Influence of membrane material on performance of a submerged membrane bioreactor

Transcription

1 Influence of membrane material on performance of a submerged membrane bioreactor 1 Jae-Hoon Choi* and How Yong Ng** * Division of Environmental Science and Engineering, National University of Singapore, Block EA #03-12, 9 Engineering Drive 1, Singapore , Singapore ( jaehoonchoi2@hanmail.net) ** Division of Environmental Science and Engineering, National University of Singapore, Block EA #03-12, 9 Engineering Drive 1, Singapore , Singapore ( esenghy@nus.edu.sg) Abstract To evaluate the impact of membrane material on filtration performance of a submerged membrane bioreactor (MBR), three types of membranes with the same pore size of 0.1 µm: polytetrafluoroethylene (PTFE), polycarbonate (PCTE) and polyester (PETE) membranes, were used in this work. Evolution of permeability of the PCTE and PTFE were similar, whereas the PETE exhibited the most rapid flux decline rate. The faster permeability decline and lower permeate TOC of the PETE could be attributable to the fact that a higher permeate flux than an average design flux led to a fast fouling due to a larger amount of fouling materials being attached onto the membrane in a shorter time. Results of surface roughness and membrane hydrophobicity showed no clear correlation with membrane fouling. No significant changes in terms of DOC fractionation in the supernatant and permeates also indicated that membrane hydrophobicity might not be a dominant influencing factor on MBR fouling in this work. The PETE had higher irreversible filtration resistance compared to the PCTE and PTFE. The organic foulants desorbed from the PCTE resulted in mostly hydrophobic fractions, whereas the PETE had much less hydrophobic fractions than expected regardless of the membranes having similar contact angle. Keywords Fouling; membrane bioreactor; membrane material; organic removal; sewage treatment INTRODUCTION Membrane bioreactor (MBR) process combines biological wastewater treatment with a membrane process to treat wastewater biologically and to separate biomass physically from mixed liquor in an integrated step. The MBR process is increasingly adopted for wastewater treatment and water reuse due to its advantages of complete solid-liquid separation, production of high-quality effluent, capability of handling wide fluctuations in influent quality and small footprint (Visvanathan et al., 2000). However, the decrease of permeate flux by membrane fouling is a major deterrent to the efficient application of the MBR process to wastewater treatment. To date, many studies have been carried out to identify and investigate membrane fouling, and to elucidate dominant impact factors on membrane fouling in the MBR. As a result, several investigations relating to biomass characteristics such as mixed liquor suspended solids (MLSS), particle size distribution and extracellular polymeric substances (EPS), have been published (Nagaoka et al., 1996; Zhang et al., 1997; Rosenberger et al., 2005). In addition, membrane fouling is affected by operating conditions, which includes hydraulic retention time (HRT), solid retention time (SRT) and permeate flux, has been studied as well (Chang et al., 2002; Ng et al., 2006). Since membrane fouling in the MBR results from interaction between membrane and mixed liquor in the bioreactor, membrane characteristics, such as membrane material, pore size and surface charge, are also important factors on membrane fouling. Particularly, experimental results using polymeric and/or ceramic membranes found that there was a close relationship between membrane material and membrane fouling in the MBR (Judd et al., 2004; Yamato et al., 2006). As there is still little information available with regard to the impact of membrane material on fouling, further in-depth investigation is required.

2 The objective of this work was to investigate the effect of membrane material on MBR performance for municipal wastewater treatment. Three different polymeric microfiltration (MF) membranes were used in this study: polytetrafluoroethylene (PTFE) membrane (Sumitomo Electric, Japan), polycarbonate (PCTE) membrane (GE-Osmonics, USA) and polyester (PETE) membrane (GE-Osmonics, USA). 2 METHODS Three flat-plate membrane modules were immersed in a bioreactor (effective volume: 24.3 L). Each membrane module has similar surface area of approximately 0.1 m 2. As this study was focused to investigate the impact of membrane material on membrane performance including fouling, the membrane modules with the same pore size of 0.1 µm were used. Characteristics of the membranes used are listed in Table 1. An extinct feature of the membranes was that pure water permeability (PWP) of the PETE membrane was lower by 30% and 43% than those of the PCTE and PTFE membranes regardless of the membranes with the same pore size. Table 1 Properties of the MF membranes used in this work Item PETE a PCTE a PTFE a Manufacturer Pore size (µm) b Zeta potential (mv) c Contact angle (º) d Mean roughness (nm) e Permeability (L m -2 h -1 kpa -1 ) f GE-Osmonics ± ± ± 2.9 GE-Osmonics ± ± ± 3.0 a PETE = Polyester; PCTE = Polycarbonate; PTFE = Polytetrafluoroethylene. b Nominal pore size. c Ionic strength = 10 mm NaCl; ph = 7.0 ± 0.2; temperature = 25ºC. d Data are expressed as mean ± standard deviation (n = 5). e Surface roughness were measured in triplicate (mean ± standard deviation) f The filtration test was performed using Milli-Q water at 30ºC. Sumitomo Electric ± ± ± 7.5 Feed wastewater from a local municipal wastewater treatment plant was used; the water qualities were ± mg L -1 for COD, ± 99.9 mg L -1 for suspended solids (SS), and ± 51.7 mg L -1 for volatile suspended solids (VSS). Seed-activated sludge was collected from the same wastewater treatment plant at a MLSS concentration of approximately 1,600 mg L -1. Permeate flux was initially set to approximately 9.1 L m -2 h -1. Intermittent suction with a cycle of 3-min run and 2-min pause was carried out for permeate production. HRT and SRT were set at 8 h and 15 d, respectively. An air diffuser was installed under each membrane module to provide similar aeration condition. A ph controller and sodium bicarbonate (NaHCO 3 ) solution of 0.25 M were used to adjust the mixed liquor ph to 7.0 ± 0.1. In order to restore permeate flux, each membrane module was cleaned physically and chemically on day 43 and day 82 throughout the operation. Physical cleaning was carried out by wiping and rinsing with a soft sponge and tap water, respectively. Then, the membranes were chemically cleaned by immersing in 1% (w/v) HCl for 0.5 h and in 0.5% (w/v) sodium hypochlorite (NaClO) for 2 h in series. During the membrane cleaning on day 82, PWP of the fouled and cleaned membranes was measured using deionized water. Subsequently, the filtration resistance at each step was calculated by the following equation: TMP Rt = Rm + Rc + Rp = 3,600 (1) µ J

3 where J is the permeate flux (m 3 m -2 h -1 ), TMP the applied transmembrane pressure (Pa), µ the permeate viscosity (Pa s), R t the total filtration resistance (m -1 ), R m the intrinsic membrane resistance, R c the cake layer resistance and R p is the pore-clogging resistance. The permeate flux was measured by filtering the pure water with (1) fouled membranes, (2) physically and (3) chemically cleaned membranes. R t was determined by the filtration resistance of steps (1) and R c could be calculated by subtracting R m from the filtration resistance of steps (2). R m was determined by filtering pure water with a fresh membrane. R p was obtained by subtracting the filtration resistance of steps (3) from the resistance of steps (2). The difference between R t from the steps (1) and the sum of filtration resistances from each filtration experiment (R m + R c + R t ) was negligible (below 2.5%). After the termination of reactor operation, inorganic and organic foulants adsorbed onto membrane were collected and examined after chemical cleaning. Inorganic foulants were desorbed from the physically cleaned membrane by soaking it in 0.5% HCl solution (ph 0.9) at 28ºC for 3 h, whereas organic foulants were remove from the membrane by immersing it in 0.1M NaOH (ph 12.7) at 28ºC for 6 h. NaOH solution was used in order to minimize the effect of cleaning chemicals on subsequent foulant analysis. For the measurement of dissolved organic and inorganic components, particles in the water samples were removed using a mixed cellulose esters membrane filter with a nominal pore size of 0.45 µm (GN-6 Metricel, Pall, USA). Total organic carbon (TOC) and dissolved organic carbon (DOC) were quantified by the NPOC (non-purgeable organic carbon) method using a TOC-V analyzer (Shimadzu, Japan). Water samples were acidified to ph 2 using a 2N HCl solution, sparged for 5 min using high-purity air, then analyzed for three to five times to produce a coefficient of variation below To fractionate the DOC in the supernatant and permeate waters, Supelite DAX-8 (Supelco, USA) and Amberlite XAD-4 (Rohm and Haas, Germany) resins were used. The organic matter in the waters was fractionated into three groups; namely, hydrophobic (DAX-8 adsorbable), transphilic (XAD-4 adsorbable), and hydrophilic (neither DAX-8 nor XAD-4 adsorbable) fractions. Zeta potential of the membrane surface was measured using a streaming potential analyzer (BI-EKA, Brookhaven Instruments, USA). Streaming potential measurements were performed using a solution of 10mM NaCl at ph values ranging from 2 to 10 and at about 25 C. The contact angle of a membrane was measured according to the sessile drop technique by Goniometer (VCA Optima Systems, AST Product Inc., USA). A drop of deionized water with a tight syringe was placed on the membrane surface. The contact angles of MF membranes were calculated from the average of the contact angles at both left and right sides of the drop. Surface roughness of the membranes used in this work was measured using an atomic force microscopy (Dimension 3100TM, Veeco Metrology Group, US). The MLSS and mixed liquor volatile suspended solids (MLVSS) were measured in accordance with Japanese Standard Methods (JSWA, 1997). 3 RESULTS AND DISCUSSION Figure 1 shows the permeability of the three membranes over an approximately 120 days. The specific permeate flux profiles of the PCTE and PTFE membranes were almost similar, whereas the PETE membrane exhibited the most rapid flux decline rate. Since the same mixed liquor was filtered and similar initial permeate flux was set, this result could be attributable to the membrane characteristics, particularly, different PWP values of the membranes. Actually, a faster decline of permeability of the PETE membrane throughout the operation indicated that the initial event of membrane fouling due to very high permeate flux determined the following progress of membrane fouling. Therefore, an MF membrane should not be operated at a higher permeate flux than the average design flux.

4 4 Figure 1 Evolution of permeability of each membrane permeate during the operation Organic matter concentrations in the supernatant of the bioreactor and the permeate from each membrane were also monitored during the operation: 9.9 ± 1.1 mg DOC L -1 for the supernatant; 7.4 ± 0.5, 7.9 ± 0.8 and 7.5 ± 0.7 mg TOC L -1 for the permeate of the PETE, PCTE and PTFE membranes, respectively. Hence, the removal efficiencies of organic matter by the membranes were 25, 20 and 24% for the PETE, PCTE and PTFE membranes, respectively. The organic removal efficiencies showed similar trend: the rates increased with decreasing permeability, indicating that the membranes were fouled gradually over time. The faster permeability decline and lower permeate TOC of the PETE could be due to the higher operating permeate flux that led to faster membrane fouling caused by a larger amount of fouling materials being attached onto the membrane in a shorter time. This resulted in the quick build-up and compaction of fouling layer on the membrane surface. In addition, a high permeate flux of the PETE may lead to a higher foulant concentration at the membrane surface due to concentration polarization, which would cause gel/cake layer formation and/or narrow the pores of the PETE membrane, increasing filtration resistance. Therefore, the amount of adsorbed foulants during the initial stage of filtration was more important than the total amount of foulants Figure 2 Changes of supernatant DOC and permeate TOC during the operation

5 5 attached on the membrane in this work. The evolution of MLSS and MLVSS concentrations were monitored throughout the operation (Fig. 3). The biomass concentrations in the MBR increased gradually from 1.6 g SS L -1 and 1.2 g VSS L -1 to 5.0 g VSS L -1 and 3.2 g VSS L -1 for the first 30 days, respectively. Then, the biomass concentration fluctuated between 3.6 g SS L -1 and 4.9 g SS L -1 and between 2.0 g VSS L -1 and 2.9 g VSS L -1 for the next 40 days. This fluctuation could be attributed to low organic and SS loading of influent wastewater. The MLSS and MLVSS concentrations increased gradually after day 70. Figure 3 Changes of MLSS and MLVSS concentrations in the bioreactor Membrane surface roughness and membrane hydrophobicity, which can influence the filtration performance, were measured to investigate the effect of membrane properties on permeate flux decline of each membrane in a submerged MBR. It can be seen from Table 1 that surface roughness of the PETE membrane had the smallest value compared to other membranes, indicating a smoother membrane surface. Some researches found that colloidal fouling could be correlated with the surface roughness of nanofiltration and reverse osmosis membranes (Elimelech et al., 1997; Vrijenhoek et al., 2001). This result could be explained by that in the initial stages of fouling, the colloidal particles preferentially accumulated in the valleys of rough membranes, resulting in valley clogging and hence, a more severe flux decline. However, Van der Bruggen et al. (2004) reported that there was no correlation between particle fouling and membrane roughness. Since the faster fouling of the PETE membrane occurred notwithstanding the smoothest membrane, membrane surface roughness did not affect fouling in this work. Contrarily, the research by Van der Bruggen et al. (2004) was likely to support the assumption that particle fouling is primarily controlled by the membrane hydrophobicity. Thus, the contact angle measurement for each membrane was carried out to evaluate the effect of membrane hydrophobicity on membrane fouling. Table 1 shows the contact angles measured for the three membranes; the PETE and PCTE membranes were more hydrophobic than the PTFE membrane. The PETE membrane showed the largest fouling effect, whereas the PCTE membrane with a similar contact angle did not exhibit effect as expected from the degree of membrane hydrophobicity. Therefore, further study is required to elucidate the impact of surface roughness and membrane hydrophobicity on membrane fouling in a submerged MBR. DOC component in the supernatant and permeates was fractionated using DAX-8 and XAD-4 resins, followed by DOC measurement for each fractionated water sample (Table 2). This analysis aimed to

6 examine the hydrophobic and hydrophilic compositions of DOC in the supernatant and permeate, which could be affected by the interaction (adsorption) between the organic matter and membrane hydrophobicity. The isolation procedure allowed determination of hydrophobic (HPO), transphilic (TPI) and hydrophilic (HPI) fractions. The HPO fraction was comprised of the least polar and generally highest molecular weight (MW) humic materials of the three fractions. The TPI fraction was of intermediate polarity and lower MW, and the HPI fraction consisted of the most polar and lowest MW organic matter (Quanrud et al., 2003). It can be seen from the results that the composition of each fraction between the supernatant and permeates did not changed significantly throughout the operation, indicating that membrane hydrophobicity might not be an dominant influencing factor on MBR fouling in this work. Table 2 Fractionation of DOC in the supernatant and permeates of the MBR Water (mg DOC L -1 ) HPO fraction (%) TPI fraction (%) HPI fraction (%) Supernatant (9.8 ± 1.2) PETE permeate (7.0 ± 0.3) PCTE permeate (7.3 ± 0.5) PTFE permeate (7.2 ± 0.5) 36.6 ± ± ± ± ± ± ± ± ± ± ± ± 5.8 Values are expressed as mean ±standard deviation (n = 6). After each membrane was cleaned in accordance with the procedures aforementioned, the filtration resistances, such as R t, R m, R c and R p, were determined to evaluate the degree of reversible and irreversible resistance in fouled membranes (Fig. 4). Reversible membrane fouling can be removed by physical methods, but irreversible fouling cannot be cleaned by physical cleaning. Regardless of a faster fouling of the PETE membrane, R t of the PETE membrane was contrarily lower than that of the PTFE membrane. This result can be explained by the filtration resistance measured after all membranes were fouled. It can be seen from the filtration resistances that the PETE membrane had the highest irreversible filtration resistance compared to those of the PCTE and PTFE membranes. Yamato et al. (2006) found that most of irreversible fouling in a submerged MBR could be attributed to the dissolved fraction in mixed liquor. Thus, a higher organic rejection rate of the PETE membrane would contribute to a higher irreversible fouling due to adsorption of dissolved organics onto the pore walls and narrowing the pores, resulting in faster membrane fouling. 6 Figure 4 Filtration resistance of fouled membranes

7 After the termination of the reactor operation, the desorbed foulants (using 0.1 M NaOH) from each membrane were separated into hydrophobic, transphilic, and hydrophilic fractions as well. The mass percentage in each fraction is listed in Table 3. The organic foulants desorbed from the PCTE membrane consisted of mostly hydrophobic fractions. In contrast, the PETE membrane had much less hydrophobic fractions than expected regardless of the membranes having similar contact angle. One possible explanation for this result is that the PETE membrane surface is more negative than that of the PCTE membrane (Table 1). Since the HPO fraction generally possesses negative charge due to deprotonated carboxylic acid groups, electrostatic repulsion between the charged membrane surface and the fractions of DOC might decrease the attachment of dissolved organic matter onto and into the membrane. The desorbed DOC mass per unit area of the membrane surface are larger for the PCTE and PTFE membranes than for the PETE membrane, which is inconsistent with the results of permeate flux decline rate and filtration resistances (measured after all membranes were fouled). Table 3 Fractionation of the organic foulants desorbed from 0.1M NaOH cleaning Membrane Desorbed DOC (mg m -2 ) HPO fraction (mass %) TPI fraction (mass %) HPI fraction (mass %) PETE PCTE PTFE CONCLUSIONS The permeate flux profiles of the PCTE and PTFE membranes were almost similar, whereas the PETE membrane exhibited the most rapid flux decline rate. The faster permeability decline and lower permeate TOC of the PETE could be due to the higher operating permeate flux that led to faster membrane fouling caused by a larger amount of fouling materials being attached onto the membrane in a shorter time. Results of surface roughness and membrane hydrophobicity exhibited no clear correlation with membrane fouling. DOC fractionation in the supernatant and permeates showed that the composition of each fraction between the supernatant and permeates did not changed significantly throughout the operation, supporting that membrane hydrophobicity might not be a dominant influencing factor on MBR fouling in this work. The PETE membrane had higher irreversible filtration resistance compared to the PCTE and PTFE membranes. The organic foulants desorbed from the PCTE membrane results in mostly hydrophobic fractions, whereas the PETE had much less hydrophobic fractions than expected regardless of the membranes having similar contact angle. REFERENCES Chang I. -S., Le-Clech P., Jefferson B. and Judd S. (2002). Membrane fouling in membrane bioreactors for wastewater treatment. J. Environ. Eng., 128, Elimelech M., Zhu X., Childress A. E. and Hong S. (1997). Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes. J. Membrane Sci., 127, Japanese Standard Methods of the Examination of Wastewater (1997). Japan Sewage Works Association, Tokyo, Japan. Judd S. J., Robinson T., Holdner J., Alvarez-Vazquez H. and Jefferson B. (2004). Impact of membrane material on membrane bioreactor permeability. In: Proceedings of the Water Environment-Membrane Technology (WEMT) Conference, Seoul, Korea.

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