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Effect of pretreatment on the fouling of membranes: application in biologically treated sewage effluent
Journal of Membrane Science 234 (2004) 111–120
Effect of pretreatment on the fouling of membranes: application in biologically treated sewage effluent H.K. Shon a , S. Vigneswaran a,∗ , In S. Kim b , J. Cho b , H.H. Ngo a a
Faculty of Engineering, University of Technology, P.O. Box 123, Broadway, Sydney, NSW 2007, Australia Water Reuse Technology Center, Kwangju Institute of Science and Technology, Gwangju, South Korea
b
Received 2 October 2003; received in revised form 7 October 2003; accepted 21 January 2004
Abstract Reuse of wastewater can help in maintaining environmental quality and relieving the unrelenting pressure on conventional and natural freshwater sources. Membrane processes find an important place in the wastewater treatment for reuse. Nonetheless, reverse osmosis (RO) and nanofiltration (NF), i.e. non-porous membranes require higher operational costs and energy. Thus, in this research NTR 7410 ultrafiltration (UF) membrane which is porous was used without and with pretreatment to treat biologically treated sewage effluent (BTSE). Four different pretreatment methods, namely, ferric chloride (FeCl3 ) flocculation, powdered activated carbon (PAC) adsorption, flocculation followed by adsorption, and granular activated carbon (GAC) biofilter were used in this study to compare their relative merits. Experimental results indicate that the most suitable pretreatment was flocculation followed by adsorption leading to a total organic carbon (TOC) removal of 90%. To assess the suitability of the membranes, it is important to conduct a detailed membrane characterization. The fouled NTR 7410 membrane surface was analyzed in terms of contact angle, zeta potential, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), flux decline, and TOC removal. The contact angle of the fouled membrane surface was lower than that of the clean membrane surface. This suggests that the majority of the foulants may have been the hydrophilic organic compounds such as polysaccharides, urea, etc. which are the extracellular enzyme of microorganisms in BTSE. But, the fouled membrane surface after the pretreatment of flocculation followed by adsorption had nearly the same contact angle as that of the clean membrane, suggesting that the hydrophobicity of the membrane is preserved by this pretreatment. According to attenuated total reflection-Fourier transform infrared spectroscopy results, the peaks observed on the fouled membrane were ether (C–O–C) and urea (R–NH–CO–NH–R). On the other hand, the peaks obtained after the pretreatment of flocculation followed by adsorption were similar to those of clean membranes. The highest effluent organic matter (EfOM) concentration on the fouled membranes without any pretreatment was measured up to 0.011 mg EfOM/cm2 membrane surface. The pretreatment of flocculation followed by adsorption reduced the EfOM concentration on the membrane to 0.005 mg EfOM/cm2 . The SEM images on the membrane cross-section revealed that there was practically no foulant layer on the membrane when a pretreatment of flocculation followed by adsorption was used. © 2004 Elsevier B.V. All rights reserved. Keywords: Flocculation; Adsorption; Membrane characterization; Biofilter; Biologically treated sewage effluent; Effluent organic matter; Pretreatment; Ultrafiltration
1. Introduction The domestic wastewater contains more than 99% of water and less than 1% of solids. Therefore, natural shortage of water can be overcome by reuse of wastewater. Membrane processes can be successfully used in obtaining water of recyclable quality. Reverse osmosis (RO) and nanofiltration ∗ Corresponding author. Tel.: +61-2-9514-2653; fax: +61-2-9514-2633. E-mail address: [email protected] (S. Vigneswaran).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.01.015
(NF) can remove all the pollutants including dissolved organics. However, their operational costs are high due to high energy requirement [1,2]. Ultrafiltration (UF) is a cost-effective option in terms of higher permeate flux compared to NF and RO. The problem is that the UF membranes are easily fouled by effluent organic matter (EfOM) present at high levels in treated wastewater. EfOM-fouling, defined as the accumulation and/or adsorption of organic materials on the surface, or in the pores of a membrane, affects membrane performance including permeability and EfOM rejection [3]. Thus, pretreatment is necessary to decrease
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the fouling on the UF membrane. Further, the pretreatment will help in achieving superior organic removal. The main objective of this study was to assess the variation in the characteristics of the NTR 7410 UF membrane surface after the membrane was subjected to filtration of biologically treated sewage effluent (BTSE) with and without pretreatment. The pretreatments used prior to the application of the UF were: (i) flocculation with FeCl3 , (ii) adsorption with PAC, (iii) flocculation followed by adsorption, and (iv) GAC biofilter. The membrane which underwent the filtration of BTSE was characterized in terms of contact angle, zeta potential, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM). The flux decline and total organic carbon (TOC) removal by the UF were also measured.
2. Background The pretreatment such as flocculation can remove 80–90% of the total suspended matter, 40–70% of the BOD5 , 30–60% of the COD, and 80–90% of the bacteria. Adsorption can remove organics which are not removed by conventional chemical and biological treatment methods [4]. The flocculation–adsorption process was able to remove 86% of chemical oxygen demand (COD) from domestic wastewater [5]. Granular activated carbon (GAC) bioadsorption is used extensively for achieving superior removals of particulate organic matter and dissolved solids from wastewater effluents by biological and adsorption processes. Since UF membrane is porous, the characterization of membrane fouling is important to choose the correct design parameters of UF and pretreatment methods. To identify the fouling on the UF membrane surface, the scanning electron microscopy, zeta potential, pyrolysis-GC/MS, attenuated total reflection-Fourier transform infrared spectroscopy, etc. are used [6–11]. The zeta potential is used as the electrokinetic value associating a realistic magnitude of surface charge [12,13]. Chun et al. [12] found that changes in membrane zeta potential could be used to examine the behavior of cake deposition and fouling during the filtration. The identification of fouling can be investigated both by extraction of organics and by ATR-FTIR analysis of deposits at the surface and in the substructure [13]. ATR-FTIR especially is possible to confirm a detailed screen of the molecular functional groups contributing to membrane fouling. SEM has been a useful instrument to identify membrane fouling and formation as well as membrane structure [14]. Kim and Fane [14] reported that the thickness of adsorbed protein could be measured in the cake layer of UF using SEM images. Hydrophobicity is suggested to be a very important parameter in membrane fouling as more hydrophobic surface will exhibit a higher degree of fouling. A number of researchers have tried to find a way to express hydrophobicity in a quantitative way [15,16]. Contact angle measurements are routinely used for dense and flat surfaces due to
Fig. 1. The schematic diagram of cross-flow ultrafiltration unit used.
their simple operations but these values cannot be extended to membranes which have a rough surface and pores. None of the previous studies characterized the EfOM removed by the pretreatment and its contribution to membrane fouling. In order to optimize the performance of the membrane filtration, it is extremely important to identify the foulants and fouling mechanisms [17]. A detailed characterization of membrane fouled with EfOM will help in the selection of a suitable pretreatment method and the optimum range of operating parameters of pretreatment.
3. Experimental 3.1. Ultrafiltration set-up In this study, the cross-flow ultrafilter unit (Nitto Denko Corp.) was used to study the effect of pretreatment on the membrane performance. The schematic diagram of cross-flow ultrafilter experimental set-up is shown in Fig. 1. The wastewater with and without pretreatment was pumped to a flat sheet membrane module (effective membrane area 0.006 m2 ). The operating pressure and cross-flow velocity were controlled at 300 kPa and 0.5 m/s by means of by-pass and regulating valves. Reynolds number and shear stress at the wall were 735.5 and 5.33 Pa, respectively. The permeate flux was recycled into feed tank. The membrane used in this study was NTR 7410 (Nitto Denko Corp., Japan) with an average pore size of 17,500 MWCO (Table 1). 3.2. Pretreatment methods The research was carried out with wastewater drawn from the sewage treatment plant after the biologically treatment step. The characteristics of biologically treated sewage Table 1 Characteristics of NTR 7410 membrane used Code Material MWCOa (Da) Contact angle (◦ ) Zeta potential at pH 7 (mV) PWPb at 300 kPa (m/days) a b
Molecular weight cut-off. Pure water permeability.
NTR 7410 Sulfonated polysulfones 17500 69 −98.63 3.01
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Table 2 Characteristics of biologically treated sewage effluent TOC (ppm) BOD5 (ppm) pH SS (ppm) TN (ppm) TP (ppm) Conductivity (S/cm)
6.5–10.4 9.1–19 6.8–7.5 3.5–5.0 23.2–40 2.2–5 200–584
effluent during the period of experimentation are shown in Table 2. The effect of PAC and GAC adsorption and FeCl3 flocculation in removing EfOM from the BTSE was investigated. 3.2.1. Flocculation The flocculation was conducted with FeCl3 of predetermined doses. BTSE was placed in six 1 l containers, and known amounts of ferric chloride were added. FeCl3 was chosen in these experiments as it has a good capability in removing both colloidal organic matter and phosphorus [8,18,19]. It also removes organic matter of small molecular weight through complexation and adsorption mechanisms. The optimum FeCl3 dose was found to be 120 mg/l. The BTSE with the optimum dose of FeCl3 was stirred rapidly for 1 min at 100 rpm, followed by 20 min of slow mixing at 30 rpm, and 30 min of settling. The supernatant was filtered through UF. 3.2.2. PAC adsorption The 1 g/l of PAC was added into a beaker containing BTSE and was stirred using a mechanical stirrer at 100 rpm for 1 h. The ambient temperature was 25 ◦ C. The supernatant was filtered through UF. The characteristics of PAC used are given in Table 3. 3.2.3. Biofilter pretreatment Long term bioadsorption column experiments were conducted using BTSE. The filter column had ports for influent feed, effluent collection and backwashing. The column was packed with 20 g (bed depth of 7 cm) of granular activated
Table 3 Characteristics of powdered activated carbon (PAC) used Specification
PAC-WB
Iodine number (mg/g min) Ash content (%) Moisture content (%) Bulk density (kg/m3 ) Surface area (m2 /g) Nominal size Type Mean pore diameter (Å) Micropore volume (cm3 /g) Mean diameter (m) Product code
900 6 (maximum) 5 (maximum) 290–390 882 80% (minimum) finer than 75 m Wood based 30.61 0.34 19.71 MD3545WB powder
Fig. 2. Schematic diagram of the fixed bed adsorption system.
carbon (Fig. 2). The physical properties of the GAC are shown in Table 4. The GAC bed was acclimatized at a constant filtration rate of 1 m/h. The filter was backwashed for 5 min every 24 h of filtration run. The backwash rate was adjusted to obtain a bed expansion of 30%. A 4-L effluent (after 45 days of operation of the biofilter) was collected from the biofilter and was used as the feed to the UF. This experiment was conducted to investigate the effect of GAC biofilter as pretreatment to the UF unit. 3.3. Effluent characterization methods 3.3.1. Total organic carbon TOC was measured by using the Dohrmann Phoenix 8000 UV-persulfate TOC analyzer with an autosampler. All samples were filtered through 0.45 m membrane prior to the TOC measurement. Thus, the TOC values obtained are, in fact, dissolved organic carbon (DOC) values. 3.3.2. Molecular weight (MW) distribution The wastewater effluent after each pretreatment was subjected to molecular weight distribution measurement. High performance size exclusion chromatography (HPSEC, Shimadzu Corp., Japan) with a SEC column (Protein-pak 125, Waters Milford, USA) was used to determine the MW distribution of EfOM. Standards of MW of various polystyrene sulfonates (PSS: 210, 1800, 4600, 8000, and 18,000 Da) were used to calibrate the equipment. Details on the measurement methodology are given elsewhere [20].
Table 4 Characteristics of granular activated carbon used Specification
GAC (kg/m3 )
Bed density Surface area (m2 /g) Mean pore diameter (Å) Micropore volume (cm3 /g) Mean diameter (mm) Product code
840 1001.2 22.55 0.269 0.75 F-400
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3.4. Membrane characterization methods 3.4.1. Contact angle and zeta potential Contact angle measurements using the sessile drop method with a contact angle meter (Tantec Co., USA) were used to determine an index of membrane hydrophobicity; 20 l of Milli-Q water was dropped onto the dried membrane surface and the contact angle was measured within approximately 10 s [15,16]. Zeta potential on the different membrane surfaces was measured by electrophoresis method (ELS 8000 Otzca, Japan) using polylatex in 10 mM NaCl solution as a standard particle [12,13,21]. The pH of the solution was adjusted with 0.1 N HCl and NaOH. 3.4.2. ATR-FTIR and SEM The clean and fouled membrane surfaces were analyzed for functional groups using attenuated total reflection-Fourier transform infrared spectroscopy. The prepared membranes were examined by FTIR (460 plus, Jasco, Japan) equipped with an ATR accessory and the IR peak was analyzed with Bio-Rad laboratories software. Scanning electron microscopy has been a useful tool for investigating membrane structure during membrane fouling [14]. SEM images of the NTR 7410 membrane were carried out using the SEM (FE-SEM S-4700, Hitachi Corp., Japan). The voltage was 5 kV and the working distance was 12 mm. The magnification was 20,000×. The top and side views of the membranes were analyzed.
4. Results and discussion Prior to the membrane characterization, the effect of the different pretreatment methods was first measured in terms of TOC removal. The TOC removal was the highest with the pretreatment of flocculation followed by adsorption (>90%) (Fig. 3). However, the removal of UF filtration after the pretreatment was negligible, suggesting that EfOM in BTSE (after pretreated) is comprised of significant quantity of organic matter of small molecular weight. It is also equally important to know the range of molecular weight size distribution of effluent after each pretreatment so that appropriate pretreatments can be used to remove all EfOM of different MW ranges. The MW of EfOM in BTSE ranged from 250 to about 3573 Da with a large fraction in the range of 250–520 Da (Fig. 4). The flocculation removed EfOM of MW of 3573–528 Da. Flocculation followed by adsorption as pretreatment removed all the organics including small MW ones. Levine et al. [22] showed size distributions of pollutants removed by various treatment processes. Flocculation is usually used to aggregate wastewater constituents in the size range from less than 0.1 to about 10 m. Adsorption is used to remove the pollutants in the range of 10−4 to 10 m. This means that organics in BTSE should have been removed by only adsorption. However, the results showed a removal of small MW organic compounds in BTSE by
Fig. 3. TOC removal by different pretreatment methods: membrane used = NTR 7410; UF membrane with MWCO of 17,500 Da, cross-flow velocity = 0.5 m/s, pressure = 300 kPa.
FeCl3 flocculation. The mechanism of small MW organic matter removal by flocculation with FeCl3 is mainly due to complexation of iron at wide range of pH (5.5–7.5) [23]. In the present experiment, the pH was between 7 and 7.5. The adsorption of small organic molecules onto iron hydroxide during flocculation also occurs at a neutral pH [24]. In this study, the UF (a loose membrane) was used with pretreatment instead of tight NF membranes. A recent study by the authors showed that pretreatment had practically no effect in improving the removal efficiency of organics when a tight NF membrane was used [1,2]. 4.1. Flux decline and TOC removal The BTSE was filtered through the NTR 7410 ultrafiltration with and without pretreatment. The direct filtration of BTSE led to rapid filtration flux (J) decline with time (Fig. 5). Here J0 is the pure water filtration flux and C0 the initial TOC concentration of BTSE. The normalized flux (J/J0 ) increased from 0.26 (without any pretreatment) to 0.86 (flocculation followed by adsorption) after 18 h UF run. The flux decline followed in following order: without pretreatment > GAC biofilter as pretreatment > PAC adsorption as pretreatment > flocculation as pretreatment > flocculation followed by PAC adsorption as pretreatment. The TOC removal increased from 54% (without pretreatment) to 90% (flocculation followed by adsorption). In other words, the C/C0 ratio of UF decreased from 0.46 (without pretreatment) to 0.1 (with flocculation followed by adsorption as pretreatment). Here, since the UF experiments were conducted in a closed loop system, the TOC concentrations
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Fig. 4. MW distributions with different pretreatments with flocculation followed by adsorption (membrane used = NTR 7410; UF membrane with a MWCO of 17,500, cross-flow velocity = 0.5 m/s and pressure = 300 kPa).
of the permeate increased over time although the increase was marginal. The pretreatment of flocculation followed by adsorption led to the highest flux improvement and the highest TOC removal.
4.2. EfOM concentration on the membrane surface The EfOM concentration of the clean and the fouled membrane surfaces was measured after extraction of the membrane with 0.1 N NaOH solution. The membrane samples
Fig. 5. Temporal variation of filtration flux and TOC removal with and without pretreatment (NTR 7410 membrane, Nitto Denko Corp., biologically treated sewage effluent).
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Fig. 6. Adsorbed EfOM on the fouled membrane surfaces for different pretreatments.
were soaked and stirred periodically for 2 days to desorb the EfOM components from the membranes. The entire EfOM on the membrane dissolved into 0.1 N NaOH solution. The TOC concentration in the NaOH solution was then measured. The highest EfOM concentration was observed for the membranes without any pretreatment. It was as high as 0.011 mg EfOM/cm2 membrane surface (Fig. 6). The pretreatment with flocculation followed by adsorption led to the lowest fouling of the membranes. The amount of EfOM on the membrane surface after this pretreatment was almost similar as that on the clean membrane. 4.3. The effect of contact angle A higher contact angle indicates higher hydrophobicity of the membrane surface [12,13,21]. After different pretreatments, the contact angle measured followed the order as presented in Table 5: without any pretreatment < flocculation as pretreatment < PAC adsorption as pretreatment < GAC Table 5 The effect of the contact angle on the clean and fouled membrane surfaces Membrane surface
Contact angle (◦ )
Clean membrane Membrane without pretreatment Flocculation + membrane GAC biofilter + membrane PAC adsorption + membrane Flocculation + adsorption + membrane
69 30 39 54 50 64
biofilter as pretreatment < flocculation followed by adsorption as pretreatment. The contact angle of the membrane after undergoing filtration of BTSE decreased from 69◦ (clean membrane) to 30◦ (without pretreatment). The contact angle of the membrane with pretreated BTSE (flocculation followed by adsorption) was 64◦ , which is almost the same as that of the clean membrane. This illustrates that the pretreatment with flocculation followed by adsorption could preserve the nature of membrane hydrophobicity on the membrane surface. The fouled membrane had lower contact angle because the foulants constitute of hydrophilic organic matter such as polysaccharides, urea, etc. which are the extracellular enzyme of microorganisms in BTSE. 4.4. Zeta potential characterization Fig. 7 presents the variation in zeta potential of clean membranes as a function of pH, which indicates that the operation at pH 6–10 is the most appropriate application with respect to charge effect. The isoelectric point (IEP) was 2.6. The zeta potential was also measured for the membranes after different pretreatments (Fig. 8). The zeta potential on the membrane surfaces with different pretreatments was higher than that on the clean membrane. The zeta potential increased up to −18 mV after the pretreatment of flocculation. Chun et al. [12] reported that the growth of the cake layer has been developed with increase in the feed concentration. This weakened the electrokinetic flow owing to a lower permeate flux, thus leading to a decrease of the
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Fig. 7. Zeta potential of NTR 7410 membrane surface as a function of solution pH (background electrolyte concentration = 10 mM NaCl).
membrane zeta potential. During FeCl3 flocculation, the Fe ion may have been adsorbed on the membrane surface. This would have caused increase in zeta potential. These results show a similar effect with Peeters’ observation [25]. As the concentrations of CaCl2 and NaCl increased, the zeta potential showed higher values on the nanofilter membranes. Soffer et al. [26] also found that the zeta potential values of all the fouled membranes were less negative. In the present study also, the zeta potential values of all the fouled membranes were observed to be less negative compared to those of the clean membranes. 4.5. ATR-FTIR spectroscopy results for different pretreatments The clean and fouled membrane surfaces were analyzed for functional groups using attenuated total reflectionFourier transform infrared spectroscopy (Figs. 9 and 10). A difference in IR spectra between clean and fouled membranes was observed due to the adsorption phenomena of
Fig. 8. The effect of pretreatments on the zeta potential of membrane.
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Fig. 9. FTIR spectra for clean membrane, and for fouled membranes without any pretreatment, and after a pretreatment of flocculation followed by adsorption (NTR 7410 membrane with biologically treated sewage effluent).
the organic foulants onto the membrane surfaces [16]. The peaks observed at wave numbers of 1540 and 1640 cm−1 are due to the functional group of aromatic carbons [21]. The peaks between 1040 and 1240 cm−1 show the presence of C–O bonds of ethers, carboxylic acids, and polysaccharides. Cho et al. [16,21] identified that the possible foulants on the membranes during the operation with BTSE at 1040, 1540, and 1640 cm−1 were humic fraction and polysaccharides. In this study, the functional groups on the clean NTR 7410 membrane surface were investigated. The main groups observed were: (i) 1625–1590 cm−1 : aromatic group (ring bond), (ii) 1525–1470 cm−1 : aromatic (ring bond), (iii) 1465–1430 cm−1 : aromatic (ring bond), (iv) 1415–1390 cm−1 : sulfur (CO–SO2 –OC), (v) 1375–1335 cm−1 : sulfur (C–SO2 –OC), (vi) 1340– 1290 cm−1 : sulfur (C–SO2 –C), (vii) 1300–1230 cm−1 : sulfur (N=S=O), (viii) 1200–1050 cm−1 : sulfur (C–SO2 –C),
Fig. 10. FTIR spectra for membranes after pretreatments of flocculation, PAC adsorption, and GAC biofilter (NTR 7410 membrane with biologically treated sewage effluent).
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(ix) 1165–1120 cm−1 : sulfur (C–SO2 –C), (x) 1125– 1090 cm−1 : ether (C–O–C), (xi) 1075–1000 cm−1 : alcohol (R–CH2 –OH), and (xii) 950–815 cm−1 : ether (C–O–C). As can be seen in Fig. 9, the FTIR absorbance intensity without any pretreatment was very low with a lot of noise. The peaks with very low absorbance intensity on the surface are too difficult to be analyzed for functional groups (Table 6). The peaks obtained for membrane surface with different pretreatments were compared with the clean membrane surface (Fig. 10). After a pretreatment of PAC adsorption, a peak at 850–775 cm−1 (ether: C–O–C) was observed. After GAC biofilter pretreatment, there were a lot of overlapped peaks
observed with strong intensity. The common feature with these pretreatments was a peak at 1721–1626 cm−1 (not defined) and 1585–1535 cm−1 (urea: R–NH–CO–NH–R). On the other hand, the peaks observed for the membranes with flocculation followed by adsorption as pretreatment were similar to the clean one. 4.6. SEM analysis of clean and fouled membranes Fig. 11 shows the SEM images of clean and fouled membranes after 18 h filtration. The membrane experienced a severe fouling when BTSE was filtered directly through the
Fig. 11. Cross-section of beam energies on filed FE-SEM images of NTR 7410 membrane (working distance of 12 mm and magnification of 20,000×): (a) without pretreatment, (b) after flocculation, (c) after PAC adsorption, (d) after GAC biofilter and (e) after flocculation followed by adsorption.
H.K. Shon et al. / Journal of Membrane Science 234 (2004) 111–120 Table 6 Functional groups obtained by IR spectra (on the fouled membrane surfaces) Pretreatment Without pretreatment PAC adsorption GAC biofilter Flocculation Adsorption + flocculation a
Wave number (cm−1 ) 850–775 1721–1626 1585–1535
Functional groups NDa Ether (C–O–C) Not defined Urea (R–NH–CO–NH–R) NDa NDa
Not detected.
membrane without any pretreatment. The SEM image of the membrane cross-section for this case showed a fouling thickness of 4.3 m (Fig. 11(a)). When a pretreatment of flocculation was used prior to the membrane filtration, the thickness of fouling layer was found to be much less (0.13 m) (Fig. 11(b)). The fouling layer thickness from the membrane surface was 0.26 m after the pretreatment of PAC adsorption (Fig. 11(c)). After the pretreatment by GAC biofilter, the fouling thickness was higher 0.52 m (Fig. 11(d)). On the other hand, the pretreatment of flocculation followed by adsorption had almost similar images as that of the clean membrane with a negligible fouling layer.
5. Conclusions A detailed membrane characterization with NTR 7410 membranes was made in terms of contact angle, zeta potential, ATR-FTIR, SEM, flux decline, and TOC removal both for clean and fouled membranes without and with pretreatment. The results led to the following conclusions: 1. The pretreatment of flocculation followed by adsorption resulted in the highest flux improvement and the TOC removal of more than 90%. 2. The highest EfOM concentration on the fouled membranes was observed to be 0.011 mg EfOM/cm2 membrane surface. This was with membrane with no pretreatment. The pretreatment with flocculation followed by adsorption led to the lowest fouling concentration (0.0052 mg EfOM/cm2 membrane surface). This amount of 0.0052 mg was similar to that for the clean membrane. 3. The contact angle on the fouled membrane was lower than that for the clean membrane. This could be due to the fact that the foulants may consist of relative hydrophilic organic matter such as polysaccharides, urea, which may be the extracellular enzyme of microorganisms in BTSE. The contact angle of the membrane (which underwent filtration of BTSE with pretreatment of flocculation followed by adsorption) was nearly the same as that of clean membrane. 4. The zeta potential of membranes used to filter the wastewater after pretreatment was higher compared to the clean membrane. The zeta potential decreased from
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−98 up to −18 mV after the pretreatment of flocculation. It may be due to the adsorption of ferric ion on the membrane surface. 5. The peaks observed on the fouled membrane were ether (C–O–C) and urea (R–NH–CO–NH–R). On the other hand, the peaks obtained after the pretreatment of flocculation followed by adsorption were similar to those of clean membranes. 6. The SEM images on the membrane cross-section revealed that there was practically no foulant layer on the membrane when a pretreatment of flocculation followed by adsorption was used. The UF without any pretreatment led to a significant large foulant layer. Acknowledgements This research was funded by Australian Research Council (ARC) discovery grant. The support of Brain Pool Korea provided to Prof. S. Vigneswaran during the period of March–June 2003 is greatly appreciated.
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[20] N.G. Her, Identification and characterization of foulants and scalants on NF membrane, Doctoral Thesis of Philosophy, University of Colorado, 2002, pp. 40–49. [21] J. Cho, G. Amy, J. Pelligrino, Y. Yoon, Characterization of clean and natural organic matter (NOM) fouled NF and UF membranes, and the foulants characterization, Desalination 118 (1998) 101– 108. [22] A.D. Levine, G. Tchobanoglous, T. Asano, Characterization of the size distribution of contaminants in wastewater: treatment and reuse implications, J. WPCF 57 (7) (1985) 805–816. [23] A. Vilge-Ritter, J. Rose, A. Masion, J.Y. Bottero, J.M. Laine, Chemistry and structure of aggregates formed with Fe-salts and natural organic matter, Colloids Surf. 147 (1999) 297–308. [24] B.A. Dempsey, R.N. Ganho, C.R. O’Melia, Coagulation of humic substances by means of aluminum salts, J. Am. Water Works Assoc. 74 (1984) 141–150. [25] J.M.M. Peeters, M.H.V. Mulder, H. Strathmann, Streaming potential measurements as a characterization method for nanofiltration membranes, Colloids Surf. A: Physicochem. Eng. Aspects 150 (1999) 247–259. [26] Y. Soffer, J. Gilron, A. Adin, Steaming potential and SEM–EDX study of UF membranes fouled by colloidal iron, Desalination 146 (2002) 115–121.
Effect of pretreatment on the fouling of membranes: application in biologically treated sewage effluent H.K. Shon a , S. Vigneswaran a,∗ , In S. Kim b , J. Cho b , H.H. Ngo a a
Faculty of Engineering, University of Technology, P.O. Box 123, Broadway, Sydney, NSW 2007, Australia Water Reuse Technology Center, Kwangju Institute of Science and Technology, Gwangju, South Korea
b
Received 2 October 2003; received in revised form 7 October 2003; accepted 21 January 2004
Abstract Reuse of wastewater can help in maintaining environmental quality and relieving the unrelenting pressure on conventional and natural freshwater sources. Membrane processes find an important place in the wastewater treatment for reuse. Nonetheless, reverse osmosis (RO) and nanofiltration (NF), i.e. non-porous membranes require higher operational costs and energy. Thus, in this research NTR 7410 ultrafiltration (UF) membrane which is porous was used without and with pretreatment to treat biologically treated sewage effluent (BTSE). Four different pretreatment methods, namely, ferric chloride (FeCl3 ) flocculation, powdered activated carbon (PAC) adsorption, flocculation followed by adsorption, and granular activated carbon (GAC) biofilter were used in this study to compare their relative merits. Experimental results indicate that the most suitable pretreatment was flocculation followed by adsorption leading to a total organic carbon (TOC) removal of 90%. To assess the suitability of the membranes, it is important to conduct a detailed membrane characterization. The fouled NTR 7410 membrane surface was analyzed in terms of contact angle, zeta potential, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), flux decline, and TOC removal. The contact angle of the fouled membrane surface was lower than that of the clean membrane surface. This suggests that the majority of the foulants may have been the hydrophilic organic compounds such as polysaccharides, urea, etc. which are the extracellular enzyme of microorganisms in BTSE. But, the fouled membrane surface after the pretreatment of flocculation followed by adsorption had nearly the same contact angle as that of the clean membrane, suggesting that the hydrophobicity of the membrane is preserved by this pretreatment. According to attenuated total reflection-Fourier transform infrared spectroscopy results, the peaks observed on the fouled membrane were ether (C–O–C) and urea (R–NH–CO–NH–R). On the other hand, the peaks obtained after the pretreatment of flocculation followed by adsorption were similar to those of clean membranes. The highest effluent organic matter (EfOM) concentration on the fouled membranes without any pretreatment was measured up to 0.011 mg EfOM/cm2 membrane surface. The pretreatment of flocculation followed by adsorption reduced the EfOM concentration on the membrane to 0.005 mg EfOM/cm2 . The SEM images on the membrane cross-section revealed that there was practically no foulant layer on the membrane when a pretreatment of flocculation followed by adsorption was used. © 2004 Elsevier B.V. All rights reserved. Keywords: Flocculation; Adsorption; Membrane characterization; Biofilter; Biologically treated sewage effluent; Effluent organic matter; Pretreatment; Ultrafiltration
1. Introduction The domestic wastewater contains more than 99% of water and less than 1% of solids. Therefore, natural shortage of water can be overcome by reuse of wastewater. Membrane processes can be successfully used in obtaining water of recyclable quality. Reverse osmosis (RO) and nanofiltration ∗ Corresponding author. Tel.: +61-2-9514-2653; fax: +61-2-9514-2633. E-mail address: [email protected] (S. Vigneswaran).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.01.015
(NF) can remove all the pollutants including dissolved organics. However, their operational costs are high due to high energy requirement [1,2]. Ultrafiltration (UF) is a cost-effective option in terms of higher permeate flux compared to NF and RO. The problem is that the UF membranes are easily fouled by effluent organic matter (EfOM) present at high levels in treated wastewater. EfOM-fouling, defined as the accumulation and/or adsorption of organic materials on the surface, or in the pores of a membrane, affects membrane performance including permeability and EfOM rejection [3]. Thus, pretreatment is necessary to decrease
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the fouling on the UF membrane. Further, the pretreatment will help in achieving superior organic removal. The main objective of this study was to assess the variation in the characteristics of the NTR 7410 UF membrane surface after the membrane was subjected to filtration of biologically treated sewage effluent (BTSE) with and without pretreatment. The pretreatments used prior to the application of the UF were: (i) flocculation with FeCl3 , (ii) adsorption with PAC, (iii) flocculation followed by adsorption, and (iv) GAC biofilter. The membrane which underwent the filtration of BTSE was characterized in terms of contact angle, zeta potential, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM). The flux decline and total organic carbon (TOC) removal by the UF were also measured.
2. Background The pretreatment such as flocculation can remove 80–90% of the total suspended matter, 40–70% of the BOD5 , 30–60% of the COD, and 80–90% of the bacteria. Adsorption can remove organics which are not removed by conventional chemical and biological treatment methods [4]. The flocculation–adsorption process was able to remove 86% of chemical oxygen demand (COD) from domestic wastewater [5]. Granular activated carbon (GAC) bioadsorption is used extensively for achieving superior removals of particulate organic matter and dissolved solids from wastewater effluents by biological and adsorption processes. Since UF membrane is porous, the characterization of membrane fouling is important to choose the correct design parameters of UF and pretreatment methods. To identify the fouling on the UF membrane surface, the scanning electron microscopy, zeta potential, pyrolysis-GC/MS, attenuated total reflection-Fourier transform infrared spectroscopy, etc. are used [6–11]. The zeta potential is used as the electrokinetic value associating a realistic magnitude of surface charge [12,13]. Chun et al. [12] found that changes in membrane zeta potential could be used to examine the behavior of cake deposition and fouling during the filtration. The identification of fouling can be investigated both by extraction of organics and by ATR-FTIR analysis of deposits at the surface and in the substructure [13]. ATR-FTIR especially is possible to confirm a detailed screen of the molecular functional groups contributing to membrane fouling. SEM has been a useful instrument to identify membrane fouling and formation as well as membrane structure [14]. Kim and Fane [14] reported that the thickness of adsorbed protein could be measured in the cake layer of UF using SEM images. Hydrophobicity is suggested to be a very important parameter in membrane fouling as more hydrophobic surface will exhibit a higher degree of fouling. A number of researchers have tried to find a way to express hydrophobicity in a quantitative way [15,16]. Contact angle measurements are routinely used for dense and flat surfaces due to
Fig. 1. The schematic diagram of cross-flow ultrafiltration unit used.
their simple operations but these values cannot be extended to membranes which have a rough surface and pores. None of the previous studies characterized the EfOM removed by the pretreatment and its contribution to membrane fouling. In order to optimize the performance of the membrane filtration, it is extremely important to identify the foulants and fouling mechanisms [17]. A detailed characterization of membrane fouled with EfOM will help in the selection of a suitable pretreatment method and the optimum range of operating parameters of pretreatment.
3. Experimental 3.1. Ultrafiltration set-up In this study, the cross-flow ultrafilter unit (Nitto Denko Corp.) was used to study the effect of pretreatment on the membrane performance. The schematic diagram of cross-flow ultrafilter experimental set-up is shown in Fig. 1. The wastewater with and without pretreatment was pumped to a flat sheet membrane module (effective membrane area 0.006 m2 ). The operating pressure and cross-flow velocity were controlled at 300 kPa and 0.5 m/s by means of by-pass and regulating valves. Reynolds number and shear stress at the wall were 735.5 and 5.33 Pa, respectively. The permeate flux was recycled into feed tank. The membrane used in this study was NTR 7410 (Nitto Denko Corp., Japan) with an average pore size of 17,500 MWCO (Table 1). 3.2. Pretreatment methods The research was carried out with wastewater drawn from the sewage treatment plant after the biologically treatment step. The characteristics of biologically treated sewage Table 1 Characteristics of NTR 7410 membrane used Code Material MWCOa (Da) Contact angle (◦ ) Zeta potential at pH 7 (mV) PWPb at 300 kPa (m/days) a b
Molecular weight cut-off. Pure water permeability.
NTR 7410 Sulfonated polysulfones 17500 69 −98.63 3.01
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Table 2 Characteristics of biologically treated sewage effluent TOC (ppm) BOD5 (ppm) pH SS (ppm) TN (ppm) TP (ppm) Conductivity (S/cm)
6.5–10.4 9.1–19 6.8–7.5 3.5–5.0 23.2–40 2.2–5 200–584
effluent during the period of experimentation are shown in Table 2. The effect of PAC and GAC adsorption and FeCl3 flocculation in removing EfOM from the BTSE was investigated. 3.2.1. Flocculation The flocculation was conducted with FeCl3 of predetermined doses. BTSE was placed in six 1 l containers, and known amounts of ferric chloride were added. FeCl3 was chosen in these experiments as it has a good capability in removing both colloidal organic matter and phosphorus [8,18,19]. It also removes organic matter of small molecular weight through complexation and adsorption mechanisms. The optimum FeCl3 dose was found to be 120 mg/l. The BTSE with the optimum dose of FeCl3 was stirred rapidly for 1 min at 100 rpm, followed by 20 min of slow mixing at 30 rpm, and 30 min of settling. The supernatant was filtered through UF. 3.2.2. PAC adsorption The 1 g/l of PAC was added into a beaker containing BTSE and was stirred using a mechanical stirrer at 100 rpm for 1 h. The ambient temperature was 25 ◦ C. The supernatant was filtered through UF. The characteristics of PAC used are given in Table 3. 3.2.3. Biofilter pretreatment Long term bioadsorption column experiments were conducted using BTSE. The filter column had ports for influent feed, effluent collection and backwashing. The column was packed with 20 g (bed depth of 7 cm) of granular activated
Table 3 Characteristics of powdered activated carbon (PAC) used Specification
PAC-WB
Iodine number (mg/g min) Ash content (%) Moisture content (%) Bulk density (kg/m3 ) Surface area (m2 /g) Nominal size Type Mean pore diameter (Å) Micropore volume (cm3 /g) Mean diameter (m) Product code
900 6 (maximum) 5 (maximum) 290–390 882 80% (minimum) finer than 75 m Wood based 30.61 0.34 19.71 MD3545WB powder
Fig. 2. Schematic diagram of the fixed bed adsorption system.
carbon (Fig. 2). The physical properties of the GAC are shown in Table 4. The GAC bed was acclimatized at a constant filtration rate of 1 m/h. The filter was backwashed for 5 min every 24 h of filtration run. The backwash rate was adjusted to obtain a bed expansion of 30%. A 4-L effluent (after 45 days of operation of the biofilter) was collected from the biofilter and was used as the feed to the UF. This experiment was conducted to investigate the effect of GAC biofilter as pretreatment to the UF unit. 3.3. Effluent characterization methods 3.3.1. Total organic carbon TOC was measured by using the Dohrmann Phoenix 8000 UV-persulfate TOC analyzer with an autosampler. All samples were filtered through 0.45 m membrane prior to the TOC measurement. Thus, the TOC values obtained are, in fact, dissolved organic carbon (DOC) values. 3.3.2. Molecular weight (MW) distribution The wastewater effluent after each pretreatment was subjected to molecular weight distribution measurement. High performance size exclusion chromatography (HPSEC, Shimadzu Corp., Japan) with a SEC column (Protein-pak 125, Waters Milford, USA) was used to determine the MW distribution of EfOM. Standards of MW of various polystyrene sulfonates (PSS: 210, 1800, 4600, 8000, and 18,000 Da) were used to calibrate the equipment. Details on the measurement methodology are given elsewhere [20].
Table 4 Characteristics of granular activated carbon used Specification
GAC (kg/m3 )
Bed density Surface area (m2 /g) Mean pore diameter (Å) Micropore volume (cm3 /g) Mean diameter (mm) Product code
840 1001.2 22.55 0.269 0.75 F-400
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3.4. Membrane characterization methods 3.4.1. Contact angle and zeta potential Contact angle measurements using the sessile drop method with a contact angle meter (Tantec Co., USA) were used to determine an index of membrane hydrophobicity; 20 l of Milli-Q water was dropped onto the dried membrane surface and the contact angle was measured within approximately 10 s [15,16]. Zeta potential on the different membrane surfaces was measured by electrophoresis method (ELS 8000 Otzca, Japan) using polylatex in 10 mM NaCl solution as a standard particle [12,13,21]. The pH of the solution was adjusted with 0.1 N HCl and NaOH. 3.4.2. ATR-FTIR and SEM The clean and fouled membrane surfaces were analyzed for functional groups using attenuated total reflection-Fourier transform infrared spectroscopy. The prepared membranes were examined by FTIR (460 plus, Jasco, Japan) equipped with an ATR accessory and the IR peak was analyzed with Bio-Rad laboratories software. Scanning electron microscopy has been a useful tool for investigating membrane structure during membrane fouling [14]. SEM images of the NTR 7410 membrane were carried out using the SEM (FE-SEM S-4700, Hitachi Corp., Japan). The voltage was 5 kV and the working distance was 12 mm. The magnification was 20,000×. The top and side views of the membranes were analyzed.
4. Results and discussion Prior to the membrane characterization, the effect of the different pretreatment methods was first measured in terms of TOC removal. The TOC removal was the highest with the pretreatment of flocculation followed by adsorption (>90%) (Fig. 3). However, the removal of UF filtration after the pretreatment was negligible, suggesting that EfOM in BTSE (after pretreated) is comprised of significant quantity of organic matter of small molecular weight. It is also equally important to know the range of molecular weight size distribution of effluent after each pretreatment so that appropriate pretreatments can be used to remove all EfOM of different MW ranges. The MW of EfOM in BTSE ranged from 250 to about 3573 Da with a large fraction in the range of 250–520 Da (Fig. 4). The flocculation removed EfOM of MW of 3573–528 Da. Flocculation followed by adsorption as pretreatment removed all the organics including small MW ones. Levine et al. [22] showed size distributions of pollutants removed by various treatment processes. Flocculation is usually used to aggregate wastewater constituents in the size range from less than 0.1 to about 10 m. Adsorption is used to remove the pollutants in the range of 10−4 to 10 m. This means that organics in BTSE should have been removed by only adsorption. However, the results showed a removal of small MW organic compounds in BTSE by
Fig. 3. TOC removal by different pretreatment methods: membrane used = NTR 7410; UF membrane with MWCO of 17,500 Da, cross-flow velocity = 0.5 m/s, pressure = 300 kPa.
FeCl3 flocculation. The mechanism of small MW organic matter removal by flocculation with FeCl3 is mainly due to complexation of iron at wide range of pH (5.5–7.5) [23]. In the present experiment, the pH was between 7 and 7.5. The adsorption of small organic molecules onto iron hydroxide during flocculation also occurs at a neutral pH [24]. In this study, the UF (a loose membrane) was used with pretreatment instead of tight NF membranes. A recent study by the authors showed that pretreatment had practically no effect in improving the removal efficiency of organics when a tight NF membrane was used [1,2]. 4.1. Flux decline and TOC removal The BTSE was filtered through the NTR 7410 ultrafiltration with and without pretreatment. The direct filtration of BTSE led to rapid filtration flux (J) decline with time (Fig. 5). Here J0 is the pure water filtration flux and C0 the initial TOC concentration of BTSE. The normalized flux (J/J0 ) increased from 0.26 (without any pretreatment) to 0.86 (flocculation followed by adsorption) after 18 h UF run. The flux decline followed in following order: without pretreatment > GAC biofilter as pretreatment > PAC adsorption as pretreatment > flocculation as pretreatment > flocculation followed by PAC adsorption as pretreatment. The TOC removal increased from 54% (without pretreatment) to 90% (flocculation followed by adsorption). In other words, the C/C0 ratio of UF decreased from 0.46 (without pretreatment) to 0.1 (with flocculation followed by adsorption as pretreatment). Here, since the UF experiments were conducted in a closed loop system, the TOC concentrations
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Fig. 4. MW distributions with different pretreatments with flocculation followed by adsorption (membrane used = NTR 7410; UF membrane with a MWCO of 17,500, cross-flow velocity = 0.5 m/s and pressure = 300 kPa).
of the permeate increased over time although the increase was marginal. The pretreatment of flocculation followed by adsorption led to the highest flux improvement and the highest TOC removal.
4.2. EfOM concentration on the membrane surface The EfOM concentration of the clean and the fouled membrane surfaces was measured after extraction of the membrane with 0.1 N NaOH solution. The membrane samples
Fig. 5. Temporal variation of filtration flux and TOC removal with and without pretreatment (NTR 7410 membrane, Nitto Denko Corp., biologically treated sewage effluent).
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Fig. 6. Adsorbed EfOM on the fouled membrane surfaces for different pretreatments.
were soaked and stirred periodically for 2 days to desorb the EfOM components from the membranes. The entire EfOM on the membrane dissolved into 0.1 N NaOH solution. The TOC concentration in the NaOH solution was then measured. The highest EfOM concentration was observed for the membranes without any pretreatment. It was as high as 0.011 mg EfOM/cm2 membrane surface (Fig. 6). The pretreatment with flocculation followed by adsorption led to the lowest fouling of the membranes. The amount of EfOM on the membrane surface after this pretreatment was almost similar as that on the clean membrane. 4.3. The effect of contact angle A higher contact angle indicates higher hydrophobicity of the membrane surface [12,13,21]. After different pretreatments, the contact angle measured followed the order as presented in Table 5: without any pretreatment < flocculation as pretreatment < PAC adsorption as pretreatment < GAC Table 5 The effect of the contact angle on the clean and fouled membrane surfaces Membrane surface
Contact angle (◦ )
Clean membrane Membrane without pretreatment Flocculation + membrane GAC biofilter + membrane PAC adsorption + membrane Flocculation + adsorption + membrane
69 30 39 54 50 64
biofilter as pretreatment < flocculation followed by adsorption as pretreatment. The contact angle of the membrane after undergoing filtration of BTSE decreased from 69◦ (clean membrane) to 30◦ (without pretreatment). The contact angle of the membrane with pretreated BTSE (flocculation followed by adsorption) was 64◦ , which is almost the same as that of the clean membrane. This illustrates that the pretreatment with flocculation followed by adsorption could preserve the nature of membrane hydrophobicity on the membrane surface. The fouled membrane had lower contact angle because the foulants constitute of hydrophilic organic matter such as polysaccharides, urea, etc. which are the extracellular enzyme of microorganisms in BTSE. 4.4. Zeta potential characterization Fig. 7 presents the variation in zeta potential of clean membranes as a function of pH, which indicates that the operation at pH 6–10 is the most appropriate application with respect to charge effect. The isoelectric point (IEP) was 2.6. The zeta potential was also measured for the membranes after different pretreatments (Fig. 8). The zeta potential on the membrane surfaces with different pretreatments was higher than that on the clean membrane. The zeta potential increased up to −18 mV after the pretreatment of flocculation. Chun et al. [12] reported that the growth of the cake layer has been developed with increase in the feed concentration. This weakened the electrokinetic flow owing to a lower permeate flux, thus leading to a decrease of the
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Fig. 7. Zeta potential of NTR 7410 membrane surface as a function of solution pH (background electrolyte concentration = 10 mM NaCl).
membrane zeta potential. During FeCl3 flocculation, the Fe ion may have been adsorbed on the membrane surface. This would have caused increase in zeta potential. These results show a similar effect with Peeters’ observation [25]. As the concentrations of CaCl2 and NaCl increased, the zeta potential showed higher values on the nanofilter membranes. Soffer et al. [26] also found that the zeta potential values of all the fouled membranes were less negative. In the present study also, the zeta potential values of all the fouled membranes were observed to be less negative compared to those of the clean membranes. 4.5. ATR-FTIR spectroscopy results for different pretreatments The clean and fouled membrane surfaces were analyzed for functional groups using attenuated total reflectionFourier transform infrared spectroscopy (Figs. 9 and 10). A difference in IR spectra between clean and fouled membranes was observed due to the adsorption phenomena of
Fig. 8. The effect of pretreatments on the zeta potential of membrane.
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Fig. 9. FTIR spectra for clean membrane, and for fouled membranes without any pretreatment, and after a pretreatment of flocculation followed by adsorption (NTR 7410 membrane with biologically treated sewage effluent).
the organic foulants onto the membrane surfaces [16]. The peaks observed at wave numbers of 1540 and 1640 cm−1 are due to the functional group of aromatic carbons [21]. The peaks between 1040 and 1240 cm−1 show the presence of C–O bonds of ethers, carboxylic acids, and polysaccharides. Cho et al. [16,21] identified that the possible foulants on the membranes during the operation with BTSE at 1040, 1540, and 1640 cm−1 were humic fraction and polysaccharides. In this study, the functional groups on the clean NTR 7410 membrane surface were investigated. The main groups observed were: (i) 1625–1590 cm−1 : aromatic group (ring bond), (ii) 1525–1470 cm−1 : aromatic (ring bond), (iii) 1465–1430 cm−1 : aromatic (ring bond), (iv) 1415–1390 cm−1 : sulfur (CO–SO2 –OC), (v) 1375–1335 cm−1 : sulfur (C–SO2 –OC), (vi) 1340– 1290 cm−1 : sulfur (C–SO2 –C), (vii) 1300–1230 cm−1 : sulfur (N=S=O), (viii) 1200–1050 cm−1 : sulfur (C–SO2 –C),
Fig. 10. FTIR spectra for membranes after pretreatments of flocculation, PAC adsorption, and GAC biofilter (NTR 7410 membrane with biologically treated sewage effluent).
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(ix) 1165–1120 cm−1 : sulfur (C–SO2 –C), (x) 1125– 1090 cm−1 : ether (C–O–C), (xi) 1075–1000 cm−1 : alcohol (R–CH2 –OH), and (xii) 950–815 cm−1 : ether (C–O–C). As can be seen in Fig. 9, the FTIR absorbance intensity without any pretreatment was very low with a lot of noise. The peaks with very low absorbance intensity on the surface are too difficult to be analyzed for functional groups (Table 6). The peaks obtained for membrane surface with different pretreatments were compared with the clean membrane surface (Fig. 10). After a pretreatment of PAC adsorption, a peak at 850–775 cm−1 (ether: C–O–C) was observed. After GAC biofilter pretreatment, there were a lot of overlapped peaks
observed with strong intensity. The common feature with these pretreatments was a peak at 1721–1626 cm−1 (not defined) and 1585–1535 cm−1 (urea: R–NH–CO–NH–R). On the other hand, the peaks observed for the membranes with flocculation followed by adsorption as pretreatment were similar to the clean one. 4.6. SEM analysis of clean and fouled membranes Fig. 11 shows the SEM images of clean and fouled membranes after 18 h filtration. The membrane experienced a severe fouling when BTSE was filtered directly through the
Fig. 11. Cross-section of beam energies on filed FE-SEM images of NTR 7410 membrane (working distance of 12 mm and magnification of 20,000×): (a) without pretreatment, (b) after flocculation, (c) after PAC adsorption, (d) after GAC biofilter and (e) after flocculation followed by adsorption.
H.K. Shon et al. / Journal of Membrane Science 234 (2004) 111–120 Table 6 Functional groups obtained by IR spectra (on the fouled membrane surfaces) Pretreatment Without pretreatment PAC adsorption GAC biofilter Flocculation Adsorption + flocculation a
Wave number (cm−1 ) 850–775 1721–1626 1585–1535
Functional groups NDa Ether (C–O–C) Not defined Urea (R–NH–CO–NH–R) NDa NDa
Not detected.
membrane without any pretreatment. The SEM image of the membrane cross-section for this case showed a fouling thickness of 4.3 m (Fig. 11(a)). When a pretreatment of flocculation was used prior to the membrane filtration, the thickness of fouling layer was found to be much less (0.13 m) (Fig. 11(b)). The fouling layer thickness from the membrane surface was 0.26 m after the pretreatment of PAC adsorption (Fig. 11(c)). After the pretreatment by GAC biofilter, the fouling thickness was higher 0.52 m (Fig. 11(d)). On the other hand, the pretreatment of flocculation followed by adsorption had almost similar images as that of the clean membrane with a negligible fouling layer.
5. Conclusions A detailed membrane characterization with NTR 7410 membranes was made in terms of contact angle, zeta potential, ATR-FTIR, SEM, flux decline, and TOC removal both for clean and fouled membranes without and with pretreatment. The results led to the following conclusions: 1. The pretreatment of flocculation followed by adsorption resulted in the highest flux improvement and the TOC removal of more than 90%. 2. The highest EfOM concentration on the fouled membranes was observed to be 0.011 mg EfOM/cm2 membrane surface. This was with membrane with no pretreatment. The pretreatment with flocculation followed by adsorption led to the lowest fouling concentration (0.0052 mg EfOM/cm2 membrane surface). This amount of 0.0052 mg was similar to that for the clean membrane. 3. The contact angle on the fouled membrane was lower than that for the clean membrane. This could be due to the fact that the foulants may consist of relative hydrophilic organic matter such as polysaccharides, urea, which may be the extracellular enzyme of microorganisms in BTSE. The contact angle of the membrane (which underwent filtration of BTSE with pretreatment of flocculation followed by adsorption) was nearly the same as that of clean membrane. 4. The zeta potential of membranes used to filter the wastewater after pretreatment was higher compared to the clean membrane. The zeta potential decreased from
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−98 up to −18 mV after the pretreatment of flocculation. It may be due to the adsorption of ferric ion on the membrane surface. 5. The peaks observed on the fouled membrane were ether (C–O–C) and urea (R–NH–CO–NH–R). On the other hand, the peaks obtained after the pretreatment of flocculation followed by adsorption were similar to those of clean membranes. 6. The SEM images on the membrane cross-section revealed that there was practically no foulant layer on the membrane when a pretreatment of flocculation followed by adsorption was used. The UF without any pretreatment led to a significant large foulant layer. Acknowledgements This research was funded by Australian Research Council (ARC) discovery grant. The support of Brain Pool Korea provided to Prof. S. Vigneswaran during the period of March–June 2003 is greatly appreciated.
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