- Email: [email protected]
Anaerobic membrane bioreactor (AnMBR) for domestic wastewater treatment
Desalination 243 (2009) 251–257
Anaerobic membrane bioreactor (AnMBR) for domestic wastewater treatment B. Lewa*, S. Tarreb, M. Beliavskib, C. Dosoretzb, M. Greenb a
Agricultural Research Organization, PO Box 6, 50250 Bet Dagan, Israel Tel. +972 (3) 968-3453; Fax: +972 (3) 960-4704; email: [email protected] b Faculty of Civil and Environmental Engineering, Technion, 32000 Haifa, Israel Received 27 August 2007; Accepted 15 April 2008
Abstract The effect of backwash frequency (15, 30 and 60 min) and influent flux (3.75, 7.50 and 11.25 L/h/m2) on fouling amelioration in an innovative external side-stream dead-end microfiltration anaerobic membrane bioreactor (AnMBR) for the treatment of domestic wastewater at 25EC was investigated. During 6 months of reactor operation, a constant COD removal of 88% and an accumulation of 350 mg TSS/L/d inert matter in the reactor were observed. Experiments with different backwash frequency and fluxes showed that the increase in transmembrane pressure with time (fouling rate) follows in a two-step manner: a slow linear rise at the beginning, followed by a sudden increase in transmembrane pressure. Based on the fouling rates during the slow linear phase, the best backwash frequency for energy savings and fouling amelioration was determined to be in between 30–60 min. Moreover, the effect of particulate matter load on fouling rate was determined. Keywords: AnMBR; Backwash frequency; Domestic wastewater; Fouling; particulate matter load
1. Introduction Anaerobic treatment of domestic wastewater is an attractive option for secondary wastewater treatment. The high costs of aeration and sludge handling associated with aerobic sewage treatment are dramatically lower in anaerobic treatment as no oxygen is needed and the production *Corresponding author.
of sludge is lower. In addition, greenhouse gas emissions during anaerobic treatment are lower relative to aerobic technologies if methane is used as an energy source. Domestic wastewater has typically high concentrations of suspended solids, which can have a negative effect on anaerobic reactor performance, particularly during periods of low temperature when the extracellular degradation
0011-9164/09/$– See front matter © 2008 Elsevier B.V All rights reserved. doi:10.1016/j.desal.2008.04.027
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B. Lew et al. / Desalination 243 (2009) 251–257
rate (hydrolysis) of suspended solids and colloidal fractions is the rate limiting step [1,2]. Therefore, an increase in the suspended solids retention time in an anaerobic reactor should increase the degradation efficiency. The anaerobic membrane bioreactor (AnMBR) can provide for short hydraulic retention times while maintaining high solids retention time as no particulate matter is expelled from the system. As a consequence, the particulate organics retained in the reactor can eventually be hydrolyzed and decomposed because of the long solids retention time. Also the AnMBR allows the anaerobic microbes (which have relatively low growth rates compared with the aerobes, especially at low temperatures) to proliferate without being washed out from the process. Most common membrane bioreactors consist of a cell growth reactor and a membrane filtration device combined into a single unit process. The membrane unit can either be placed externally, as in side-stream operation, or submerged in the reactor. In the external system (side-stream) the membrane unit works independently of the reactor. Influent enters the bioreactor and is degraded by microorganisms. The bioreactor effluent is then conducted into a membrane filtration unit, which generally works in a cross-flow mode, where a pump provides the velocity and transmembrane pressure to enhance flux and prevent fouling during filtration. The membrane permeate is the treated product and the retentate is continuously returned to the bioreactor. Fouling prevention is usually achieved by a high rate of re-circulation and therefore the transmembrane pressure is often high. The high re-circulation rate not only requires high electric power input but also decreases the activity of methanogenic bacteria [3–5]. In submerged systems, there is no circulation loop as the membrane separation unit is immersed within the bioreactor itself. Under this circumstance, the transmembrane pressure is driven by the hydrostatic head of the liquid level above the
membrane or, if inadequate, an auxiliary suction pump. Fouling control is achieved by continuous biogas bubbling scouring the membrane surface coupled. Both fouling control and the use of a suction pump contribute to electric power demand. This project concentrates on the study of the performance of an external side-stream dead-end AnMBR in a configuration that consumes less electrical power than the two main alternatives mentioned above. The AnMBR was tested at different backwash frequencies and influent flow rates for fouling amelioration during domestic wastewater treatment.
2. Material and methods An innovative AnMBR concept was used in the investigation. The traditional cross-flow external membrane unit was replaced by a 4.00 m2 area, side-stream, microfiltration (MF) (0.20 µm pore size), hollow fiber, dead-end external unit, placed 2 m below the bioreactor effluent exit (Fig. 1). The height difference between the bioreactor and the membrane unit provided enough transmembrane pressure for constant flux filtration; i.e. no pump was used for recirculation or transmembrane pressure enhancement, promoting energy savings. Fouling amelioration was achieved by intermittent backwash of the membrane. For each backwash, 3 L of permeate were pumped through the membrane for 5 s. To prevent high upflow velocities in the bioreactor, which can cause methanogenic activity deterioration, backwash retentate flowed back to the feed container, located before the bioreactor. The 180 L AnMBR scale reactor was filled with 75 L of granular sludge taken from a fullscale UASB reactor treating food wastewater. The TSS concentration in the reactor immediately after filling was 14 g TSS/l. The reactor was fed with pre-settled domestic wastewater (540 mg COD/l) from the Neve Sha’anan neighborhood,
B. Lew et al. / Desalination 243 (2009) 251–257
253
Fig. 1. Experimental anaerobic side-stream membrane bioreactor with dead-end configuration. Full line indicates AnMBR continuous operation and dash line indicates intermittent backwash procedure.
Haifa, and operated at 25EC for a year at three different backwash frequencies (15, 30 and 60 min) and three different fluxes (3.75, 7.50 and 11.25 L/h/m2) with the corresponding loading rates (1.08, 2.16 and 4.32 g COD/L/d) and hydraulic retention times (12, 6 and 4.5 h). At the end of each backwash frequency and flow rate experiment, the membrane was chemically cleaned. Chemical cleaning procedure was accomplished with the membrane unit disconnected from the bioreactor and the used chemical solution was discarded at the end of the operation. Inflow, bioreactor effluent, retentate and permeate samples were tested on a regular basis for pH, BOD, TSS, VSS, total COD, soluble COD, VFA, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma (ICP) spectroscopy. Every 20 days a 50 ml sludge sample was taken and tested for SVI, TSS and VSS. All analyses were performed according to Standard Methods for the Examination of Water and Wastewater [6]. The volatile fatty acids concentration was measured using the five-point titration method [7].
3. Results and discussion During the first 6 months of reactor operation, a constant flux of 7.50 L/h/m2 was maintained, with the corresponding HRT of 6 h. This flux was well below the nominal membrane critical flux (500 L/h/m2). An average COD removal of 88% was measured, based on the influent (540 mg COD/l) and a constant 65 mg COD/l permeate concentration, which was composed of soluble COD only, e.g., no particulate COD was observed in the permeate flow. The membrane module promoted biomass and influent particulate matter retention in the bioreactor, as sludge, which concentration increased almost linearly from 14 to 80 g/l during the 6 months of operation. The high sludge retention time provided for particulate matter degradation, which was observed by a decrease in the sludge VSS/TSS ratio with time, from 0.80 at the beginning of the experiments to 0.71 at the end. During this period the reactor was operated at three different backwash frequencies: 15, 30 and 60 min. It is important to cite that the experimental sequence was 30, 60 and 15 min. The
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B. Lew et al. / Desalination 243 (2009) 251–257
Fig. 2. Increase in transmembrane pressure with time for different backwash frequencies (constant flux of 7.50 L/h/m2).
transmembrane pressure was measured continuously for each experiment, with the run ending when the transmembrane pressure reached 2 m. In all three cases, the transmembrane pressure increased with time (fouling rate) in a two-step manner: a slow linear rise at the beginning, followed by a sudden increase in transmembrane pressure. To overcome the different transmembrane pressures observed in a day of each experiment, the normalized results of transmembrane pressure (based on the pressure on day one) versus time are given in Fig. 2. FTIR and ICP spectroscopy of the influent, permeate and retentate samples showed an accumulation of sulfate and aliphatic material on the membrane during the slow linear phase of transmembrane pressure increase with time. This accumulation of sulfate and aliphatic material on the membrane suggests extracellular polysaccharides (EPS) as the fouling agent, as shown in similar works [8–10]. The increase in the transmembrane pressure with time (fouling rate) during the slow-linear phase was calculated for each backwash frequency. A similar fouling rate of 1.5%/d was observed for 15 and 30 min backwash frequencies. However, a higher fouling rate was observed
for the 60 min backwash frequency, 3.4%/d. These results indicate that the critical backwash frequency for this system is between 30 and 60 min backwash frequency. At higher backwash frequencies a constant and slower fouling rate is expected, while at lower backwash frequencies an exponential increase in the fouling rate is expected. Moreover, increase in fouling rate cannot be associated with experimental sequence, which was 30, 60 and 15 min. During the second half-year of reactor operation, the effect of flux on the transmembrane pressure was studied. In the previous experiments with constant flux, a constant particulate matter load into the membrane module was maintained; however, this was not the case for experiments with different fluxes. In order to determine the effect of particulate matter load on the membrane fouling process, the membrane module only was operated at constant flux, 6.00 L/h/m2, and fed with domestic wastewater diluted to three different particulate matter concentrations: 415, 545 and 690 mg COD/l. No backwash was applied in the experiments. The transmembrane pressure was measured continuously for each experiment, with the run ending when the transmembrane pressure reached 2 m.
B. Lew et al. / Desalination 243 (2009) 251–257
255
Fig. 3. Increase in transmembrane pressure with time for different fluxes (constant backwash frequency, every 15 min).
In all three cases, the same linear transmembrane pressure increased with the amount of particulate matter reaching the membrane was observed, 1.05×10!2 %/g COD, indicating that transmembrane pressure increases in a direct proportion to the amount of particulate matter reaching the membrane module when no backwash was applied. After the determination of the effect of particulate matter load on the membrane fouling process, the AnMBR was operated at a constant 15 min backwash frequency and at different fluxes for 6 months: 3.75, 7.50 and 11.25 L/h/m2. At each flux experiment, the particulate matter concentration reaching the membrane module varied, 275, 420 and 360 mg COD/l for 3.75, 7.50 and 11.25 L/h/m2, respectively. In each experiment the transmembrane pressure was measured continuously, with the run ending when the transmembrane pressure reached 2 m. As in the previous experiment with constant flux and different backwash frequencies (Fig. 2), in all three cases the transmembrane pressure increased as time progressed in a two -pattern: a slow linear rise at the beginning, followed by a sudden increase in transmembrane pressure. The normalized results of transmembrane pressure
(based on the pressure on day one) versus time are given in Fig. 3. The fouling rate during the slow linear phase was calculated for each experiment and an increase in fouling rate with the increase flux was observed: 0.99, 1.50 and 2.56%/d at 3.75, 7.50 and 11.25 L/h/m2, respectively. However, these fouling rate results do not take in consideration the varying concentration of particulate matter reaching the membrane module. The calculated fouling during the slow-linear phase as function of the amount of particulate matter reaching the membrane for the three experiments is shown in Fig. 4. The same linear transmembrane pressure increase was observed for all experiments, 3.52×10!5 %/g COD, indicating that as in the previous experiment, transmembrane pressure increases in a direct proportion to the amount of particulate matter reaching the membrane module. In the previous experiment, the fouling per amount of particulate matter reaching the membrane was higher, 1.05×10!2 %/g COD. The lower fouling in this experiment was probably due to the backwash applied, which reduced the amount of particulate matter depositing on the membrane.
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Fig. 4. Increase in transmembrane pressure as function of particulate matter reaching the membrane module (constant backwash frequency, every 15 min).
At the end of each experiment, e.g., when the transmembrane pressure reached 2 m, different chemical cleaning methods were used on the membrane module (0.1 M NaOH, 1% HCl, 1% H2O2, 1% oxalic acid, or a mix) to determine the best cleaning process for transmembrane pressure recover. A mix of 0.1 M NaOH and 1% H2O2 interspersed with 1% HCl gave the best results, with a recovery of 75%.
4. Conclusions Based on the results of this research project, it can be concluded that the external side-stream dead-end AnMBR configuration can be a viable alternative for domestic wastewater treatment at 25E. Backwash reduces the amount of particulate matter settling on the membrane and reducing the fouling. The best backwash frequency for energy savings and fouling amelioration was determined to be in between 30–60 min. At this reactor operational parameter, an 88% COD removal and an accumulation of 350 mg TSS/l/day inert particulate matter in the reactor were observed.
References [1] T.A. Elmitwalli, G. Zeeman and G. Lettinga, Anaerobic treatment of domestic sewage at low temperature, Water Sci. Technol., 44(4) (2001) 33. [2] B. Lew, M. Belavski, S. Admon, S. Tarre and M. Green, Temperature effect on UASB reactor operation for domestic wastewater treatment in temperate climate regions, Water Sci. Technol., 48(3) (2003) 25. [3] M. Brockman and C.F. Seyfried, Sludge activity and cross-flow microfiltration—a non-beneficial relationship, Water Sci. Technol., 34(9) (1996) 205. [4] K.H. Choo and C.H. Lee, Membrane fouling mechanisms in the membrane-coupled anaerobic bioreactor, Water Res., 30(8) (1996) 1771. [5] W.R. Ghyoot and W.H. Verstraete, Coupling membrane filtration to anaerobic primary sludge digestion, Environ. Technol., 18 (1997) 569. [6] APHA Standard Methods for the Examination of Water and Wastewater, 19th ed., APHA, AWWA, WPCF, Washington, DC, 1995. [7] O. Lahav, B.E. Morgan and R.E. Loewenthal, Rapid, simple, and accurate method for measurement of VFA and carbonate alkalinity in anaerobic reactors, Environ. Sci. Technol., 36(12) (2002) 2736. [8] I. Chang, P. LeClech, B. Jefferson and S. Judd,
B. Lew et al. / Desalination 243 (2009) 251–257 Membrane fouling in membrane bioreactors for wastewater treatment, J. Environ. Eng., 128(11) (2002) 1018. [9] B.D. Cho and A.G. Fane, Fouling transients in nominally sub-critical flux operation of a membrane bioreactor, J. Membr. Sci., 209(2) (2002) 391.
257
[10] E. Germain, T. Stephenson and P. Pearce, Biomass characteristics and membrane aeration: toward a better understanding of membrane fouling in submerged membrane bioreactors (MBRs), Biotechnol. Bioeng., 90(3) (2005) 316.
Anaerobic membrane bioreactor (AnMBR) for domestic wastewater treatment B. Lewa*, S. Tarreb, M. Beliavskib, C. Dosoretzb, M. Greenb a
Agricultural Research Organization, PO Box 6, 50250 Bet Dagan, Israel Tel. +972 (3) 968-3453; Fax: +972 (3) 960-4704; email: [email protected] b Faculty of Civil and Environmental Engineering, Technion, 32000 Haifa, Israel Received 27 August 2007; Accepted 15 April 2008
Abstract The effect of backwash frequency (15, 30 and 60 min) and influent flux (3.75, 7.50 and 11.25 L/h/m2) on fouling amelioration in an innovative external side-stream dead-end microfiltration anaerobic membrane bioreactor (AnMBR) for the treatment of domestic wastewater at 25EC was investigated. During 6 months of reactor operation, a constant COD removal of 88% and an accumulation of 350 mg TSS/L/d inert matter in the reactor were observed. Experiments with different backwash frequency and fluxes showed that the increase in transmembrane pressure with time (fouling rate) follows in a two-step manner: a slow linear rise at the beginning, followed by a sudden increase in transmembrane pressure. Based on the fouling rates during the slow linear phase, the best backwash frequency for energy savings and fouling amelioration was determined to be in between 30–60 min. Moreover, the effect of particulate matter load on fouling rate was determined. Keywords: AnMBR; Backwash frequency; Domestic wastewater; Fouling; particulate matter load
1. Introduction Anaerobic treatment of domestic wastewater is an attractive option for secondary wastewater treatment. The high costs of aeration and sludge handling associated with aerobic sewage treatment are dramatically lower in anaerobic treatment as no oxygen is needed and the production *Corresponding author.
of sludge is lower. In addition, greenhouse gas emissions during anaerobic treatment are lower relative to aerobic technologies if methane is used as an energy source. Domestic wastewater has typically high concentrations of suspended solids, which can have a negative effect on anaerobic reactor performance, particularly during periods of low temperature when the extracellular degradation
0011-9164/09/$– See front matter © 2008 Elsevier B.V All rights reserved. doi:10.1016/j.desal.2008.04.027
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B. Lew et al. / Desalination 243 (2009) 251–257
rate (hydrolysis) of suspended solids and colloidal fractions is the rate limiting step [1,2]. Therefore, an increase in the suspended solids retention time in an anaerobic reactor should increase the degradation efficiency. The anaerobic membrane bioreactor (AnMBR) can provide for short hydraulic retention times while maintaining high solids retention time as no particulate matter is expelled from the system. As a consequence, the particulate organics retained in the reactor can eventually be hydrolyzed and decomposed because of the long solids retention time. Also the AnMBR allows the anaerobic microbes (which have relatively low growth rates compared with the aerobes, especially at low temperatures) to proliferate without being washed out from the process. Most common membrane bioreactors consist of a cell growth reactor and a membrane filtration device combined into a single unit process. The membrane unit can either be placed externally, as in side-stream operation, or submerged in the reactor. In the external system (side-stream) the membrane unit works independently of the reactor. Influent enters the bioreactor and is degraded by microorganisms. The bioreactor effluent is then conducted into a membrane filtration unit, which generally works in a cross-flow mode, where a pump provides the velocity and transmembrane pressure to enhance flux and prevent fouling during filtration. The membrane permeate is the treated product and the retentate is continuously returned to the bioreactor. Fouling prevention is usually achieved by a high rate of re-circulation and therefore the transmembrane pressure is often high. The high re-circulation rate not only requires high electric power input but also decreases the activity of methanogenic bacteria [3–5]. In submerged systems, there is no circulation loop as the membrane separation unit is immersed within the bioreactor itself. Under this circumstance, the transmembrane pressure is driven by the hydrostatic head of the liquid level above the
membrane or, if inadequate, an auxiliary suction pump. Fouling control is achieved by continuous biogas bubbling scouring the membrane surface coupled. Both fouling control and the use of a suction pump contribute to electric power demand. This project concentrates on the study of the performance of an external side-stream dead-end AnMBR in a configuration that consumes less electrical power than the two main alternatives mentioned above. The AnMBR was tested at different backwash frequencies and influent flow rates for fouling amelioration during domestic wastewater treatment.
2. Material and methods An innovative AnMBR concept was used in the investigation. The traditional cross-flow external membrane unit was replaced by a 4.00 m2 area, side-stream, microfiltration (MF) (0.20 µm pore size), hollow fiber, dead-end external unit, placed 2 m below the bioreactor effluent exit (Fig. 1). The height difference between the bioreactor and the membrane unit provided enough transmembrane pressure for constant flux filtration; i.e. no pump was used for recirculation or transmembrane pressure enhancement, promoting energy savings. Fouling amelioration was achieved by intermittent backwash of the membrane. For each backwash, 3 L of permeate were pumped through the membrane for 5 s. To prevent high upflow velocities in the bioreactor, which can cause methanogenic activity deterioration, backwash retentate flowed back to the feed container, located before the bioreactor. The 180 L AnMBR scale reactor was filled with 75 L of granular sludge taken from a fullscale UASB reactor treating food wastewater. The TSS concentration in the reactor immediately after filling was 14 g TSS/l. The reactor was fed with pre-settled domestic wastewater (540 mg COD/l) from the Neve Sha’anan neighborhood,
B. Lew et al. / Desalination 243 (2009) 251–257
253
Fig. 1. Experimental anaerobic side-stream membrane bioreactor with dead-end configuration. Full line indicates AnMBR continuous operation and dash line indicates intermittent backwash procedure.
Haifa, and operated at 25EC for a year at three different backwash frequencies (15, 30 and 60 min) and three different fluxes (3.75, 7.50 and 11.25 L/h/m2) with the corresponding loading rates (1.08, 2.16 and 4.32 g COD/L/d) and hydraulic retention times (12, 6 and 4.5 h). At the end of each backwash frequency and flow rate experiment, the membrane was chemically cleaned. Chemical cleaning procedure was accomplished with the membrane unit disconnected from the bioreactor and the used chemical solution was discarded at the end of the operation. Inflow, bioreactor effluent, retentate and permeate samples were tested on a regular basis for pH, BOD, TSS, VSS, total COD, soluble COD, VFA, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma (ICP) spectroscopy. Every 20 days a 50 ml sludge sample was taken and tested for SVI, TSS and VSS. All analyses were performed according to Standard Methods for the Examination of Water and Wastewater [6]. The volatile fatty acids concentration was measured using the five-point titration method [7].
3. Results and discussion During the first 6 months of reactor operation, a constant flux of 7.50 L/h/m2 was maintained, with the corresponding HRT of 6 h. This flux was well below the nominal membrane critical flux (500 L/h/m2). An average COD removal of 88% was measured, based on the influent (540 mg COD/l) and a constant 65 mg COD/l permeate concentration, which was composed of soluble COD only, e.g., no particulate COD was observed in the permeate flow. The membrane module promoted biomass and influent particulate matter retention in the bioreactor, as sludge, which concentration increased almost linearly from 14 to 80 g/l during the 6 months of operation. The high sludge retention time provided for particulate matter degradation, which was observed by a decrease in the sludge VSS/TSS ratio with time, from 0.80 at the beginning of the experiments to 0.71 at the end. During this period the reactor was operated at three different backwash frequencies: 15, 30 and 60 min. It is important to cite that the experimental sequence was 30, 60 and 15 min. The
254
B. Lew et al. / Desalination 243 (2009) 251–257
Fig. 2. Increase in transmembrane pressure with time for different backwash frequencies (constant flux of 7.50 L/h/m2).
transmembrane pressure was measured continuously for each experiment, with the run ending when the transmembrane pressure reached 2 m. In all three cases, the transmembrane pressure increased with time (fouling rate) in a two-step manner: a slow linear rise at the beginning, followed by a sudden increase in transmembrane pressure. To overcome the different transmembrane pressures observed in a day of each experiment, the normalized results of transmembrane pressure (based on the pressure on day one) versus time are given in Fig. 2. FTIR and ICP spectroscopy of the influent, permeate and retentate samples showed an accumulation of sulfate and aliphatic material on the membrane during the slow linear phase of transmembrane pressure increase with time. This accumulation of sulfate and aliphatic material on the membrane suggests extracellular polysaccharides (EPS) as the fouling agent, as shown in similar works [8–10]. The increase in the transmembrane pressure with time (fouling rate) during the slow-linear phase was calculated for each backwash frequency. A similar fouling rate of 1.5%/d was observed for 15 and 30 min backwash frequencies. However, a higher fouling rate was observed
for the 60 min backwash frequency, 3.4%/d. These results indicate that the critical backwash frequency for this system is between 30 and 60 min backwash frequency. At higher backwash frequencies a constant and slower fouling rate is expected, while at lower backwash frequencies an exponential increase in the fouling rate is expected. Moreover, increase in fouling rate cannot be associated with experimental sequence, which was 30, 60 and 15 min. During the second half-year of reactor operation, the effect of flux on the transmembrane pressure was studied. In the previous experiments with constant flux, a constant particulate matter load into the membrane module was maintained; however, this was not the case for experiments with different fluxes. In order to determine the effect of particulate matter load on the membrane fouling process, the membrane module only was operated at constant flux, 6.00 L/h/m2, and fed with domestic wastewater diluted to three different particulate matter concentrations: 415, 545 and 690 mg COD/l. No backwash was applied in the experiments. The transmembrane pressure was measured continuously for each experiment, with the run ending when the transmembrane pressure reached 2 m.
B. Lew et al. / Desalination 243 (2009) 251–257
255
Fig. 3. Increase in transmembrane pressure with time for different fluxes (constant backwash frequency, every 15 min).
In all three cases, the same linear transmembrane pressure increased with the amount of particulate matter reaching the membrane was observed, 1.05×10!2 %/g COD, indicating that transmembrane pressure increases in a direct proportion to the amount of particulate matter reaching the membrane module when no backwash was applied. After the determination of the effect of particulate matter load on the membrane fouling process, the AnMBR was operated at a constant 15 min backwash frequency and at different fluxes for 6 months: 3.75, 7.50 and 11.25 L/h/m2. At each flux experiment, the particulate matter concentration reaching the membrane module varied, 275, 420 and 360 mg COD/l for 3.75, 7.50 and 11.25 L/h/m2, respectively. In each experiment the transmembrane pressure was measured continuously, with the run ending when the transmembrane pressure reached 2 m. As in the previous experiment with constant flux and different backwash frequencies (Fig. 2), in all three cases the transmembrane pressure increased as time progressed in a two -pattern: a slow linear rise at the beginning, followed by a sudden increase in transmembrane pressure. The normalized results of transmembrane pressure
(based on the pressure on day one) versus time are given in Fig. 3. The fouling rate during the slow linear phase was calculated for each experiment and an increase in fouling rate with the increase flux was observed: 0.99, 1.50 and 2.56%/d at 3.75, 7.50 and 11.25 L/h/m2, respectively. However, these fouling rate results do not take in consideration the varying concentration of particulate matter reaching the membrane module. The calculated fouling during the slow-linear phase as function of the amount of particulate matter reaching the membrane for the three experiments is shown in Fig. 4. The same linear transmembrane pressure increase was observed for all experiments, 3.52×10!5 %/g COD, indicating that as in the previous experiment, transmembrane pressure increases in a direct proportion to the amount of particulate matter reaching the membrane module. In the previous experiment, the fouling per amount of particulate matter reaching the membrane was higher, 1.05×10!2 %/g COD. The lower fouling in this experiment was probably due to the backwash applied, which reduced the amount of particulate matter depositing on the membrane.
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B. Lew et al. / Desalination 243 (2009) 251–257
Fig. 4. Increase in transmembrane pressure as function of particulate matter reaching the membrane module (constant backwash frequency, every 15 min).
At the end of each experiment, e.g., when the transmembrane pressure reached 2 m, different chemical cleaning methods were used on the membrane module (0.1 M NaOH, 1% HCl, 1% H2O2, 1% oxalic acid, or a mix) to determine the best cleaning process for transmembrane pressure recover. A mix of 0.1 M NaOH and 1% H2O2 interspersed with 1% HCl gave the best results, with a recovery of 75%.
4. Conclusions Based on the results of this research project, it can be concluded that the external side-stream dead-end AnMBR configuration can be a viable alternative for domestic wastewater treatment at 25E. Backwash reduces the amount of particulate matter settling on the membrane and reducing the fouling. The best backwash frequency for energy savings and fouling amelioration was determined to be in between 30–60 min. At this reactor operational parameter, an 88% COD removal and an accumulation of 350 mg TSS/l/day inert particulate matter in the reactor were observed.
References [1] T.A. Elmitwalli, G. Zeeman and G. Lettinga, Anaerobic treatment of domestic sewage at low temperature, Water Sci. Technol., 44(4) (2001) 33. [2] B. Lew, M. Belavski, S. Admon, S. Tarre and M. Green, Temperature effect on UASB reactor operation for domestic wastewater treatment in temperate climate regions, Water Sci. Technol., 48(3) (2003) 25. [3] M. Brockman and C.F. Seyfried, Sludge activity and cross-flow microfiltration—a non-beneficial relationship, Water Sci. Technol., 34(9) (1996) 205. [4] K.H. Choo and C.H. Lee, Membrane fouling mechanisms in the membrane-coupled anaerobic bioreactor, Water Res., 30(8) (1996) 1771. [5] W.R. Ghyoot and W.H. Verstraete, Coupling membrane filtration to anaerobic primary sludge digestion, Environ. Technol., 18 (1997) 569. [6] APHA Standard Methods for the Examination of Water and Wastewater, 19th ed., APHA, AWWA, WPCF, Washington, DC, 1995. [7] O. Lahav, B.E. Morgan and R.E. Loewenthal, Rapid, simple, and accurate method for measurement of VFA and carbonate alkalinity in anaerobic reactors, Environ. Sci. Technol., 36(12) (2002) 2736. [8] I. Chang, P. LeClech, B. Jefferson and S. Judd,
B. Lew et al. / Desalination 243 (2009) 251–257 Membrane fouling in membrane bioreactors for wastewater treatment, J. Environ. Eng., 128(11) (2002) 1018. [9] B.D. Cho and A.G. Fane, Fouling transients in nominally sub-critical flux operation of a membrane bioreactor, J. Membr. Sci., 209(2) (2002) 391.
257
[10] E. Germain, T. Stephenson and P. Pearce, Biomass characteristics and membrane aeration: toward a better understanding of membrane fouling in submerged membrane bioreactors (MBRs), Biotechnol. Bioeng., 90(3) (2005) 316.