Ultrafiltration membrane fouling caused by extracellular organic matter (EOM) from Microcystis aeruginosa: Effects of membrane pore size and surface hydrophobicity

Ultrafiltration membrane fouling caused by extracellular organic matter (EOM) from Microcystis aeruginosa: Effects of membrane pore size and surface hydrophobicity

Journal of Membrane Science 449 (2014) 58–66 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.co...

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Journal of Membrane Science 449 (2014) 58–66

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Ultrafiltration membrane fouling caused by extracellular organic matter (EOM) from Microcystis aeruginosa: Effects of membrane pore size and surface hydrophobicity Fangshu Qu a, Heng Liang a,n, Jian Zhou b, Jun Nan a, Senlin Shao a, Jianqiao Zhang a, Guibai Li a a b

State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), Harbin Institute of Technology, Harbin 150090, PR China Key Laboratory of the Three Gorges Reservoir Region's Eco-Environment, Chongqing University, Chongqing 400045, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 18 March 2013 Received in revised form 5 July 2013 Accepted 29 July 2013 Available online 24 August 2013

This study focused on the effects of membrane pore size and surface hydrophobicity on ultrafiltration (UF) membrane fouling caused by extracellular organic matter (EOM) from Microcystis aeruginosa. A hydrophilic membrane (cellulose acetate) with a molecular weight cutoff (MWCO) of 100 kDa and hydrophobic membranes (polyethersulfone) with MWCO of 100, 30 and 10 kDa were employed for UF experiments. The results indicated that the hydrophobic membrane suffered more adsorptive fouling, faster flux decline and worse fouling reversibility than the hydrophilic membrane when treating EOM solution. The membrane with larger pores exhibited worse flux reduction but less adsorptive fouling and superior flux recovery. Mass balances of dissolved organic carbon (DOC) content implied that more EOM passed through the hydrophilic membrane owing to a lack of hydrophobic adsorption and that the larger pore membrane allowed for higher EOM retention and a greater capacity for irreversibly deposited EOM. Fluorescence excitation-emission matrix (EEM) spectra coupling with regional integration were used to further analyze the fates of protein-like and humic-like substances during UF. Membrane pore size and surface hydrophobicity apparently influenced the transportation of protein-like substances, but they were of less importance for humic-like substances. In addition, four classic filtration models were introduced to analyze the fouling mechanisms. Cake formation was identified as the main mechanism for flux decline caused by EOM in this study, independent of membrane pore size and surface hydrophobicity. & 2013 Elsevier B.V. All rights reserved.

Keywords: Ultrafiltration (UF) Membrane fouling Extracellular organic matter (EOM) Pore size Surface hydrophobicity

1. Introduction Ultrafiltration (UF) technology is increasingly applied in drinking water production and wastewater reclamation owing to its high performance in the removal of particles, colloids and microorganisms [1]. However, membrane fouling, which will remarkably deteriorate membrane permeability and consequently result in high energy consumption, has become a major obstacle to the further application of UF technology in water treatment [2,3]. It is widely considered that natural dissolved organics in surface water are important foulants for low-pressure membranes such as microfiltration (MF) and UF [4,5]. Moreover, macromolecular organics such

n

Corresponding author. Tel./fax: þ 86 451 86283001. E-mail addresses: [email protected] (F. Qu). [email protected] (H. Liang), [email protected] (J. Zhou). [email protected] (J. Nan), [email protected] (S. Shao). [email protected] (J. Zhang), [email protected] (G. Li). 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.07.070

as polysaccharides have proved to be responsible for the irreversible fouling [6]. Algae are ubiquitous in rivers and reservoirs polluted by nutrients, causing several problems such as odors, toxins, formation of disinfection by-products and dysfunction of water treatment process [7–9]. As UF can effectively remove algal cells without destroying them, it has a great potential for algae-rich water treatment [10]. However, the deposition of algal cells and extracellular organic matter (EOM) on the membrane can cause severe membrane fouling, especially for EOM which is a typical type of autochthonous natural organic matter (NOM) [11,12]. Henderson et al. extracted and characterized the EOM from cyanobacteria, green algae and diatoms, and found that the EOM was comprised of proteins, polysaccharides and humic-like substances [13]. In addition, the molecular weight (MW) distribution and the hydrophobicity of algal EOM were also investigated. It was reported that the EOM was characterized by high MW and strong hydrophilicity with the hydrophilic portion accounting for 57.3% [14]. Meanwhile, membrane fouling caused by EOM was of particular concern. Chiou et al. [15] has reported that algae with more EOM

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led to faster flux decline. Moreover, EOM was found to be responsible for the severe fouling of a ceramic MF membrane with a high-MW fraction as the dominant foulant [16]. In our previous study, the relationship between the characteristics of EOM and membrane fouling were investigated based on a complete characterization of EOM, and the high-MW portion of EOM was also found to play an important role during fouling formation [17]. Moreover, the hydrophobic fraction of EOM was found to contribute more to the irreversible fouling, whereas the hydrophilic fraction exhibited a stronger impact on the membrane flux. Overall, most studies focused on the effects of the amount and characteristics of EOM on the membrane fouling, but the influences of membrane properties on the EOM fouling have not been investigated. Membrane properties include pore size, surface hydrophobicity, surface charge, and surface roughness. All of the properties can influence the membrane fouling to some extent. Kwang et al. [18] has investigated the effect of pore size on the microfiltration membrane fouling caused particles, and found that the loose membrane was subjected to more severe fouling. Costa et al. [19] compared the humic acid fouling of membranes with different pore sizes, and found that the tight membrane displayed stronger anti-fouling ability. Lin et al. [20] investigated the UF membrane fouling by dissolved organic matter (DOM), and also observed that the fouling of the loose membrane was much more severe due to more pore blockage. With respect to the surface hydrophilicity, Ochoa et al. [21] reported that hydrophilicity modification could help to enhance the anti-fouling of PVDF membranes when they were used to treat emulsified oil wastewater. Xiao et al. [22] studied the combined effect of membrane hydrophobicity and surface charge on adsorptive fouling during microfiltration, and discovered that the surface hydrophobic interaction rather than the electrostatic interaction was the predominant mechanism affecting adsorptive fouling by soluble microbial products (SMP). However, these studies were performed with different membranes (i.e., nanofiltration, UF and MF membranes) and different foulants (i.e., NOM, SMP, secondary effluent organic matter (EfOM) or model foulants such as humic acid, alginate and bovine serum albumin). Thus, the literature cannot directly contribute to understanding the UF membrane fouling caused by EOM when the UF membrane is used to treat algae-rich water for drinking water production. It was widely considered that EOM was analogous to SMP, but the UF system for water treatment and the MBR for wastewater treatment were operated under apparently different conditions. Therefore, it is necessary to investigate the effect of on membrane pore size and surface hydrophobicity on UF membrane fouling by EOM, which may contribute to the design of UF system for reservoir water treatment. In this study, four types of UF membrane were employed to study the fouling by EOM extracted from Microcystis aeruginosa, which usually dominated in seasonal algae blooms. The effects of membrane pore size and surface hydrophobicity on the flux decline, fouling reversibility, and fates of proteins, polysaccharides and humic-like substances during UF of EOM solution were systematically investigated.

2. Materials and methods 2.1. Algae culture and EOM extraction Microcystis aeruginosa was purchased from the Culture Collection of Algae at the Institute of Hydrobiology, Chinese Academy of Sciences, China. The instructions for both algae cultivation and EOM extraction can be found in Qu et al. [17]. Axenic cultures were conducted in batch mode in 1 L conical flasks with BG11 medium. The conical flasks were placed in an incubator at 25 1C with an illumination of 5000 l  provided for 14 h every day.

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Microcystis aeruginosa was cultured for 42 d to allow the cells to grow into a stationary phase. Algal EOM was extracted by centrifuging the cell suspension at 10,000g and 4 1C for 15 min and by subsequently filtering the supernatant through a 0.45 μm mixed cellulose filter (Taoyuan Co. Ltd, China). EOM was carefully characterized in a previous study [17]. EOM was comprised of protein-like, polysaccharide-like and humic-like substances. The DOC concentration of the EOM ranged from 15.82– 18.46 mg/L due to the dissimilarity in growth rates. The molecular weight distribution of EOM was characterized by bimodal peaks having high-MW (4100 kDa) and low-MW (o100 kDa) fractions accounting for 45.0% and 33.4%, respectively. The hydrophilic fraction of the EOM (61.5%) was significantly greater than the hydrophobic fraction (34.8%). 2.2. Membranes and experimental setup Four types of UF membranes with a surface area of 45 cm2 were used in this study. A cellulose acetate (CA) membrane (Millipore, USA) with a molecular weight cutoff (MWCO) of 100 kDa and polyethersulfone (PES) membranes (Pall, USA) with MWCO of 100, 30 and 10 kDa were employed in filtration experiments and were denoted CA100, PES100, PES30 and PES10, respectively. The PES membrane with a contact angle greater than 57.11 was much more hydrophobic than the CA membrane which had a contact angle of 19.31 (refer to Table 1). Moreover, these membranes were all negatively charged with the average zeta potential ranging from 13.92–16.88 mV, and their average roughness varied from 16.5–22.8 nm (shown in Table 1). The contact angle of the membrane was measured by the standard sessile drop method with de-ionized water serving as the probe liquid, using an optical contact measurement system (DSA-100, KRUSS, Germany) [23]. Membrane zeta potential was determined by a streaming current electro kinetic analyzer (SurPass, Anton Paar GmbH, Graz, Austria). The electrolyte solution was 10 mM KCl (pH 7.0) and the membranes were cleaned with Milli-Q water for 1 d prior to measurement [22]. Membrane roughness was obtained by analyzing atomic force microscopy (AFM) images of new membranes [23]. The AFM employed was from Digital Instruments (Veeco, USA), and the data were analyzed with Nanoscope V5.80 software. AFM observation was performed under the tapping mode, by using a tip made of etched single crystal silicon. The membrane samples were scanned over a range of 10 μm  10 μm. The experimental setup, which was introduced in detail in Qu et al. [17], included a nitrogen cylinder, a stirred cell (Amicon 8400, Millipore Corp., USA), and an electronic balance connected to a computer. Nitrogen gas at a constant pressure of 0.03 MPa was utilized to drive the feed solution through the membrane. 2.3. Experimental protocol Fouling tests were comprised of two parts, adsorptive fouling and filtration fouling. To investigate the adsorptive fouling, membranes were placed in the stirred cell with its glossy side exposed to the EOM solution (100 mL, pH 7.070.1) for 2 h. Preliminary adsorptive fouling tests had shown that 2 h was sufficient to achieve saturation of the surface adsorption capacity for the membranes used in this Table 1 Characteristics of membranes used in this study. Membrane MWCO (kDa)

Contact angle (deg)

Zeta potential (mv)

Roughness (nm)

CA100 PES100 PES30 PES10

19.3 7 1.6 58.2 7 0.9 60.5 7 2.3 57.17 1.5

 14.917 0.47  16.88 7 1.13  13.92 7 1.35  14.737 0.95

22.8 7 1.2 19.9 7 1.7 16.5 7 1.2 18.4 7 1.9

100 100 30 10

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study (data not shown). To strengthen the adsorption of EOM, a very high dissolved organic carbon (DOC) concentration (10.0 mg/L) was used. Additionally, the stirrer was kept running at a rate of 300 rpm to enhance contact between foulants and the membrane [24]. During this process, the outlet of the stirred cell was sealed to prevent water from passing through the membrane driven by the level difference. The adsorptive fouling was evaluated by relative flux reduction (RFR), which could be calculated with the Milli-Q water flux before and after adsorptive fouling (Eq.(1)). The adsorptive fouling experiment was conducted in triplicate. RFRð%Þ ¼

J 0 J′  100% J0

ð1Þ

where J0 and J′ refer to the Milli-Q water fluxes of the membranes before and after adsorptive fouling, respectively. In the second part of the process, a series of UF experiments was also carried out in the stirred cell. EOM solutions were diluted with Milli-Q water to 3.0 mg/L in DOC and their pH values were adjusted to 7.070.1. Prior to filtrating the EOM solution, the Milli-Q water flux of each membrane was measured and designated J0. Every UF experiment contained three continuous filtration cycles. Each cycle included three steps: (1) filter 300 mL of the 350 mL feed solution, (2) turn over the membrane and backwash it for 2 min with Milli-Q water, and (3) filter 100 mL Milli-Q water. The flux at the end of each filtration process was classified as Je(n) with the number n (1–3) representing the cycle number. The Milli-Q water flux after rinsing was denoted Jn. Then, the reversible fouling (RF) in each filtration cycle and the accumulative irreversible fouling (IF) could be calculated as follows according to Jermann et al. [25]. The filtration test was duplicated for each type of membrane. IF n ¼ ðJ 0 J n Þ=J 0

ð2Þ

RF n ¼ ðJ 0 J eðnÞ Þ=J 0 ðJ 0 J n Þ=J 0 ¼ ðJ n J eðnÞ Þ=J 0

ð3Þ

water was subtracted from all spectra to eliminate water Raman scattering. Moreover, spectral correction was taken to minimize the instrumental bias potentially including the wavelength-dependent variability in the transmission efficiency of monochromators and the fluctuations in spectrometer light intensity. All spectra were obtained at a solution pH of 7.070.1 and an ambient temperature of 2171 1C. To interpret the EEM spectral data, regional integration was used for semi-quantitative analysis of various components in the EOM [26]. Because EOM from lab-cultured cyanobacteria was much simpler in component diversity than NOM and EfOM, the Ex and Em boundaries were separated into three regions based on the locations of the main peaks rather than the five regions in Chen et al. The integration area was between the two Raman peaks. The wave length ranges of region T1, A and B were Ex 240–360 nm and Em 250–380 nm, Ex 300– 450 nm and Em 380–550 nm, and Ex 240–300 nm and Em 380– 550 nm, respectively. The integration of volumes beneath the EEM spectra was performed via MATLAB afterwards. Then, the volumetric values of the different regions could be used to evaluate the fates of those fluorophores. In addition, the fate of polysaccharide-like substances during UF was also calculated by measuring the polysaccharide concentrations using the phenol-sulfuric method [28]. Glucose was used for calibration (0–100 mg/L) and the results were given as glucose equivalents. The analyses were performed in triplicate. 2.5. Modeling for membrane fouling process Four classic filtration models including complete blocking, standard blocking, intermediate blocking and cake filtration have been widely adopted to interpret the flux decline of low-pressure membrane filtration in dead-end mode under constant pressure, and their schematic diagrams are presented in Fig. 1 [29]. The assumptions for the filtration models were introduced well in Shen et al. [30] and their

2.4. Mass balances during UF based on DOC and fluorescence excitation-emission matrix (EEM) spectrum Mass balance, which indicates the fate of organics during UF, is critically important for the investigation of membrane fouling. In a filtration process, organics may pass through the membrane, remain in retentate, reversibly accumulate on the membrane or irreversibly adhere onto/into the membrane. When the membrane property and water quality vary, the transportation of organics may apparently be changed. To investigate the fate of EOM during UF, the feed solution, permeate, retentate solution and backwash waste were collected and their DOC concentrations were measured by a total organic matter (TOC) analyzer (multi N/C 2100S, Analytic Jena, Germany). Then, the mass balance of organic carbon during UF could be calculated. The measurements of the DOC concentration were conducted in triplicate. As UF is a physical separation process, the fates of the subfractional components (protein-like, polysaccharide-like and humiclike substances) of the EOM should comply with the mass conservation principle. Mass balance analysis of EOM components can provide valuable information for identifying their fouling contributions and is, thus, of great importance. Three dimensional fluorescence EEM spectrum coupling with regional integration was employed to quantify the amounts of humic-like and protein-like organics in the feed solution, permeate, retentate and backwash waste [26]. Fluorescence spectra were obtained using a fluorescence spectrophotometer (F7000, Hitachi, Japan) at excitation (Ex) wavelengths of 200–450 nm in 5 nm increments and emission (Em) wavelengths of 250–550 nm in 1 nm increments. To prevent the inner filter effect, which may result in a non-linear relationship between the fluorescence intensity and foulant concentration, all samples were diluted to a concentration below 5.0 mg/L [27]. A controlled fluorescence spectrum of Milli-Q

Fig. 1. Schematic diagrams of four filtration models: (a) complete blocking, (b) standard blocking, (c) intermediate blocking, and (d) cake filtration.

Table 2 Equations of four classic filtration models. Models

Equations

Complete blocking Standard blocking Intermediate blocking Cake filtration

J0  J ¼A*V 1/t þB ¼J0/V ln J0  ln J ¼C*V (1/J)  (1/J0) ¼D*V

Note: V, accumulative volume; t, filtration time; A, B, C and D are constants, respectively.

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equations are listed in Table 2. The experimental results were fitted according to these equations with the R-squared (R2) value indicating the goodness of the fit.

3. Results and discussion 3.1. Effects of membrane pore size and surface hydrophobicity on the adsorptive fouling by EOM Fig. 2 shows the relative flux reductions of the four types of membranes after adsorptive fouling by EOM. It can be observed that the relative flux reduction of the CA100 membrane (6.4%) after adoptive fouling was smaller than that of the PES100 membrane (8.5%). This result can be explained by the stronger hydrophobicity of the PES100 membrane, which may enhance the hydrophobic interactions between the EOM and the membrane surface [22,31]. Fig. 2 also shows that the relative flux reduction apparently expanded from 8.5% to 21.5% as the MWCO of PES membrane decreased from 100 to 10 kDa. This result implies that tight membranes (i.e., the PES30 and PES10 membranes) were more susceptible to adsorptive fouling. There was 33.4% of the EOM less than 1 kDa [17], and the low-MW EOM could penetrate into PES10 and PES30 membranes as well as PES100 membrane, causing pore constriction. In addition, at the same thickness of adsorptive fouling layer, a tighter UF membrane would be more affected than a looser membrane in terms of irreversible fouling. Thus, more severe adsorptive fouling occurred when a tighter membrane was used. 3.2. Effects of membrane pore size and surface hydrophobicity on the flux decline by EOM Fig. 3 shows normalized flux curves during filtration of the EOM solution with four types of membranes. Compared to the CA100 membrane, the PES100 membrane exhibited a faster decrease in flux with the normalized flux reduced to as low as 0.10. It is consistent with the general recognition that stronger surface hydrophilicity favors membrane fouling control [22,32]. It can be seen in Fig. 3 that less flux reduction occurred during UF of the EOM solution with lower MWCO membranes. The normalized fluxes at the end of filtration were 0.25 and 0.44 for the PES30 and PES10 membranes, respectively. In other studies, the membranes with larger pores were also subjected to more severe fouling caused by humic acids or soluble microbial products [21,33]. There are two probable interpretations for this phenomenon. Firstly, as reported by Hwang et al. [18], larger membrane pores were more vulnerable to blockage and constriction, which had much greater impacts on the flux than the cake layer that formed

Fig. 2. Effects of membrane pore size and surface hydrophobicity on the adsorptive fouling caused by EOM.

Fig. 3. Effects of membrane pore size and surface hydrophobicity on the flux decline by EOM.

on tighter membranes. The second interpretation is associated with the characteristics of the cake layer. Because membrane flux increases with pore size, the cake layer on the loose membrane may be compacted to be less porous by stronger permeate drag [34]. Thus, higher cake resistance occurred when loose membranes (i.e., PES100) were used, although the amounts of EOM in the feed solutions were comparable for different membranes. Meanwhile, the membrane filtration process is comprised of membrane filtration and cake filtration. The total resistance is the sum of membrane resistance and cake resistance according to the Darcy laws. When the membrane resistance is much larger than that of the cake layer, the accumulation of foulants plays a limited role in flux reduction. Therefore, the loose membrane (i.e., PES100), which had a lower membrane resistance but higher cake resistance, encountered much faster flux decline during UF of the EOM solution. 3.3. Effects of membrane pore size and surface hydrophobicity on the reversibility of fouling by EOM Both reversible fouling and irreversible fouling, which occurred during filtration of the EOM solution with four types of membranes, are presented in Fig. 4. The reversible fouling of the PES100 membrane was comparable to that of the CA100 membrane; however, the PES100 membrane suffered worse reversibility. As the PES100 membrane was more hydrophobic than CA100 membrane, the hydrophobic interaction between the PES100 membrane and the EOM might be much stronger, leading to higher irreversible fouling. Gray et al. [32] also found that the hydrophilic membrane obtained greater flux recovery after backwashing than the hydrophobic membrane when investigating the effects of membrane properties on MF performance. Fig. 4 also demonstrates that fouling reversibility gradually deteriorated as the MWCO of the membrane decreased from 100 to 10 kDa, though the total fouling was dramatically alleviated. This result suggests that the membrane with smaller pores was more susceptible to the irreversible adhesion of EOM. A similar phenomenon was found that irreversible fouling resistance increased as the membrane permeability decreased when UF membranes of various sizes were used to filter the supernatant of membrane bioreactor (MBR) [35]. Moreover, Peeva et al. [36] also reported that a higher MWCO membrane was easier to clean in a study of membrane fouling by humic acids. They claimed that a loose membrane was better backwashed due to a higher flow rate, because its resistance was smaller than that of the tight membrane. This reason can also explain the results in the current study. The PES30 and PES10 membranes, which had greater membrane resistance, were less effectively backwashed than the PES100 membrane under the same pressure. Moreover, irreversible adhesion is an important way for organics to cause irreversible fouling [22]. The adsorptive fouling test has demonstrated that the EOM was

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inclined to adhere in the PES30 and PES10 membranes as shown in Fig. 2. Therefore, the PES30 and PES10 membranes suffered more irreversible fouling than the PES100 membrane. 3.4. Effects of membrane pore size and surface hydrophobicity on mass balance based on DOC The mass balances based on DOC during UF of the EOM solution with four types of membranes are shown in Fig. 5. The terms permeate, retentate, reversibly deposited and irreversibly deposited refer to the organics in permeate, retentate solution, backwash waste and irreversibly deposited on membrane, respectively. The organics passing through and irreversibly depositing on

Fig. 4. Effects of membrane pore size and surface hydrophobicity on the reversibility of EOM fouling.

the CA100 membrane accounted for 37.1% and 11.0% of the EOM, respectively. For the PES100 membrane, the ratio of organics in permeate slightly decreased to 36.2%, while the irreversible deposition of organics increased (14.3%). This result demonstrates that stronger membrane hydrophobicity would lead to worse adhesion of EOM on the membrane surface [37]. In Fig. 5, the organics in permeate slightly increased from 36.2% to 39.4% with a decrease in membrane pore size from 100 to 10 kDa. The result indicates that the membrane with lower MWCO retained less EOM. Usually, the retention of organics is strongly dependent on the membrane pore size [38], which is contrary to the result of the current study. Nevertheless, similar results were also found in another study.

Fig. 5. Effects of pore size and the surface hydrophobicity of membranes on the mass balance of organics during UF of the EOM solution. Error bar indicates the standard error of triplicate measurements.

Fig. 6. EEM spectra of feed solution (a, dilution factor 1:1) permeate (b, dilution factor 1:1), concentrate (c, dilution factor 1:2) and backwash waste (d, dilution factor 1:6) during filtration of EOM solution with PES100 membrane.

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Lin et al. [20] has reported that the retention rate of DOM by a 100 kDa membrane was dramatically higher than that by a 10 kDa membrane. This contradiction is ascribed to the shift of organic matter removal mechanisms from membrane retention to cake interception. In other words, the cake layer on the membrane surface may trap some lowMW molecules and enhance the removal of organics during membrane filtration [34,39]. For the PES100 membrane, the cake layer might be compacted to be less porous than those on the PES30 and PES10 membranes due to the higher flow rate towards the membrane, thus resulting in the reduced diffusion of solutes across membrane. Overall, membrane retention contributed more to the removal of EOM by the PES30 and PES10 membranes, whereas cake interception served as the main mechanism in the case of the PES100 membrane. As shown in Fig. 5, the irreversibly deposited organics accounted for 14.3%, 10.0% and 9.1% of the total EOM during filtration with the PES100, PES30 and PES10 membranes, respectively. This result indicates that the loose membrane exhibited a greater capacity for irreversibly deposited EOM. The result is presumably related to the stronger permeate drag, which can make more foulants move towards and deposit on membrane surface, during filtration with the loose membrane [35]. Thus, more EOM irreversibly deposited on the PES100 membrane. However, this result seems contradictory to the less irreversible fouling of the PES100 membrane shown in Fig. 4. Irreversible fouling depends not only on the amount of foulants but also on their locations (inside the membrane pore or on the membrane surface) [6]. For the PES100 membrane, more organics irreversibly deposited on the membrane surface due to cake layer interception, and fewer low-MW molecules penetrated into the membrane pores. With respect to PES30 and PES10 membranes, less cake layer interception would allow for more low-MW organics to adhere to the pore walls, causing pore narrowing. Therefore, the PES30 and PES10 membranes suffered worse fouling reversibility, even though fewer organics irreversibly deposited on these membranes than that on the PES100 membrane.

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To evaluate the fates of the various components of EOM during UF, three regions around the main peaks (i.e., T1, A and C) were chosen for volumetric integration. Then, mass balances of these components were calculated according to the volumetric values which were in proportion to their amounts under low foulant concentrations [25]. Specifically, the amount of humic-like substances was represented by the sum of the volumetric values of regions A and C, whereas the volumetric values of region T1 were used to assess the amount of protein-like substances because peak T2 disappeared in the EEM spectra of the feed solution, permeate and retentate solution.

3.5. Effect of membrane pore size and surface hydrophobicity on the transportation of humic-like, protein-like and polysaccharide-like substances Three-dimensional fluorescence EEM spectrum has been widely utilized to identify the chemical composition of NOM and the organic foulants in MBR because of its ability to distinguish among certain classes of organic matter [27]. Five key fluorescence peaks referred to as A, C, T1, T2 and B are commonly observed in natural water samples. Peaks T and B are related to tryptophan-like and tyrosine-like materials, respectively [26]. Peaks A and C are associated with humic substances derived from the breakdown of plant material [40]. Fig. 6 presents the fluorescence EEM spectra of the feed solution, permeate, retentate and backwash waste during filtration of the EOM solution with the PES100 membrane. In Fig. 6 (a), there are peaks T1 (Ex/Em: 280 nm/330 nm), A (Ex/Em: 340 nm/430 nm) and C (Ex/ Em: 265 nm/440 nm) in the EEM spectrum of the feed solution. Peak T1 in the EEM spectrum is related to protein-like substances released by cyanobacterial cells including amino acids, peptides and enzymes [14]. As Milli-Q water was used to prepare the culture medium, there were no humic acids or fulvic acids in the lab-cultured cyanobacteria solution. Thus, Peaks A and C in the EEM spectra were related to humic-like substances, which might have similar functional groups to humic acids and fulvic acids, that originated from the breakdown of dead cells. Moreover, peak T2 (Ex/Em: 225 nm/330 nm), which is also associated with tryptophan-like materials, is observed in the EEM spectrum of backwashing waste (Fig. 6(d)). The peak disappears in the EEM spectra of samples containing residual culture medium due to the strong absorbance of excitation light by nitrate which is known to absorb ultraviolet light with wavelength shorter than 254 nm [41].

Fig. 7. Effects of membrane pore size and surface hydrophobicity on the mass balance of protein-like (a), polysaccharide-like (b) and humic-like substances (c) during UF of EOM solution. Error bar indicates the standard error of triplicate measurements.

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Fig. 7 presents the mass balances of protein-like, polysaccharidelike and humic-like substances during UF of EOM solution with different membranes. In Fig. 7(a), more protein-like substances (25.7%) passed through the CA100 membrane than that through the PES100 membrane (22.9%). In terms of irreversibly deposited organics, fewer protein-like substances (17.4%) deposited on the CA100 membrane in comparison with the PES100 membrane (23.4%). Because stronger membrane surface hydrophobicity could strengthen the hydrophobic adhesion between protein-like substances and the membrane surface, the PES membrane, which was stronger in hydrophobicity, exhibited superior retention of protein-like substances [17]. With respect to membrane pore size, the fates of protein-like and polysaccharide-like substances during UF were similar to that of DOC as shown in Fig. 5. Specifically, protein-like and polysaccharide-like substances in permeate slightly increased with decreasing pore size, but those irreversibly deposited on the membrane were apparently reduced due to weaker permeate drag during filtration with tight membranes (i.e., PES30 and PES10 membranes) [38]. For humic-like substances, neither pore size nor surface hydrophobicity showed marked impacts on their fates during UF with at least 75.4% of these substances penetrating through all membranes. Comparing the fates of the protein-like, polysaccharide-like and humic-like substances, most parts of the proteinlike and polysaccharide-like substances were retained, whereas the more humic-like substances diffused across the membranes. This result agrees well with the conclusion found in our previous study that protein-like and polysaccharide-like substances in EOM were mainly distributed in the high-MW fraction whereas humic-like substances were much lower in MW [17].

3.6. Effects of membrane pore size and surface hydrophobicity on the mechanisms for flux decline caused by EOM To investigate influences of membrane surface hydrophobicity and pore size on the mechanisms for flux decline caused by EOM, the four

classic filtration models were used to fit the experimental results. The regression results of the PES100 membrane are present in Fig. 8. The R2 values were 0.793, 0.919, 0.949 and 0.996 for complete blocking, standard blocking, intermediate blocking and cake filtration, respectively. This result indicates that the mechanism for flux decline caused by EOM was mainly ascribed to cake formation, allowing for the possibility that standard blocking and intermediate blocking were involved to some extent as well. Conversely, Shen et al. [30] has reported that complete blocking rather than cake filtration was the major mechanism of fouling caused by the hydrophilic fraction of soluble microbial products from the MBR. There are two possible reasons for this disparity. Firstly, the amount of macromolecules such as proteins and polysaccharides in the EOM (45.0%) was much greater than that in the hydrophilic fraction of SMP (25.8%) [17]. Secondly, UF membranes with an MWCO of, or less than, 100 kDa were used in this study, whereas Shen et al. [30] chose a MF membrane with pore size of 0.22μm for filtration. Thus, a much thicker cake layer formed on the UF membranes and gave rise to worse flux deterioration in this study. Moreover, humic-like substances in EOM, which were proved to be low in MW but quite strong in hydrophobicity, could lead to hydrophobic adhesion inside the membrane pores [17]. Therefore, standard blocking also played a role during the formation of fouling in the current study. Overall, cake formation together with standard and intermediate blocking governed fouling formation during UF of the EOM solution with the PES100 membrane. Table 3 R2 values of regression analyses of EOM fouling of the four types of membranes. Membranes Complete blocking

Standard blocking

Intermediate blocking

Cake filtration

CA100 PES100 PES30 PES10

0.922 0.919 0.962 0.975

0.944 0.949 0.979 0.990

0.996 0.996 0.999 0.998

0.809 0.793 0.917 0.970

Fig. 8. Regression analyses of EOM fouling of PES100 membrane: (a) complete blocking, (b) standard blocking, (c) intermediate blocking, and (d) cake filtration.

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The R2 values of the regression analyses of the experimental results obtained with the four types of membranes are shown in Table 3. It can be found that R2 values for cake filtration were all above 0.996, indicating that cake formation was the major mechanism for flux decline caused by EOM, independent of surface hydrophobicity and pore size. In terms of surface hydrophobicity, no obvious variations in R2 values were found between the PES100 and CA100 membranes. It can also be observed that the fouling behaviors of the smaller pore membrane were better described by complete blocking, standard blocking and intermediate blocking with R2 values clearly increasing as the pore size decreases (Table 3). Two reasons can be found for this phenomenon. Firstly, smaller membrane pores are more inclined to be completely blocked. Secondly, the enhanced adsorption in minor pores may be advantageous for standard blocking.

4. Conclusions The effects of membrane pore size and surface hydrophobicity on the adsorptive fouling and filtration fouling caused by EOM from Microcystis aeruginosa were systematically investigated. The following conclusions can be drawn. (1) The adsorptive fouling caused by EOM increased with the membrane surface hydrophobicity. UF membrane with smaller pores suffered worse adsorptive fouling when treating the EOM solution. (2) Both flux decline and fouling reversibility deteriorated during UF of the EOM solution, when the membrane surface was stronger in hydrophobicity. Membranes with larger pores exhibited faster flux decline but superior flux recovery. (3) Membrane with stronger hydrophobicity retained more EOM because of enhanced hydrophobic adhesion. The membrane larger pore showed slightly higher retention of EOM due to the formation of a less porous cake layer. Meanwhile, the membrane with larger with larger pores exhibited a greater capacity for irreversible deposition of EOM during UF. (4) Larger pore size and stronger hydrophobicity promoted the irreversible deposition of protein-like substances, but they were of less significance for the transportation of humic-like substances in the EOM during UF. (5) Cake formation was the main mechanism for the flux decline caused by EOM, independent of membrane pore size and surface hydrophobicity.

Acknowledgments This research was jointly supported by National Natural Science Foundation of China (Grants 51138008), State Key Laboratory of Urban Water Resource and Environment (Grants 2012DX07), China Postdoctoral Science Foundation funded project (Grants 2013M540293) and the Funds for Creative Research Groups of China (Grants 51121062). References [1] H.O. Huang, T.A. Young, K.J. Schwab, J.G. Jacangelo, Mechanisms of virus removal from secondary wastewater effluent by low pressure membrane filtration, J. Membr. Sci. 409–410 (2012) 1–8. [2] G. Amy, Fundamental understanding of organic matter fouling of membranes, Desalination 231 (2008) 44–51. [3] F.G. Meng, A. Drews, R. Mehrez, V. Iversen, M. Ernst, F.L. Yang, M. Jekel, M. Kraume, Occurrence, source, and fate of dissolved organic matter (DOM) in a pilot-scale membrane bioreactor, Environ. Sci. Technol. 43 (2009) 8821–8826.

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