Membrane fouling caused by lipopolysaccharides: A suggestion for alternative model polysaccharides for MBR fouling research

Accepted Manuscript Membrane fouling caused by lipopolysaccharides: a suggestion for alternative model polysaccharides for MBR fouling research Katsuki Kimura, Takayuki Kakuda, Hiroyuki Iwasaki PII: DOI: Reference:

S1383-5866(19)30598-2 https://doi.org/10.1016/j.seppur.2019.04.059 SEPPUR 15529

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

14 February 2019 18 April 2019 18 April 2019

Please cite this article as: K. Kimura, T. Kakuda, H. Iwasaki, Membrane fouling caused by lipopolysaccharides: a suggestion for alternative model polysaccharides for MBR fouling research, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.04.059

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Membrane fouling caused by lipopolysaccharides: a suggestion for alternative model polysaccharides for MBR fouling research

Katsuki Kimura*, Takayuki Kakuda, Hiroyuki Iwasaki Division of Environmental Engineering, Hokkaido University N13W8, Kita-ku, Sapporo 060-8628, Japan

*Corresponding author. Tel & Fax: +81-11-706-6271 Email: [email protected]

1

Abstract Polysaccharides were identified as key foulants in membrane bioreactors (MBRs) in previous studies. Polysaccharides used for MBR fouling research should represent real foulants, and alginate and dextran have often been used as model polysaccharides. However, their properties are not necessarily similar to those of polysaccharides that actually cause membrane fouling in MBRs. In recent studies, it was shown that polysaccharides causing membrane fouling in MBRs had the structures of lipopolysaccharides (LPSs), which are components of the cell wall of gram-negative bacteria. In the present study, therefore, membrane fouling caused by commercially available LPSs was investigated and was compared with that caused by model polysaccharides. In a series of batch filtration tests using four different microfiltration (MF) and ultrafiltration (UF) membranes, regardless of the membrane used, LPSs caused more severe membrane fouling than did alginate or dextran when they were filtered under the condition of the same organic carbon concentration. The properties of LPSs were found to be considerably different from those of alginate and dextran. In particular, affinity of LPS to PVDF polymer assessed by quartz crystal microbalance (QCM) mass

2

measurement was remarkably high, and it would account for the severe fouling caused by LPSs. In experiments with filtering of mixtures of the model polysaccharides and a biomass suspension collected from a pilot-scale MBR, which were conducted to mimic the sudden increase in dissolved polysaccharides in real MBRs, LPS induced significant irreversible fouling, while alginate and dextran did not. The results obtained in this study clearly demonstrated high fouling potential of LPSs. Use of LPSs in future MBR fouling studies is therefore recommended.

Keywords: wastewater treatment, soluble microbial products; alginate; dextran; LC-OCD

3

1. Introduction

Membrane bioreactors (MBRs) have many advantages over conventional wastewater treatment systems including high quality of treated water, ease of maintenance and small footprint. However, MBRs are still not the mainstream wastewater treatment technologies mainly due to their high operational costs, which are closely related to membrane fouling. Membrane fouling results in an increase in energy costs and the necessity for frequent chemical membrane cleaning and membrane replacement. Thus, for more widespread application of MBRs, it is necessary to address problems associated with membrane fouling. The issue of membrane fouling in MBRs has therefore drawn much attention [1-7]. Membrane fouling in MBRs is caused by internal pore clogging and formation of a foulant layer on the membrane. Interfacial interactions (i.e., thermodynamic forces) working between foulants and membranes were investigated in detail in previous studies [8-10]. A foulant layer can be further distinguished as a gel layer and a cake layer [11]. It was reported that the specific filtration resistance (SFR) of a gel layer was much larger than that of a cake layer [12]. Such high resistance of a gel layer

4

might be explained by variations of chemical potential inside/outside the gel layer [11, 13].

To address the problems associated with membrane fouling, it is necessary to identify the components that cause membrane fouling in MBRs. There is now a consensus among researchers that soluble microbial products (SMP)/extracellular polymeric substances (EPS) excreted by microorganisms are the major players in membrane fouling in MBRs [14-17]. Polysaccharides account for a large portion of SMP/EPS [18, 19] and are thought to be important foulants in MBRs [20-22]. Many studies have therefore been conducted using model polysaccharides such as alginate and dextran to investigate fouling in MBRs [23-27].

To mimic membrane fouling in MBRs, model compounds (polysaccharides) should be similar to the components that actually cause membrane fouling in MBRs. As demonstrated in previous studies using advanced molecular biological techniques [28, 29], microbial communities in wastewater treatment systems including MBRs are very

5

complex. Thus, a diversity of polysaccharides must also be assumed for MBRs as a result of the diversity of their sources (i.e., microorganisms). In a previous study, such a diversity of polysaccharides in an MBR was partially revealed by using lectin-affinity chromatography [30], and it was shown that fouling potentials of polysaccharides in an MBR were considerably different depending on the types of polysaccharides. Thus, model polysaccharides used for MBR fouling studies should be similar to those that actually cause membrane fouling in MBRs. To the best of our knowledge, however, there is little evidence that model polysaccharides used for MBR fouling studies (e.g., alginate) are similar to polysaccharides that actually cause membrane fouling in MBRs. Experiments with such “unrealistic” model polysaccharides might not lead to the establishment of efficient methods for control of membrane fouling in MBRs. Kim and Dempsey [31] demonstrated that alginate was a poor surrogate for effluent organic matter (EfOM), in which SMP should account for a large portion.

In our recent study [32], structures of polysaccharides that had been extracted from membranes fouled in a pilot-scale MBR were revealed by using the glyco-blotting

6

method [33], which was originally developed for analysis of oligo-saccharides in biological samples. In the glyco-blotting method, oligosaccharides produced by partial hydrolyzation of polysaccharides were efficiently purified by utilizing a specific interaction between aldehydes in polysaccharides and aminooxy groups. By using the glyco-blotting method, it was possible to isolate oligosaccharides derived from crude foulants, which are extremely complex mixtures of organic matter [32]. Then matrix-assisted laser desorption/ionization (MALDI) – time of flight (TOF)/ mass spectrometry (MS)

analysis could be applied.

The results suggested that

lipopolysaccharides (LPSs), which are found in cell walls of gram-negative bacteria, were major players in membrane fouling in the pilot-scale MBR [32]. The results of a separate study focusing on structures of proteins causing membrane fouling in MBRs also suggested that LPSs were greatly involved in membrane fouling [34]. LPSs are thought to be released into the biomass suspension during decay/lysis. It was reported that the dry weight of LPS is about 3.6% of a bacterial cell [35]. LPSs may induce membrane fouling when they are liberated into a suspension. Previous studies showed that shocks (e.g., salt stress or temperature) to the biomass in MBRs could cause membrane fouling [36-38].

7

Fouling observed in those studies might have been due to LPSs released by such shocks.

So far, however, there have been few studies in which membrane fouling caused by LPSs was directly assessed. In the present study, membrane fouling caused by commercially available LPSs was investigated for the first time and compared with that caused by alginate and dextran. As shown later in this paper, membrane fouling caused by LPSs was much greater than that caused by the model polysaccharides, suggesting that LPSs should be used for future MBR fouling studies to establish an efficient method for control of fouling.

2. Experimental methods

2.1 Model polysaccharides

Alginate (designated as ALG hereafter) and dextran (designated as DEX hereafter) were used as model polysaccharides in previous fouling studies [23-27] and were also used in

8

this study. ALG and DEX were purchased from Sigma-Aldrich Japan (Tokyo, Japan) and Wako Pure Chemical Industries (Osaka, Japan), respectively. LPSs were also examined in this study. Various LPSs (different sources and different grades of purity, etc.) are commercially available. In this study, LPSs produced by Pseudomonas and Klebsiella were chosen because these genera have been reported to be abundant in biomass suspensions in activated sludge [39, 40]. Feng et al. [41] reported that Klebsiella oxytoca was a dominant strain in the fouling layer of MF membranes used in a pilot submerged MBR. LPS produced by Pseudomonas aeruginosa (designated as P-LPS hereafter) and that produced by Klebsiella pneumoniae (designated as K-LPS hereafter) were purchased from Wako Pure Chemical Industries (Osaka, Japan). Both LPSs used in this study were phenol-extracted grade.

Stock solutions of the model polysaccharides were prepared by dissolving 1 g of a polysaccharide into 1 L of Milli-Q water. Although complete dissolution of polysaccharides could be achieved with ALG and DEX, it was difficult to completely dissolve LPSs. A trace amount of LPS remained suspended after intensive mixing. In the

9

experiments using LPSs, both suspended and dissolved LPSs were filtered (i.e., suspended LPS was not removed prior to experiments). The stock solution was then stored at 4 ˚C. The feed solution used for filtration experiments (15 mg-TOC/L) was prepared by dilution of the stock solution with Milli-Q water. Sodium hydroxide was added to the feed solution so that pH was around 7. Solutions of the model polysaccharides were filtered by a 0.45-µm filter made from PTFE, and the solutions were analyzed before and after filtration to check the solubility of the model polysaccharides. In the case of ALG and DEX, all of the polysaccharides were thought to be dissolved in the feed solution: the DOC concentration was 15 mg/L. In the case of LPSs, a small portion of the LPS remained suspended in the feed solution. On average, 2% and 17% of P-LPS and K-LPS, respectively, were present in suspended forms in the feed solution. In some experiments, to investigate the influence of calcium [42-44], CaCl2 (Wako Pure Chemical Industries) was also added to the feed solution so that the calcium concentration would be 15 mg/L. Concentrations of polysaccharides and calcium were set to mimic conditions observed in pilot-scale MBRs treating municipal wastewater, which the authors had investigated [20, 45-47]. As described later, the addition of

10

calcium promoted aggregation of polysaccharides, and the molecular weight distributions of polysaccharides became different. Even after the addition of calcium, all of ALG and DEX remained as dissolved forms. In the case of LPSs, however, the suspended fractions increased to 16% and 25% for P-LPS and K-LPS, respectively.

2.2 Membranes

Four different flat-sheet MF/UF membranes were used in the filtration tests with a stirred filtration cell described in the next section. The properties of the membranes used in this study are summarized in Table 1. Two MF membranes with identical nominal pore sizes (0.1 µm) and the same membrane polymer (polyvinilydene fluoride (PVDF) were used. One MF membrane was purchased from Merck-Millipore (Darmstadt, Germany, designated MF-A hereafter) and the other MF membrane was donated by Toray (Tokyo, Japan, designated as MF-B hereafter). The two UF membranes had similar molecular weight cut-offs (MWCOs) but were made from different polymers (100 kDa, polyethele sulfone (PES); 200 kDa, PVDF). The PES UF membrane was purchased from

11

Merck-Millipore (designated as UF-A hereafter) and the PVDF UF membrane was purchased from Microdyn-Nadir (designated as UF-B hereafter). Supporting information Fig. S1 shows surfaces of the membranes tested in this study taken by scanning electron microscopy (SEM).

Table 1. Membranes examined in this study. MF-A

MF-B

UF-A

UF-B

Manufacturer

Millipore

Toray

Millipore

Nadir

Model

VVLP

Not available

PBHK

UV200

0.1 µm

0.1 µm

100 kDa

200 kDa

PVDF

PVDF

PES

PVDF

-31.6

-28.2

-14.2

-15.2

N/A***

89

67

87

Nominal

pore

size/MWCO Polymer Zeta

potential

(mV) * Contact

angle

(degrees)**

* Measured by a zeta potential analyzer (ELSZ-2000ZS, Otsuka Electronics, Tokyo, Japan) under pH of 7.0 and 10 mM KCl. ** Measured by a contact angle meter (DropMaster 300, Kyowa Interface Science, Tokyo, Japan) *** Data could not be obtained because water droplets immediately disappeared during the measurements probably due to hydrophilic treatment provided by the manufacturer. Note: the material of this membrane is hydrophilic PVDF according to the manufacturer.

2.3 Batch filtration tests

12

Following previous studies dealing with fouling caused by SMP in MBRs [48, 49], fouling caused by model polysaccharides was assessed by a series of batch filtration experiments using a dead-end stirred cell system in this study. The effective membrane area in the batch filtration experiments was 3.5 cm2. The stirred cell was filled with 10 mL of feed water, and compressed nitrogen gas was used to filter the suspension under a constant pressure. The filtration pressure was fixed at 15 kPa, and the stirring speed in the cell was set at 300 rpm. During the batch test, feed water was supplied to the stirred cell to maintain a constant volume of suspension in the cell. In each filtration, virgin membranes were used. The membrane permeate flux was measured using an electronic balance. The membrane filtration resistance was calculated using the following equation: (1

where R is the membrane filtration resistance (1/m), J is the membrane permeate flux (m3/m2/s , ΔP is the trans-membrane pressure (TMP) (Pa), and µ is the viscosity of the permeate (Pa•s . The membrane resistance can be divided into two types of resistance as

13

follows:

where Rm is the membrane intrinsic resistance (m-1) and Rf is the resistance caused by membrane fouling (m-1). In this paper, Rf is shown in the related data. In the series of experiments with model polysaccharide solutions, filtration was continued for 2 hours, which was found to be sufficient to distinguish the differences among the tested polysaccharides. Each batch filtration was duplicated.

2.4 Filtration of a mixture of polysaccharides and biomass suspension

LPSs are components of the cell wall of gram-negative bacteria including Proteobacteria, which have been reported to account for a major fraction of the microbial community in MBRs [50, 51]. Therefore, LPSs are considered to be commonly present in biomass suspensions in MBRs. LPSs can be released into the biomass suspension during the decay process of gram-negative bacteria and may be involved in the evolution of membrane

14

fouling. Such a situation can be mimicked by spiking LPS into the biomass suspension. In this study, the model polysaccharides were added to a biomass suspension collected from a pilot-scale MBR treating municipal wastewater, and the mixture was filtered by a dead-end stirred cell system. The effective membrane area in the batch filtration experiments was 37.4 cm2. The stirred cell was filled with 300 mL of suspension, and compressed nitrogen gas was used to filter the suspension under a constant pressure. The filtration pressure was fixed at 15 kPa, and the stirring speed in the cell was set at 300 rpm. Tests were repeated three times with suspensions collected on three different dates. The average concentration of mixed liquor suspended solids (MLSS) in the biomass suspensions collected was 7.4 g/L (standard deviation: 0.28 g/L). Stock solutions of the polysaccharides were prepared with Milli-Q water, and they were added to biomass suspensions so that the concentration of the added polysaccharides would be 15 mg-TOC/L in each test. The amount of polysaccharides added was set on the basis of dissolved polysaccharides in pilot-scale MBRs that were installed at the same wastewater treatment plant (Soseigawa Wastewater Treatment Plant, Sapporo, Japan) and examined in previous studies [20, 45-47]. The volume of polysaccharide solution added in each test

15

was 15 mL. As a control test, the same volume of Milli-Q water was added and filtration was carried out. The filtration protocol was the same as that in the series of batch filtration tests, and the MF-A membrane was used for this test. As described later, the influence of LPSs was more clearly shown with MF-A than with the other membranes in the batch filtration tests using model polysaccharide solutions. When each filtration was terminated, degrees of irreversible fouling were assessed by physical cleaning (rinsing with distilled water followed by gently wiping the membrane surface with a soft sponge). After wiping, filtration resistance of the fouled membrane was determined by filtering pure water.

2.5 Analytical methods

The concentrations of total organic carbon (TOC) and dissolved organic carbon (DOC) were determined using a TOC analyzer (TOC-VCSH, Shimadzu, Japan). Samples containing suspended solids were subjected to TOC analysis after sonication for 3 min. Analysis of DOC was performed after the suspensions had been filtered using a polytetrafluoroethylene (PTFE) membrane (Advantec Tokyo, Japan) with a pore size of

16

0.45 µm. Filters were thoroughly pre-rinsed with deionized water to avoid leaching of organics from the filters. Liquid chromatography with an organic carbon detection (LC-OCD) system (Model 8, DOC-LABOR Dr. Huber, Germany) was used for size-based fractionation of organic matter. In this study, a tandem column set-up, in which an HW-65S column (250 mm x 20 mm, Tosoh Inc., Japan) and an HW-50S column (TSK HW-50S, 250 mm x 20 mm, Tosoh Inc., Japan) were connected, was used to further fractionate the biopolymer fraction [52]. Before performing LC-OCD analyses, the samples were filtered using 0.45-µm filters made from PTFE (Advantec, Tokyo, Japan). For FTIR studies, KBr pellets containing 1% of the sample were prepared and examined in an FTIR spectrophotometer (IR-7500S, Shimadzu, Kyoto, Japan) at a resolution of 4 cm-1. Zeta potentials of the model polysaccharides in the solutions used for filtration experiments were measured by using a zeta potential analyzer (ELSZ-2000ZS, Otsuka Electronics, Tokyo, Japan).

2.6 Assessment of the affinity of model polysaccharides with a membrane polymer

17

In this study, a quartz crystal microbalance (QCM) device with a fundamental frequency of 27 MHz (AFFINIX Q, ULVAC, Tokyo, Japan) was used to assess affinities of model polysaccharides with a membrane polymer (PVDF). A quartz crystal sensor coated with PVDF polymer was purchased from the manufacturer (ULVAC, Tokyo, Japan) and used for analysis. The coated sensors were prepared by a previously described method [53]. The amounts of adsorbed polysaccharides were calculated by Sauerbrey's equation:

(

where ∆F is the measured frequency change (Hz , F0 is the fundamental frequency of the quartz crystal (27 MHz , ∆m is the mass change (g , A is the electrode area (0.049 cm2), ρq is the density of quartz (2.65 g/cm3 , and μq is the shear modulus of quartz (2.95×1011 g/cm/s2). In Eq. (2), a decrease of 1 Hz in frequency corresponds to an increase of 0.62 ng/cm2 in mass [54]. The four model polysaccharide surrogates were dissolved in Milli-Q water to prepare a concentration of 1 g/L (corresponding TOC concentrations: 300±20 mg/L). First, 8 mL of 0.1 M phosphate buffer solution (NaH2PO4 3.5 g/L + Na2HPO4 12.8

18

g/L , pH=7) was injected into the QCM cell equipped with the PVDF sensor. The PVDF sensor was stabilized in the buffer solution by immerging it for at least 30 min. After that, 8 µL of the model polysaccharide solution was injected into the cell. Frequency change over a period of 5 minutes was determined.

3. Results and Discussion

3.1 Properties of the model polysaccharides tested

3.1.1 QCM mass measurement

19

Interactions between SMP/EPS and membranes have been considered by implementing

20

the XDLVO theory [55, 56]. In this study, QCM mass measurement was carried out to

21

directly assess the affinity between polysaccharides and membrane polymers. Analyses

22

were conducted with PVDF polymer: PES polymer was not examined in this study

23

Fig. 1. Amounts of absorbed model polysaccharides assessed by QCM mass measurements.

because a sensor coated with PES polymer was not available when this study was

24

Fig. 2. FTIR spectra of the LPSs examined in this study.

conducted. Fig. 1 shows the results of QCM mass measurement. High affinities between LPSs and PVDF polymer were found, whereas affinities between ALG/DEX and PVDF polymer were considerably low. The high affinities of LPSs might be due to hydrophobic interaction between Lipid A domains of LPSs and PVDF polymer. Barry et al. pointed out the possibility of membrane fouling caused by lipid bilayers (i.e., liposomes) during water reclamation [57]. The importance of fatty acids in membrane fouling of MBRs was also pointed out by Al-Halbouni et al. [58]. Adsorption behaviors of P-LPS and K-LPS

25

were different, as shown in Fig. 1. This reflects the differences in physico-chemical properties of the two types of LPSs.

3.1.2 FTIR analysis

Fig. 2 shows FTIR spectra measured for P-LPS and K-LPS. FTIR spectra of ALG and DEX are available in previous publications and a public database [59]. Fig. 2 clearly shows the features of LPSs such as lipid-A (around 2850 cm-1) and PO2- (1200-1265 cm-1) [60, 61]. These peaks are not seen with ALG/DEX [59]. Also, Fig. 2 implies that P-LPS and K-LPS have different features. For instance, P-LPS exhibited peaks at 1730 cm-1 (C=O [62]), 1556 cm-1 (amide-II [62]), 1116 cm-1 (not assigned) and 916 cm-1(not assigned) in its FTIR spectrum, whereas K-LPS did not. In contrast, a peak at 1465 cm-1 (C-H [60]) is remarkable in the FTIR spectrum of K-LPS. According to Erridge et al. [63], LPSs are different depending on their sources. These differences would lead to different degrees and patterns of fouling when they are examined in membrane processes used for water/wastewater treatment.

26

Fig. 3. Zeta potentials of the model polysaccharides examined in this study.

3.1.3 Zeta potential measurements

Fig. 3 shows zeta potentials of the model polysaccharides tested in this study. Measurements were carried out under the same conditions (pH, ionic strength and sodium/calcium concentration) as those set in the filtration tests. Without adding Na+ or Ca2+, the tested polysaccharides exhibited similar potentials (~-40 mV). The addition of

27

Na+ did not significantly change the potentials except for P-LPS: the negative charge of P-LPS was neutralized by Na+ to some extent. Different responses to Na+ addition would also reflect different features of K-LPS and P-LPS as pointed out above. Significant charge neutralization of all of the tested polysaccharides was seen when Ca2+ was added. The addition of Ca2+ would promote aggregation of the polysaccharides and affect membrane fouling.

3.1.4 LC-OCD analysis

The size distribution of particles/molecules filtered has a significant impact on membrane fouling [22, 64]. Some polysaccharides have very large molecular weights of >100 kDa [65]. To specifically analyze polysaccharides with such large molecular weights, the conventional setting of LC-OCD was modified in the manner described in the experimental section [52]. Pullurans were used as molecular weight markers. Fig. 4 shows molecular weight distributions of the polysaccharides tested in this study. The four polysaccharides tested in this study exhibited very different molecular weight

28

distributions, which would have had an impact on the degrees of membrane fouling in the filtration tests. As shown in the figure, the polysaccharides had very large molecular weights as expected. The signal intensity obtained for K-LPS without Ca2+ was much less than that expected, possibly indicating that complete dissolution of K-LPS was difficult as described before. Similar results were obtained in multiple tests, and no clear explanation for the weak signal is available at present. K-LPS used in this study had a very wide distribution of molecular weights, possibly reflecting the differences in lengths of the repeated structure of O-antigen in the K-LPS structure. Although details of purification processes of the product are not clear, such a wide distribution of molecular weights of K-LPS might be due to purification processes during the production. It should be noted that the locations of major peaks shown in Fig. 4 were considerably different among the polysaccharides tested. Fig. 4 also shows the molecular weight distribution of dissolved organic matter in a mixed liquor suspension collected from a pilot-scale MBR installed at an existing wastewater treatment plant (Sapporo, Japan) and used for treating municipal wastewater. Dissolved organic matter in mixed liquor suspensions in MBRs is mostly comprised of EPS/SMP. It was found that EPS/SMP in MBRs exhibited a peak

29

around 1 million Da in LC-OCD analysis. Such a peak is found with LPSs, whereas it is not found with ALG and DEX. In terms of molecular weight distribution, it can be concluded that LPSs are more suitable to represent EPS/SMP than are ALG and DEX.

30

Fig. 4. Molecular weight distributions of model polysaccharides in feed solutions without calcium ions (upper) and with calcium ions (middle). Molecular weight distribution of organic matter in the supernatant of a mixed liquor suspension collected from a pilot-scale MBR treating municipal wastewater is also shown (bottom).

31

As shown in Fig. 4, the addition of Ca2+ induced shifts of molecular weights towards larger sizes regardless of the type of polysaccharide. Ca2+ would cause aggregation of polysaccharides [64], and this was partially reflected by the changes in zeta potentials (Fig. 3). The effect of addition of Ca2+ on shift in molecular weights was most significant with K-LPS. Two peaks, found at retention times of around 60 minutes and 75 minutes, became significant after Ca2+ addition. The addition of Na+ did not change the molecular weight distributions of model polysaccharides (data not shown).

3.2. Batch filtration of model polysaccharides

Fig. 5 shows increases in fouling resistance in batch filtration tests in which model polysaccharide solutions were filtered. Each test was duplicated. Average fouling resistances with standard deviations are shown in Fig. 5. Regardless of the type of membrane, P-LPS caused the most severe membrane fouling, indicating its higher fouling potential than that of the model polysaccharides tested in this study. Degrees of membrane

32

Fig. 5. Fouling resistances caused by model polysaccharides in the batch filtration tests: (a) MF-A, (b) MF-B, (c) UF-A and (d) UF-B. Concentrations of the model polysaccharides in the feed solutions were 15 mg-TOC/L in all cases.

fouling caused by K-LPS were not remarkably high and were similar to those caused by ALG except for the case with MF-A.

33

The results shown in Fig. 5 reflect the difference in properties of P-LPS and K-LPS, suggested by FTIR analysis (Fig. 2). Fouling potentials of LPSs would be different depending on their sources, and LPSs with high fouling potential should be used for fouling research. However, K-LPS also caused significant membrane fouling of MF-A. In addition, as shown later, K-LPS caused more severe membrane fouling than did P-LPS under a specific condition. Thus, it is postulated that LPSs generally have higher fouling potentials than those of “conventional” model polysaccharides (e.g., alginate . Such high fouling potentials of LPSs can be partly explained by high affinities of LPSs with the polymer used for membrane production (PVDF) (Fig. 1). The hydrophobic domain in LPS structures (e.g., lipid-A) would contribute to the high affinity. Hydrophobic interaction of lipids with membrane materials has been suggested as a plausible explanation for fouling [58]. In contrast, charges of LPSs were excluded as an explanation for the high fouling potentials of LPSs since all of the tested polysaccharides exhibited similar charges (Fig. 3). The molecular size of LPSs is another explanation for the high fouling potentials: LPSs were present with a very large size (> 1 million Da), which would be close to the dimension of micropores of MF membranes. In contrast,

34

Table 2. TOC removal rates in the batch filtration tests with addition of calcium. Average values with standard deviation are shown (n=2)*. P-LPS

K-LPS

ALG

DEX

MF-A

0.60 ± 0.01

0.18 ± 0.01

~0

~0

MF-B

0.72 ± 0.03

0.36 ± 0.23

0.71 ± 0.12

~0

UF-A

0.86 ± 0.03

0.78 ± 0.03

0.90 ± 0.05

~0

UF-B

0.89 ± 0.01

0.54 ± 0.01

0.90± 0.02

0.06 ± 0.01

* Values in this table were determined by comparing polysaccharide concentrations in the feed solution and the permeate.

ALG and DEX were not present as such large molecules. The large molecules of LPSs might be more prone to plug micropores of membranes than those of model polysaccharides. As pointed out before, such very large biopolymers are found in mixed liquor suspensions in real MBRs (Fig. 4). To mimic real MBRs and establish methods for fouling mitigation in MBRs, model polysaccharides should have such large molecules. In this sense, ALG and DEX are not appropriate model polysaccharides. The use of LPSs might be recommended instead. TOC removal rates in each test are summarized in Table 2. Rates of P-LPS removal were high with all membranes, possibly being related to the significant membrane fouling shown in Fig.5. However, significant removal of ALG by

35

MF-B, UF-A and UF-B did not bring about fouling. It was shown that high rejection of model polysaccharides did not necessarily cause membrane fouling.

Fig. 6 shows increases in fouling resistance in batch filtration tests in which calcium was added to the polysaccharide solutions. In many previous studies, it was shown that the presence of calcium affected membrane fouling caused by polysaccharides [27, 38, 43, 44]. In this study, the presence of calcium accelerated membrane fouling except for the case of filtering DEX. A comparison of P-LPS and K-LPS showed that the responses to calcium addition were different: fouling was significantly accelerated in the case of K-LPS, whereas acceleration of the fouling rate was limited in the case of P-LPS. Again, this difference in responses to calcium addition between the two LPSs could be attributed to differences in their properties suggested by FTIR analysis (Fig. 2). Charge neutralization by calcium was more significant for P-LPS than for K-LPS (Fig. 3), possibly being associated with preferable intermolecular or intramolecular complexing by calcium. Zhang et al. [27, 67] suggested that calcium addition caused structural variation of alginate polymer in solution from linear chains to egg-box structure. As

36

Fig. 6. Fouling resistances caused by model polysaccharides in the batch filtration tests with addition of calcium: (a) MF-A, (b) MF-B, (c) UF-A and (d) UF-B. Concentrations of the model polysaccharides and calcium in the feed solutions were 15 mg-TOC/L and 15 mg/L, respectively.

suggested by LC-OCD analysis (Fig. 4), aggregation of polysaccharides occurred after calcium addition. Such aggregation probably occurred because calcium ions complexed with ionizable groups in LPSs. It was found that formation of very large aggregates (corresponding to retention time of around 60 minutes) occurred in the case of K-LPS.

37

These very large aggregates were not seen with the model polysaccharides tested and they might be responsible for the rapid evolution of membrane fouling in the filtration of K-LPS with calcium by MF-A. Accelerated fouling shown in Fig. 6 might also be attributed to the changes in chemical and physical geometrical configurations of model polysaccharides caused by calcium ion addition [67]. Fouling caused by ALG was accelerated in the presence of calcium, being in accordance with the results of previous studies [43, 44]. Fouling potential of LPSs in the presence of calcium ions, however, was remarkably higher than that of ALG, as shown in this study.

The effect of calcium ions mentioned above might simply be due to suppression of the electrical double layer. To investigate this point, another experiment using NaCl with the same electrical conductivity (0.1 mS/cm) was conducted. Fig. 7 shows increases in fouling resistance in batch filtration tests in which NaCl was added to the polysaccharide solutions. Generally, results obtained with NaCl were similar to those obtained without addition of CaCl2 or NaCl (Fig. 5). This implies that the acceleration in fouling observed with CaCl2 (Fig. 6) was not due to suppression of the electrical double layer but due to

38

Fig. 7. Fouling resistances caused by model polysaccharides in the batch filtration tests with addition of NaCl: (a) MF-A, (b) MF-B, (c) UF-A and (d) UF-B. Concentrations of the model polysaccharides and electrical conductivity in the feed solutions were 15 mg-TOC/L and 0.1mS/cm, respectively. specific interactions between calcium and polysaccharides. In the filtration of P-LPS solution with NaCl, however, membrane fouling in MF-A was almost negligible. At present, there is no reasonable explanation for this result. Molecular weight distributions of P-LPS used for the series of NaCl addition tests were slightly different (different lots of P-LPS were used for tests on the effect of NaCl (data not shown).) and the difference may

39

explain the results. Also, the more hydrophilic nature of MF-A (Table 1) may be responsible for less fouling by P-LPS with NaCl. In any case, as shown in Figs. 5-7, LPSs caused more significant fouling than did the two model polysaccharides used in previous studies, suggesting a potential of LPS as a new model polysaccharide for fouling study.

3.3 Membrane fouling caused by polysaccharides mixed with biomass suspension collected from a pilot-scale MBR treating municipal wastewater.

40

Fig. 8. Fouling resistance recorded in the batch tests in which mixtures of model polysaccharides and biomass suspension collected from a pilot-scale MBR were filtered. Experiments were repeated with different biomass suspensions collected on separate dates. Results obtained in the first test (Test 1), second test (Test 2) and third test (Test 3) are shown in the left, middle and right panels, respectively. In this study, the temporal increase in polysaccharides in an MBR biomass suspension was mimicked by addition of the model polysaccharides to a biomass suspension collected from a pilot-scale MBR installed at a municipal wastewater treatment plant. Fig. 8 shows the results of batch filtration tests for mixtures of the model polysaccharides and biomass suspension collected from the pilot-scale MBR. Filtration of the mixtures by using MF-A was repeated three times with samples collected on different dates. In the first test (Test 1), different filtration times were used for different polysaccharides due to miscommunications among operators. Based on the results obtained in Test 1, however, it was thought that filtration time of 900 seconds was sufficient. In subsequent tests (Tests 2 and 3), filtration time was fixed at 900 seconds in all cases. As shown in Fig. 8, the

41

addition of LPSs clearly enhanced the evolution of membrane fouling. Results shown in Fig. 8 suggest that LPS released from biomass during its decay into the bulk suspension can cause severe membrane fouling in an MBR via interactions with constituents in the suspension. Regarding the involvement of LPS in evolution of membrane fouling in MBRs, one possibility is that the LPS may originate from the lysis of gram-negative bacteria that were already attached to the membrane. Although the primary focus of this study was to examine the magnitudes of membrane fouling caused by LPSs, not to identify the source of LPS causing membrane fouling, the results shown in Fig. 8 indicate that LPSs originating from the bulk suspension can cause membrane fouling in MBRs. Future study should be done to determine which LPSs are important (LPSs originating from the fouling layer or LPSs from the bulk suspension) in evolution of membrane fouling in MBRs.

It should be noted that the addition of K-LPS resulted in the most severe membrane fouling regardless of the sample. This result is in contrast to the results in Figs. 5-7 showing that P-LPS mainly caused the most severe membrane fouling. Interactions of

42

LPSs with constituents in a mixed liquor suspension and subsequent involvement in membrane fouling would be different depending on the sources of LPSs. This again reflects the differences in properties of the two LPSs tested as shown in Fig. 2. Results shown in Fig. 8 are in accordance with the data shown in Fig. 6 (filtration of model solutions with calcium): K-LPS exhibited the most severe membrane fouling for MF-A via interaction with calcium. The presence of calcium in the mixed liquor suspension certainly accelerated membrane fouling in the case of K-LPS addition, although other constituents might also affect and enhance the fouling.

Fig. 9 shows filtration resistances of fouled membranes and physically cleaned membranes, assessed at the termination of filtration of the biomass suspension with model polysaccharides. Filtration resistances of new membranes are also shown in Fig. 9. Differences between physically cleaned membranes and new membranes (arrows in Fig. 9) represent the degree of irreversible fouling. It is clearly shown that addition of LPSs caused considerable irreversible fouling, whereas this type of fouling was not enhanced in other cases. These results would reflect the high fouling potential of LPSs described

43

before. Some synergetic effects of LPSs with constituents in the biomass suspension may also account for these results. Susanto et al. reported negative impacts caused by synergetic interaction of polysaccharides and proteins in membrane fouling [68]. Significant irreversible fouling in PVDF membranes caused by LPSs is in accordance with our previous results of molecule structure analysis showing that polysaccharides extracted from a fouled membrane from an MBR displayed the features of LPSs [32].

44

Fig. 9. Filtration resistance assessed at the termination of batch experiments in which mixtures of model polysaccharides and biomass suspension collected from a pilot-scale MBR were filtered. Arrows (differences between “Before filtration” and “After physical cleaning”) represent degrees of irreversible fouling. Results obtained in the first test (Test 1), second test (Test 2) and third test (Test 3) are shown in the upper, middle and bottom panels, respectively. 45

4. Conclusion

Based on our previous finding that LPSs are important polysaccharides in membrane fouling in MBRs, fouling caused by commercially available LPSs was investigated and compared with that caused by model polysaccharides in this study. LPSs caused considerably more severe membrane fouling than did the model polysaccharides tested. The severe fouling caused by LPSs would be due to their high affinity to membrane polymers. The unique molecular weight distribution of LPSs might also contribute to the severe membrane fouling. It was found that LPSs can cause severe irreversible fouling when they are released into the biomass suspension during decay processes, whereas the other polysaccharides did not cause such irreversible fouling. Experiments with LPSs would be more realistic than those with model polysaccharides, and the results of such experiments might lead to the establishment of efficient methods for control of membrane fouling in MBRs. For widespread use of LPSs in fouling research, however, economically reasonable LPSs should be available: currently available LPSs are mainly produced for medical research and they are generally expensive. Development of

46

methods for isolation of LPSs at reasonable costs is eagerly awaited.

Acknowledgement

This work was supported by JSPS KAKENHI (Grant-in-Aid for Scientific Research (A)) Grant Number 18053845.

References

[1] Chang, I.-S., Le-Clech, P., Jefferson, B., Judd, S., 2002. Membrane fouling in membrane bioreactors for wastewater treatment, J. Environ. Eng. 128 (11), 1018-1029.

[2] Le-Clech P., Chen, V., Fane, T. A. G., 2006. Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284, 17-53.

[3] Wang, Z. W., Wu, Z. C., 2009. A Review of membrane fouling in MBRs:

47

Characteristics and role of sludge cake formed on membrane surfaces, Sep. Purif. Technol. 44 (15), 3571-3596.

[4] Zuthi, M. F. R., Ngo, H. H., Guo, W. S., 2012. Modelling bioprocesses and membrane fouling in membrane bioreactor (MBR): A review towards finding an integrated model framework, Bioresource Technol. 122, 119-129.

[5] Aslam, M., Charfi A., Lesage, G., Heran M., Kim, J., 2017. Membrane bioreactors for wastewater treatment: A review of mechanical cleaning by scouring agents to control membrane fouling, Chem. Eng. J. 307, 897-913.

[6] Krzeminski, P., Leverette L., Malamis S., Katsou E., 2017. Membrane bioreactors - A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects, J. Membr. Sci. 527, 207-227.

[7] Meng, F. G., Zhang, H. M., Yang, F. L., Liu, L. F., 2007. Characterization of cake

48

layer in submerged membrane bioreactor, Environ. Sci. Technol. 41 (11), 4065-4070.

[8] Teng, J., Shen, L., He, Y., Liao, B.-Q., Wu, G., Lin, H., 2018. Novel insights into membrane fouling in a membrane bioreactor: Elucidating interfacial interactions with real membrane surface, Chemosphere 210, 769-778.

[9] Chen, J., Lin, H., Shen, L., He, Y., Zhang, M., Liao, B.-Q., 2017. Realization of quantifying interfacial interactions between a randomly rough membrane surface and a foulant particle, Bioresour. Technol. 226, 220-228.

[10] Teng, J., Zhang, M., Leung, K.-M., Chen, J., Hong, H., Lin, H., Liao, B-Q., 2019. A unified thermodynamic mechanism underlying fouling behaviors of soluble microbial products (SMPs) in a membrane bioreactor, Water Res. 149, 477-487.

[11] Chen, J., Zheng, M., Li, F., Qian, L., Lin, H., Yang, L., Wu, X., Zhou, X., He, Y., Liao, B.-Q., 2016. Membrane fouling in a membrane bioreactor: High filtration resistance of

49

gel layer and its underlying mechanism, Water Res. 102, 82-89.

[12] Hong, H., Zhang, M., He, Y., Chen, J., Lin, H., 2014. Fouling mechanisms of gel layer in a submerged membrane bioreactor, Bioresour. Technol. 166, 295-302.

[13] Teng, J., Shen, L., Yu, G., Wang, F., Li, F., Zhou, X., He, Y., Lin, H., 2018. Mechanism analyses of high specific filtration resistance of gel and roles of gel elasticity related with membrane fouling in a membrane bioreactor, Bioresour. Technol. 257, 39-46.

[14] Rosenberger, S., Laabs, C., Lesjean, B., Gnirss, R., Amy, G., Jekel, M., Schrotter, J. C., 2006. Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment, Water Res. 40 (4),710-720.

[15] Al-Halbouni, D., Traber, J., Lyko, S., Wintgens, T., Melin, T., Tacke, D., Janot, A., Dott, W., Hollender, J., 2007. Correlation of EPS content in activated sludge at different

50

sludge retention times with membrane fouling phenomena, Water Res. 42 (6-7), 1475-1488.

[16] Kimura, K., Naruse, T., Watanabe, Y., 2009. Changes in characteristics of soluble microbial products in membrane bioreactors associated with different solid retention times: Relation to membrane fouling, Water Res. 43 (4), 1033-1039.

[17] Drews A., 2010. Membrane fouling in membrane bioreactors-Characterisation, contradictions, cause and cures, J. Membr. Sci. 363 (1-2), 1-28.

[18] Barker, D. J., Stuckey, D. C., 1999. A review of soluble microbial products (SMP) in wastewater treatment systems, Water Res. 33 (14), 3063-3082.

[19] Frølund, B., Palmgren, R., Keiding, K., Nielsen, P. H., 1996. Extraction of extracellular polymers from activated sludge using a cation exchange resin, Water Res. 30 (8), 1749-1758.

51

[20] Kimura, K., Yamato, N., Yamamura, H., Watanabe, Y., 2005. Membrane fouling in pilot-scale membrane Bioreactors (MBRs) treating municipal wastewater, Environ. Sci. Technol. 39 (16), 6293-6299.

[21] Meng, F. G., Zhang, S. Q., Oh Y., Zhou Z. B., Shin H. S., Chae S. R., 2017. Fouling in membrane bioreactors: An updated review, Water Res. 114, 151-180.

[22] Wang, Z. W., Wu, Z. C., Yin, X., Tian, L. M., 2008. Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization, J. Membr. Sci. 325 (1), 238-244.

[23] Ye, Y., Le-Clech, P., Chen, V., Fane, A. G., Jefferson, B., 2005. Fouling mechanisms of alginate solutions as model extracellular polymeric substances, Desalination, 175 (1), 7-20.

52

[24] Ye, Y., Le-Clech, P., Chen, V., Fane, A. G., 2005. Evolution of fouling during crossflow filtration of model EPS solutions, J. Membr. Sci. 264 (1-2), 190-199.

[25] Zator, M., Fernando, M., Lopez, F., Guell C., 2007. Membrane fouling characterization by confocal microscopy during filtration of BSA/dextran mixtures, J. Membr. Sci. 301 (1-2), 57-66.

[26] Astaraee, R. S., Mohammadi, T., Kasiri N., 2015. Analysis of BSA, dextran and humic acid fouling during microfiltration, experimental and modeling, Food Bioprod. Process. 94, 331-341.

[27] Zhang, M. J., Lin, H. J., Shen, L. G., Liao, B. Q., Wu, X. L., Li, R. J., 2017. Effect of calcium ions on fouling properties of alginate solution and its mechanisms, J. Membr. Sci. 525, 320-329.

[28] Molina-Munoz, M., Poyatos, J. M., Sanchez-Peinado, M., Hontoria, E.,

53

Gonzalez-Lopez, J., Rodelas, B., 2009. Microbial community structure and dynamics in a pilot-scale submerged membrane bioreactor aerobically treating domestic wastewater under real operation conditions, Sci. Tot. Environ. 407 (13), 3994-4003.

[29] Hu, M., Wang, X. H., Wen, X. H., Xia, Y., 2012. Microbial community structures in different wastewater treatment plants as revealed by 454-pyrosequencing analysis, Bioresource Technol. 117, 72-79.

[30] Kimura, K., Tanaka, I., Nishimura, S.-I., Miyoshi, R., Miyoshi, T., Watanabe, Y., 2012. Further examination of polysaccharides causing membrane fouling in membrane bioreactors

(MBRs):

Application

of

lectin

affinity

chromatography

and

MALDI-TOF/MS, Water Res. 46, 5725-5734.

[31] Kim, H.-C., Dempsey, B. A., 2013. Membrane fouling due to alginate, SMP, EfOM, humic acid, and NOM, J. Membr. Sci. 428, 190-197.

54

[32] Kimura, K., Nishimura, S.-I., Miyoshi, R., Hoque, A., Miyoshi, T., Watanabe, Y., 2015. Application of glyco-blotting for identification of structures of polysaccharides causing membrane fouling in a pilot-scale membrane bioreactor treating municipal wastewater, Bioresource Technol. 179, 180-186.

[33] Hirose, K., Amano, M., Hashimoto, R., Lee, Y. C., Nishimura, S.-I., 2011. Insight into glycan diversity and evolutionary lineage based on comparative avio-N-glycomics and sialic acid analysis of 88 egg whites of galloanserae, Biochemistry 50(21), 4757-4774.

[34] Miyoshi, T., Aizawa, T., Kimura, K., Watanabe, Y., 2012. Identification of proteins involved in membrane fouling in membrane bioreactors (MBRs) treating municipal wastewater, Int Biodeterior Biodegradation 75, 15-22.

[35] Narita, H., Isshiki, I., Funamizu, N., Takakuwa, T., Nakagawa, H., Nishimura, S.-I., 2005. Organic matter released from activated sludge bacteria cells during their decay

55

process, Environ. Technol. 26, 433–440.

[36] Van den Brink, P., Satpradit, O.-A., Van Bentem A., Zwijnenburg, A., Temmink, H., Van Loosdrecht, M., 2011. Effect of temperature shocks on membrane fouling in membrane bioreactors, Water Res. 45 (15), 4491-4500.

[37] De Temmerman, L., Maere, T., Temmink, H., Zwijnenburg, A., Nopens, I., 2014. Salt stress in a membrane bioreactor: Dynamics of sludge properties, membrane fouling and remediation through powdered activated carbon dosing, Water Res. 63, 112-124.

[38] Zhang, H. F., Fan, X., Wang, B., Song, L. F., 2015. Calcium ion on membrane fouling reduction and bioflocculation promotion in membrane bioreactor at high salt shock, Bioresource Technol. 200, 535-540.

[39] Kampfer, P., Erhart, R., Beimfohr, C., Bohringer, J., Wagner, M., Amann, R., 1996. Characterization of bacterial communities from activated sludge: Culture-dependent

56

numerical identification versus in situ identification using group- and genus-specific rRNA-targeted oligonucleotide probes, Microb. Ecol. 32, 101-121.

[40] Shannon, K. E., Lee, D.-Y., Trevors, J. T., Beaudette, L. A., 2007. Application of real-time quantitative PCR for the detection of selected bacterial pathogens during municipal wastewater treatment, Sci. Tot. Environ. 382, 121-129.

[41] Feng, L., Li, X., Du G., Chen, J., 2009. Characterization and fouling properties of exopolysaccharide produced by Klebsiella oxytoca, Bioresource Technol. 100, 3387-3394.

[42] Jermann, D., Pronk, W., Meylan, S., Boller, M., 2007. Interplay of different NOM fouling mechanisms during ultrafiltration for drinking water production, Water Res. 41 (8), 1713-1722.

[43] Katsoufidou, K., Yiantsios, S. G., Karabelas, A. J., 2007. Experimental study of

57

ultrafiltration membrane fouling by sodium alginate and flux recovery by backwashing, J. Membr. Sci. 300 (1-2), 137-146.

[44] Van den Brink, P., Zwijnenburg, A., Smith, G., Temmink, H., Van Loosdrecht, M., 2009. Effect of free calcium concentration and ionic strength on alginate fouling in cross-flow membrane filtration, J. Membr. Sci. 345 (1-2), 207-216.

[45] Yamato, N., Kimura, K., Miyoshi, T., Watanabe, Y., 2006. Difference in membrane fouling in membrane bioreactors (MBRs) caused by membrane polymer materials, J. Membr. Sci. 280 (1-2), 911-919.

[46] Kurita, T., Kimura, K., Watanabe, Y., 2015. Energy saving in the operation of submerged MBRs by the insertion of baffles and the introduction of granular materials, Sep. Purif. Technol. 141, 207-213.

[47] Kimura, K., Okazaki, S., Ohashi, T., Watanabe, Y., 2016. Importance of the

58

co-presence of silica and organic matter in membrane fouling for RO filtering MBR effluent, J. Membr. Sci. 501, 60-67.

[48] Delrue, F., Stricker, A. E., Mietton-Peuchot, M., Racault, Y., 2011. Relationships between mixed liquor properties, operating conditions and fouling on two full-scale MBR plants, Desalination, 272, 9-19.

[49] Sun, J., Xiao, K., Mo, Y., Liang, P., Shen, Y., Zhu, N., Huang, X., 2014. Seasonal characteristics of supernatant organics and its effect on membrane fouling in a full-scale membrane bioreactor, J. Membr. Sci. 453, 168-174.

[50] Miura, Y., Hiraiwa, M. N., Ito, T., Itonaga, T., Watanabe, Y., Okabe, S., 2007. Bacterial community structures in MBRs treating municipal wastewater: Relationship between community stability and reactor performance, Water Res. 41 (3), 627-637.

[51] Ahmed, Z., Cho, J., Lim, B.-R., Song, K.-G., Ahn, K.-H., 2007. Effects of sludge

59

retention time on membrane fouling and microbial community structure in a membrane bioreactor, J. Membr. Sci. 287 (2), 211-218.

[52] Kimura, K., Shikato, K., Oki, Y., Kume, K., Huber, S. A., 2018. Surface water biopolymer fractionation for fouling mitigation in low-pressure membranes, J. Membr. Sci. 554, 83-89.

[53] Kamihira, M., Nakazawa, H., Kira, A., Mizutani, Y., Nakamura, M., Nakayama, T., 2008. Interaction of tea catechins with lipid bilayers iInvestigated by a quartz-crystal microbalance analysis, Biosci. Biotechnol. Biochem. 72 (5), 1372-1375.

[54] Ozeki, T., Verma, V., Uppalapati, M., Suzuki, Y., Nakamura, M., Catchmark, J. M., Hancock, W. O., 2009. Surface-bound casein modulates the adsorption and activity of kinesin on SiO2 surfaces, Biophys. J. 96, 3305-3318.

[55] Ying, W., Yang, F., Bick, A., Oron, G., Herzberg, M., 2010. Extracellular polymer

60

substances (EPS) in a hybrid growth membrane bioreactor (HG-MBR): viscoelastic and adherence characteristics, Environ. Sci. Technol. 44 (22), 8636-8643.

[56] Tian, Y., Li, Z., Ding, Y., Lu, Y., 2013. Identification of the change in fouling potential of soluble microbial products (SMP) in membrane bioreactor coupled with worm reactor, Water Res. 47, 2015-2024.

[57] Barry, M. C., Hristovski, K., Westerhoff, P., 2014. Membrane fouling by vesicles and prevention through ozonation, Environ. Sci. Technol. 48, 7349-7356.

[58] Al-Halbouni, D., Dott, W., Hollender, J., 2009. Occurrence and composition of extracellular lipids and polysaccharides in a full-scale membrane bioreactor, Water Res. 43, 97-106.

[59]

AIST

Spectral

Database

https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

61

for

Organic

Compounds.

[60] Parikh, S. J., Chorover J., 2007. Infrared spectroscopy studies of cation effects onlipopolysaccharides in aqueous solution, Colloid Surf. B-Biointerfaces 55 (2), 241-250.

[61] Kamaladevi, A., Balamurugan, K., 2016. Lipopolysaccharide of Klebsiella pneumoniae attenuates immunity of Caenorhabditis elegans and evades by altering its supramolecular structure, RSC Adv. 6 (36), 30070-30080.

[62] Silverstein, R. M., Bassler, G. C., Morrill, T. C., 1991. Spectrometric identification of organic compounds (Fifth Edition), Wiley, New York.

[63] Erridge, C., Bennett-Guerrero, E., Poxton, I. R., 2002. Structure and function of lipopolysaccharides, Microbes Infect. 4 (8), 837-851.

[64] Waite, T. D., Schäfer, A. I., Fane, A. G., Heuer, A., 1999. Colloidal fouling of

62

ultrafiltration membranes: Impact of aggregate structure and size, J. Colloid Interf. Sci. 212 (2), 264-274.

[65] Shen, Y. X., Xiao, K., Liang, P., Sun, J. Y., Sai, S. J., Huang, X., 2012. Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China, J. Membr. Sci. 415, 336-345.

[66] Xin, Y. J., Bligh, M. W., Kinsela, A. S., Wang, Y., Waite, T. D., 2015. Calcium-mediated polysaccharide gel formation and breakage: Impact on membrane foulant hydraulic properties, J. Membr. Sci. 475, 395-405.

[67] Zhang, M., Hong, H., Lin, H., Shen, L., Yu, H., Ma, G., Chen, J., Liao, B.-Q., 2018. Mechanistic insights into alginate fouling caused by calcium ions based on terahertz time-domain spectra analyses and DFT calculations, Water Res. 129, 337-346.

[68] Susanto, H., Arafat, H., Janssen, E. M. L., Ulbricht, M., 2008. Ultrafiltration of

63

polysahhcride-protein mixtures: Elucidation of fouling mechanisms and fouling control by membrane surface modification, Sep. Pur. Technol. 63 (3), 558-565.

64

Graphical abstract

66

Highlights Membrane fouling caused by lipopolysaccharides (LPSs) was firstly assessed. Evolution of fouling by LPSs could be several-fold faster than that by alginate. Large molecules (>1 MDa) found in LPSs would be responsible for fouling. QCM analysis showed that affinity of LPSs to PVDF polymer was remarkably high. Addition of LPSs to MBR sludge apparently accelerated irreversible fouling.

65