory-scale system were calculated to be 0.012 and 0.037 kWh/m3 for the AFBR and AFMBR, respectively, resulting in a total electrical energy requirement of 0.049 kWh/m3. More energy for the AFMBR is needed to fully fluidize the GAC over the membrane surfaces, while only 40% GAC expansion was needed in the AFBR to achieve adequate mass transfer efficiency. Electrical energy can be produced from combustion of the produced methane. Gaseous methane production from both the AFBR and AFMBR during Mode III was sufficient to generate 0.119 kWh/m3 of electrical energy, assuming an efficiency of converting methane to electricity in a co-generating system of about 33%. This is more than the 0.049 kWh/m3 required to operate the SAF-MBR system. Furthermore, an additional 0.063 kWh/m3 of electricity could be generated from the effluent dissolved methane and more still from anaerobic treatment of primary sludge, which together could provide excess electrical energy. Capturing dissolved methane is essential, not only because it is a potential energy source, but also because it is a strong greenhouse gas that should not be allowed to escape to the atmosphere. However, some energy might be required for management of dissolved methane and sulfide from the effluent, a requirement yet needing to be addressed.
CONCLUSIONS The SAF-MBR system was applied for the treatment of a primary-settled domestic wastewater at a total HRT of 2.3 h with temperature decreasing from 25 to 10 C. Overall COD and BOD5 removals were little affected by low W
Figure 3
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Variations in trans-membrane pressure with time and operational mode.
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temperature, and averaged higher than 89% and 94% with average effluent concentrations equal to or lower than 30 and 7 mg/L, respectively. Biosolids production was estimated to be 0.01–0.03 g VSS/g COD; far less than that for a comparable aerobic system. Although the rate of TMP increase doubled with temperature decrease from 20 to 15 C, the SAF-MBR was operated for longer than 200 d before chemical cleaning was required because of the efficient scouring effect of the GAC particles on the membrane surfaces. However, a sixfold TMP increase rate was found as temperature decreased from 15 to 10 C, indicating that the increased membrane fouling at such temperatures needs to be considered in the design. For such temperatures a reduced membrane flux may be desirable. For the influent COD of 235–300 mg/L, electrical energy potential from combustion of all methane produced (gaseous and dissolved) was 3.7 times that required for total system operation. Therefore, the SAF-MBR system has good potential as a low-energy, low-biosolids-producing, high-efficiency domestic wastewater treatment system for wastewater temperatures of 10 C and above. W
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ACKNOWLEDGEMENTS This research was supported by Korean Ministry of the Environment as ‘Global Top Project’ (Project No: GT–12– B–01–012–0) and the World Class University Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology (grant number R33-2008-000-10043-0).
REFERENCES APHA Standard Methods for the Examination of Water and Wastewater, 20th edn. American Public Health Association, Washington, DC. Bae, J., Yoo, R., Lee, E. & McCarty, P. L. Two-staged anaerobic fluidized-bed membrane bioreactor treatment of settled domestic wastewater. Water Science & Technology 68 (2), 394–399.
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Jiang, T., Kennedy, M. D., Guinzbourg, B. F., Vanrolleghem, P. A. & Schippers, J. C. Optimising the operation of a MBR pilot plant by quantitative analysis of the membrane fouling mechanism. Water Science and Technology 51 (6–7), 19–25. Kim, J., Kim, K., Ye, H., Lee, E., Shin, C., McCarty, P. L. & Bae, J. Anaerobic fluidized bed membrane bioreactor for wastewater treatment. Environmental Science and Technology 45 (2), 576–581. Kobayashi, H. A., Stenstrom, M. K. & Mah, R. A. Treatment of low strength domestic wastewater using the anaerobic filter. Water Research 17 (8), 903–909. Lettinga, G., Roersma, R. & Grin, P. Anaerobic treatment of raw domestic sewage at ambient-temperatures using a granular bed UASB reactor. Biotechnology and Bioengineering 25 (7), 1701–1723. Lin, H. J., Xie, K., Mahendran, B., Bagley, D. M., Leung, K. T., Liss, S. N. & Liao, B. Q. Sludge properties and their effects on membrane fouling in submerged anaerobic membrane bioreactors (SAnMBRs). Water Research 43 (15), 3827–3837. Lin, H., Peng, W., Zhang, M., Chen, J., Hong, H. & Zhang, Y. A review on anaerobic membrane bioreactors: applications, membrane fouling and future perspectives. Desalination 314, 169–188. Rittmann, B. E. & McCarty, P. L. Environmental Biotechnology, Principles and Applications. McGraw-Hill, New York, 754 pp. Skouteris, G., Hermosilla, D., Lopez, P., Negro, C. & Blanco, A. Anaerobic membrane bioreactors for wastewater treatment: a review. Chemical Engineering Journal 198, 138–148. Smith, A. L., Stadler, L. B., Love, N. G., Skerlos, S. J. & Raskin, L. Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: a critical review. Bioresource Technology 122, 149–159. Stuckey, D. C. Recent developments in anaerobic membrane reactors. Bioresource Technology 122, 137–148. Trzcinski, A. P. & Stuckey, D. C. Treatment of municipal solid waste leachate using a submerged anaerobic membrane bioreactor at mesophilic and psychrophilic temperatures: analysis of recalcitrants in the permeate using GC–MS. Water Research 44 (3), 671–680. Weber Jr., W. J., Hopkins, C. B. & Bloom Jr., R. Physicochemical treatment of wastewater. Journal of Water Pollution Control Federation 42, 83–99. Yoo, R., Kim, J., McCarty, P. L. & Bae, J. Anaerobic treatment of municipal wastewater with a staged anaerobic fluidized membrane bioreactor (SAF-MBR) system. Bioresource Technology 120, 133–139.
First received 13 July 2013; accepted in revised form 4 December 2013. Available online 18 December 2013
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