Fouling behaviour of soluble microbial products and extracellular polymeric substances in a submerged anaerobic membrane bioreactor treating low-strength wastewater at room temperature

Journal of Membrane Science 531 (2017) 1–9

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Fouling behaviour of soluble microbial products and extracellular polymeric substances in a submerged anaerobic membrane bioreactor treating lowstrength wastewater at room temperature

MARK

Rong Chena,b,1, Yulun Niec,1, Yisong Hua, Rui Miaoa, Tetsuya Utashirob, Qian Lia, Manjuan Xua, ⁎ Yu-You Lib, a International S & T Cooperation Center for Urban Alternative Water Resources Development, School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, No.13 Yanta Road, Xi'an 710055, PR China b Department of Civil and Environmental Engineering, Graduate School of Engineering, Tohoku University, 6-6-06 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan c Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: Fouling behaviour SMP EPS Macromolecules Adhesion forces

The properties of soluble microbial products (SMP) and extracellular polymeric substances (EPS) were determined and their role in membrane fouling behaviour was considered in an anaerobic membrane bioreactor (AnMBR) treating low-strength wastewater at room temperature. The SMP/EPS properties of interest were specific production, molecular weight distribution and adhesion force. The results showed the largest factor affecting the SMP/EPS properties and their diverse membrane fouling performances was the organic loading rate (OLR). An increase in the OLR resulted in an increase in the production of specific EPS and macromolecules in the SMP/EPS fractions, in effect exacerbating the flocculation ability of the mixed liquor in the AnMBR and thus facilitating the fast formation of cake layers. Furthermore, the EPS tended to be more viscoelastic and hydrophobic at a higher OLR and because the adhesion forces of the EPS-membrane and EPSEPS were significantly enhanced as the OLR increased, cake fouling was significantly accelerated. The results indicated that the main cause of fouling was SMP-induced pore blockages, and that membrane resistance increased gradually until an OLR of 0.7 gCOD/L/d, but increased remarkably when the OLR was higher than 1.4 gCOD/L/d, caused by the EPS-induced fast-growth and compact cake layer on the membrane surface.

1. Introduction The anaerobic membrane bioreactor (AnMBR) integrates anaerobic digestion and membrane technology and produces high biomass through separating solid retention time (SRT) from hydraulic retention time (HRT). This process has been given much attention by researchers due to its highly efficient degradation of organics and its ability to recover energy in wastewater treatment [1]. Typically, the operated HRT for treating high-strength wastewater in an anaerobic reactor is measured in days, whereas that for low-strength wastewater, such as sewage, is measured in hours in order to maintain an appropriate organic loading rate (OLR) for effective anaerobic digestion. Because of this and the relatively low cross-membrane flux in such systems, membrane fouling is not a main concern for AnMBRs applied to high-strength wastewater treatment. However, it is a matter which

⁎

1

deserves more attention in low-strength wastewater treatment since a high flux across the membrane may be required for a short HRT. Membrane fouling behaviour is typically attributed to both pore blocking and cake formation, regardless of whether the membrane bioreactor is aerobic or anaerobic [2]. Pore blocking or adsorption tends to occur when the foulant is smaller than or comparable in size to the membrane pores, whereas a cake layer is formed on the membrane surface if the size of the foulant is larger than the pores. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) have been shown to have a great impact on sludge/biomass properties and further membrane fouling [3]. SMP-based fouling results in pore blocking due to the soluble state of SMP fractions. SMPs can be divided into utilization-associated products (UAP) and biomass-associated products (BAP) [4]: UAP are directly produced from substrate utilization and are relatively small in size, while BAP are

Corresponding author. E-mail address: [email protected] (Y.-Y. Li). These two authors contributed equally to this work and should be considered co-first authors.

http://dx.doi.org/10.1016/j.memsci.2017.02.046 Received 15 December 2016; Received in revised form 13 February 2017; Accepted 27 February 2017 Available online 28 February 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.

Journal of Membrane Science 531 (2017) 1–9

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Cole-Parmer, USA) were individually used to feed influent into the reactor and withdraw permeate from the membrane. The biogas produced was recycled by a constant-speed diaphragm pump (APN085 LV-1, Iwaki, Japan) to scour the membrane surface for fouling control via an air diffuser located below the membrane sheet, with a specific gas flow rate of 2.6 m3/m2 h per unit membrane area. A digital pressure meter (Keyence, AP-V85) was installed between the membrane module and the permeate pump to record trans-membrane pressure (TMP). Biogas production was measured according to the volume of biogas collected in a wetted gas holder. The gas holder was maintained at atmospheric pressure. The seed sludge was from a mesophilic anaerobic digestion tank for excess sludge in a wastewater treatment plant in Sendai, Japan, with a treatment scheme composed of anaerobic, anoxic and aerobic processes. The operating temperature was kept at 25 °C by means of a water bath. The AnMBR was operated successively under five OLRs of 0.35, 0.70, 1.05, 1.40 and 2.10 gCOD/ L/d, and five HRTs of 48, 24, 16, 12 and 8 h respectively, by setting the suction cycle of the permeate pump for intermittent filtration. No mixed liquor was discharged except for periodical sampling for analysis, due to the low yield of mixed liquor volatile suspended solid (MLVSS), determined as 0.06–0.09 over the whole experimental duration. The operating conditions are shown in Table 1. The composition of the synthetic wastewater used in this study is given in Table 2. It contained SS of 150 ± 50 mg/L, total COD (TCOD) of 700 ± 100 mg/L, soluble COD (SCOD) of 500 ± 60 mg/L, NH4+ of 40 ± 15 mg/L and PO43- of 5 ± 2 mg/L.

produced by dissolving EPS [5] and are relatively large in size. BAP have been reported to have hydrophilic-colloid properties [2,6]. In contrast, EPS-based fouling leads to the formation of a cake layer since the colloidal state of the EPS makes the sludge flocculate on the membrane surface, generating a biofilm [7]. SMP/EPS, as microbial metabolic products and also organic matters, both serve the microbial substrate, and cause organic fouling to the membrane. From this perspective, the three main issues when investigating the fouling behaviour of SMP/EPS are the production capacity, organic composition and fouling potential. Because the anaerobic microbial community differs remarkably from that in an aerobic reactor, the microbial products including SMP and EPS are also believed to be different [8]. With a focus on side-stream membrane reactors, a colloidal probe was used to measure the atomic forces between a membrane probe and model foulants, and mitigation measures against fouling were proposed by determining the fouling behaviour of model foulants on the membrane surface [9–12]. However, because SMP and EPS are more complicated than the model foulants, their fouling behaviour needs to be further investigated based on the results obtained for model-foulants. Furthermore, the fouling performances in a submerged membrane bioreactor can be assumed to differ considerably from those of a side-stream membrane reactor because the significantly different hydrodynamic regime results in the diverse fouling behaviour of the mixed liquor, a complex of sludge flocs, substrates, SMP and EPS. In this paper, an AnMBR, with a submerged flat-sheet membrane module, was configured to treat low-strength wastewater at room temperature, and the following were investigated: (1) treatment performance with a focus on the removal of organics and energy production; (2) the physical-chemical properties of anaerobic SMP and EPS in the reactor; and (3) the fouling behaviour of SMP and EPS under various OLRs. The results are expected to contribute to the comprehensive understanding of the properties of SMP/EPS and their fouling behaviour, and for further development in the control of membrane fouling caused by SMP/EPS in AnMBRs.

2.2. Samples collection and analytic methods Influent and effluent samples were regularly taken from the outlets of the feeding and permeate pumps to analyze the COD concentrations. Periodically, samples of mixed liquor from the reactor were taken to determine the concentrations of mixed liquor suspended solid (MLSS) and MLVSS. The Standard Method [13] was used to measure the COD, MLSS and MLVSS. The composition of N2, CH4 and CO2 in the collected biogas was measured using a gas chromatograph (Shimadzu, GC-8A, Japan). All the measurements of CH4 were normalized to standard state (0 °C, 1 atm).

2. Materials and methods 2.1. AnMBR and operating conditions

2.3. SMP and EPS properties detection The experimental system consisted of a 6-L completely mixed submerged AnMBR with a substrate tank, and is shown in Fig. 1. The flat sheet membrane was made of chlorinated polyethylene with a nominal pore size of 0.2 µm, and a total area of 0.116 m2 (Kubota Membrane Cartridge, Japan). Two peristaltic pumps (Model 7518-10,

SMP and EPS concentrations were measured as those of carbohydrate and protein. A mixed liquor sample with 60 ml volume was centrifuged for 15 min at 12,000g at 4 °C and the supernatant was then filtered through a 0.45 µm filter. The obtained filtrate represented the

Fig. 1. Submerged anaerobic membrane bioreactor.

2

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matter in the membrane pores, and finally 10 g/L citric acid solution was used as an acidic reagent to soak the membrane to clear inorganic ions in the pores. A filtration test was carried out on the membrane before any cleaning, after water cleaning, and after NaClO and citric acid soaking, to test the filtering flux for tap water. The chemical cleaning and filtration tests were implemented in two dates including the end of operating OLR of 1.4 and 2.1 gCOD/L/d respectively, when the TMP reached 20kPa. Eq. (1) was used to determine the in-situ total membrane resistance for various flux, or OLRs, over the whole experimental process of the AnMBR. In addition, this equation was also used to determine the distribution of different resistances induced by the cake layer, pore blocking and the membrane itself, based on the ex-situ filtration test results [19].

Table 1 AnMBR operating conditions. OLR (gCOD/L/d) HRT (h) Duration (day) Permeation rate (ml/ min) Permeation cycle 2

Flux (L/m h)

0.35 48 1–30 25

0.70 24 31–70 25

1.05 16 71–90 25

1.40 12 91–121 25

2.10 8 123–153 25

1 min on 9 min off 1.08

1 min on 5 min off 2.17

1 min on 3 min off 3.25

1 min on 2 min off 4.29

1 min on 1 min off 6.46

SMP [14]. EPS was obtained using a cation exchange resin (DOWEX R Marathon C, Na+ form, Sigma-Aldrich, USA) extraction method [15]. A mixed liquor sample with 60 ml volume was centrifuged for 15 min at 2000g at 4 °C and the sediments were re-suspended with a buffer solution (2 mmol/L Na3PO4, 4 mmol/L NaH2PO4, 9 mmol/L NaCl and 1 mmol/L KCl). Afterwards, resin (80g/g VSS) was added and mixed for 1 h at 900 rpm and 4 °C. The mixture was first centrifuged for 1 min at 12,000g and the obtained supernatant was then re-centrifuged for 20 min at 12,000g. The finally obtained supernatant represented the EPS. The carbohydrate in SMP and EPS was measured using H2SO4/ phenol oxidation and a colorimeter method, and the protein was measured using the Folin–Ciocalteu method [16]. All analyses were conducted in two replicates. The molecular weight (MW) distributions of SMP and EPS were determined by a gel filtration chromatography (GFC) analyser (LC2010A, Shimadzu Corporation, Japan) using a UV detector (SPD-10, Shimadzu Corporation, Japan). A Zenix SEC-100 type gel column (Sepax Technologies Corporation, USA) was used with 150 mM sodium phosphate buffer (including Na2HPO4 and NaH2PO4) as eluent at a flow rate of 1.0 ml/min [17]. To determine the fouling behaviour of SMP/EPS, a MultiMode 8.0 at. force microscope (Bruker, Germany) in conjunction with a colloidal probe was used to quantify the interaction forces of membrane material-SMP/EPS and SMP/EPS-SMP/EPS. Commercial membrane material (chlorinated polyethylene) in powder form was used to prepare membrane probes. The obtained SMP and EPS were used as foulants. The preparation methods of the membrane probes were based on a previously published procedure [9]. The SMP/EPS-coated probes were prepared by adsorbing the corresponding SMP/EPS fractions on the surface of the membrane probe that had been sintered onto the cantilever. For each sample, force measurements were carried out at six locations, and the averages of more than 10 force curves for each location were used for analysis. The process of interaction force measurements and data normalization were referred to an earlier related study [18].

J=

TMP TMP = μR t μ(Rm +Rc +Rp −org +Rp −inorg )

(1)

where J is the permeate flux (m /m /s), TMP is mentioned above (Pa), and μ is the dynamic viscosity of permeate (Pa s). The total membrane resistance Rt (per m) was composed of intrinsic membrane resistance Rm (per m), cake layer resistance Rc (per m), organic pore blocking resistance Rp-org (per m), and inorganic pore blocking resistance Rpinorg (per m). 3

2

3. Results 3.1. Treatment performance at various OLRs Table 3 shows the results of the treatment performance. The MLSS & MLVSS tended to increase gradually as the OLR increased, and the increase was greater when the OLR was higher than 1.05 gCOD/L/d. The AnMBR presented a low biomass yield of 0.06-0.09gMLVSS/ gCOD, mainly because the anaerobic biomass yield was relatively slow compared to aerobic one [20]. The high TCOD removal efficiency ( > 90%) achieved under all OLRs was attributed to the membrane completely retaining microorganisms in the mixed liquor for efficient degradation to produce a high-quality effluent. A higher biogas production rate was observed when the OLR was higher than 1.05 gCOD/L/d, and the percentage of CH4 in the biogas was kept high in a range of 75–80% after stabilizing from start-up. The start-up took 30 days, and another 3–5 days was needed for stabilization after every change of the OLR. The conversion from the influent TCOD to CH4 was maintained at a high percentage and kept increasing as the OLR increased. Furthermore, the normalized CH4 yield, at 0.328 L/gCOD at OLR 2.1 gCOD/L/d, approached the theoretical value of 0.35 L/gCOD [21]. Because the specific CH4 yield per gram of MLVSS was largely dependent on OLRs, it was inferred that an increase in the OLR resulted in a greater methanogenic capacity of the biomass. These results conformed to many investigations related to AnMBRs [1,20,22] which revealed that the anaerobic digestion efficiency can be significantly improved by membrane installation since the retained anaerobic biomass is prevented from being washed out. Nonetheless, membrane fouling may become a limiting factor to sustain high efficiency for longterm and stable operation under a condition of high flux.

2.4. Fouled membrane cleaning and filtration test When TMP increased to 20 kPa, the membrane was withdrawn from the reactor for chemical cleaning to recover its permeability. Firstly, tap water was used to wash away the cake sludge on the membrane surface by a sponge, then 0.1% NaClO solution was used as an alkaline reagent to soak the membrane for 24 h to clear the organic Table 2 Composition of the synthetic wastewater. Chemical compounds

Concentration (mg/L)

Food ingredients

Concentration (mg/L)

Trace metals

Concentration (mg/L)

Toilet paper Urea Sodium acetate NH4HCO3 KH2PO4 FeSO4·7H2O MgCl2·6H2O

150.0 88.2 220.6 219.4 21.9 5.0 5.0

Yeast extract Beef extract Peptone Glucose

56.8 64.6 56.8 220.6

ZnSO4·7H2O CoCl2·6H2O MnCl2·4H2O CuSO4·5H2O (NH4)6Mo7O24·4H2O NiCl2·6H2O Na2SeO4

0.215 0.12 0.495 0.125 0.089 0.095 0.078

3

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Table 3 Average treatment performance under steady condition at various OLRs. OLR (g-COD/L/d)

0.35

0.7

1.05

1.4

2.1

Operational conditions

Day MLSS (g/L) MLVSS (g/L) Biomass yield (gMLVSS/gCOD) MLVSS/MLSS

1–30 6.40 ± 0.51 5.58 ± 0.45 0.09 0.872 ± 0.045

31–70 6.38 ± 0.62 5.91 ± 0.55 0.07 0.926 ± 0.063

71–90 7.16 ± 0.73 6.05 ± 0.59 0.08 0.845 ± 0.053

90–121 8.31 ± 0.82 7.32 ± 0.63 0.06 0.881 ± 0.036

123–153 9.26 ± 0.81 8.15 ± 0.77 0.06 0.880 ± 0.028

Organic removal

Effluent COD (mg/L) TCOD removal (%)

43.1 ± 5.4 94.5 ± 2.5

24.2 ± 6.9 94.9 ± 3.8

29.4 ± 5.4 94.3 ± 1.7

23.6 ± 3.3 95.2 ± 3.1

20.8 ± 5.2 95.5 ± 1.9

Methane production

Biogas production (L/L/d) CH4 in biogas (%) Conversion of TCOD to CH4 (%) CH4 yield (L-CH4/g-COD) Specific CH4 yield (L-CH4/g-MLVSS/d)

0.14 ± 0.02 71.5 ± 0.3 83.6 ± 1.8 0.277 ± 0.021 0.016 ± 0.009

0.28 ± 0.04 76.4 ± 1.2 85.1 ± 2.1 0.295 ± 0.015 0.033 ± 0.007

0.42 ± 0.02 79.1 ± 1.0 89.0 ± 3.2 0.307 ± 0.018 0.050 ± 0.005

0.56 ± 0.02 79.6 ± 1.5 91.6 ± 1.3 0.306 ± 0.021 0.056 ± 0.011

0.90 ± 0.02 79.7 ± 1.1 93.8 ± 1.6 0.328 ± 0.017 0.081 ± 0.013

0.35

0.7

1.05

1.4

2.1

Table 4 The fouling rate at various OLRs.

24

24 Trans-membrane pressure (kPa)

Trans-membrane pressure Total membrane resistance

20

20

16

16

12

12

8

8 Membrane cleaning

4

4 0

0 0

20

40

60

80

Total membeane resistance (×1012 per meter)

OLR

OLR (gCOD/L/d) 12

Rt (10 per m) Time (day) dRt/dt (1012 per m/d)

0.35

0.70

1.05

1.40

2.10

10.8–18.5 1–30 0.19

9.2–14.2 31–70 0.12

9.0–13.7 71–90 0.23

9.8–17.7 91–121 0.28

1.7–12.3 123–153 0.34

fouling rate was significant from OLR 0.7–1.05 gCOD/L/d. Fig. 3 provided the resistances distribution obtained from the results of filtration tests for the membrane which was withdrawn in the aforementioned two dates. Regarding the resistances at the end of duration under the operating OLR of 1.4 gCOD/L/d (Fig. 3a), Rp-org accounts for 49% and Rc for 43% of Rt, which indicates that the fouling from both cake layer and organic pore blocking accounted for the majority of the total resistances, while pore blocking played the most important role. In contrast, for the membrane resistances at OLR 2.1 gCOD/L/d (Fig. 3b), Rc accounts for 89% of Rt, whereas Rp-org only accounted for 5%, even though both of these two resistances also accounted for the majority of Rt. The intrinsic membrane resistance Rm and inorganic pore blocking resistance Rp-inorg was far less than that Rc and Rp-org. Furthermore, Fig. 3(c) and (d) provided sufficient evidence for this observation. The membrane surface at OLR 1.4 gCOD/L/d appeared to have slight cake fouling. On the contrary, the fouled membrane at OLR 2.1 gCOD/L/d appeared to have a heavy cake layer.

100 120 140

Time (days) Fig. 2. Tans-membrane pressure and total membrane resistance at different operating OLRs.

3.2. Fouling performance at various OLRs 3.3. Specific SMP and EPS production TMP and membrane resistance are two important indicators of membrane fouling performance. As shown in Fig. 2, while the OLR increased from 0.35 to 0.7 gCOD/L/d, the TMP gradually increased, and after the OLR exceeded1.05 gCOD/L/d, the TMP increased at a much faster rate. The membrane module was withdrawn from the reactor for the first cleaning and filtration test on day 121 when the OLR was 1.4 gCOD/L/d. Afterwards, it was installed inside the reactor again for operation at OLR 2.1 gCOD/L/d. While the increase in TMP was steady during the first 15 days, it spiked thereafter. On day 153, TMP reached 20 kPa again and the membrane module was withdrawn for a second cleaning and filtration test. With regard to the membrane resistance calculated by using Eq. (1), Rt showed a positively linear dependency on operation time at any specific OLR, which was derived from the increase in the amount of permeate with time for a specific operating flux. The fouling rate at various OLRs, expressed as dRt/dt, was determined and the results are given in Table 4. The fouling rate was 0.19×1012, 0.12×1012, 0.23×1012, 0.28×1012 and 0.34×1012 per meter per day at the operating OLR 0.35, 0.7, 1.05, 1.4 and 2.1 gCOD/L/d respectively, indicating an increase as the OLR increased, except at a very low OLR of 0.35 gCOD/L/d. Note also that the increase in the

The specific production of SMP and EPS expressed by protein and carbohydrate production per gram MLVSS is shown in Fig. 4. As shown, the specific protein was more than three times higher than the specific carbohydrate, however, the carbohydrate was higher than protein in the synthetic wastewater. Since hydrolysis is considered the rate-limiting stage in an anaerobic process, it is important to note the hydrolysis of protein was much slower than that of carbohydrate [23]. For this reason, protein, particularly the protein in the form of biomass like tightly-bound EPS (TB-EPS), tended to accumulate in the mixed liquor. Furthermore, with the increase in the number of microorganisms when the OLR was increased, EPS accumulation was heightened. The specific production of EPS, including protein and carbohydrates, was more obvious than that of SMP. In particular, specific EPS protein presented a 54.7%, 33.1% and 30.9% increase at OLR 1.05, 1.4 and 2.1 gCOD/L/d respectively over levels in the OLR of 0.7, 1.05 and 1.4 gCOD/L/d. These values were significantly higher than those for specific SMP protein. This may be attributed to the increase in the biomass, which may produce more EPS and SMP with increased OLR. Meanwhile, EPS tended to accumulate in the mixed liquor by mem4

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Rc (a)

Rp-org

Rm

Rp-inorg

2%

(b)

6%

2% 4%

5%

43% 89%

49%

(d)

(c)

OLR 2.1 gCOD/L/d

OLR 1.4 gCOD/L/d

Fig. 3. Results of membrane resistances distribution based on filtration tests at the end of duration under operating OLR 1.4 and 2.1 gCOD/L/d including resistances distribution at (a) OLR 1.4 gCOD/L/d and (b) OLR 2.1 gCOD/L/d; and membrane surface at (c) OLR 1.4 gCOD/L/d and (d) OLR 2.1 gCOD/L/d.

d, the number of small molecules decreased whereas there was a marked increase in the number of medium-sized molecules (Fig. 5(b): 13–14 min). This may be due to the growth of UAP within SMP with increased OLR. The occurrence of large molecules (Fig. 5(b): 6–8 min) may have been due to BAP occurrence. When the OLR changed to 2.1 gCOD/L/d, the intensity of the medium-sized molecules increased significantly, most probably due to the fast production of the UAP under a high OLR. Besides the decrease in the number of small molecules (Fig. 5(d) and 5(e): 16–19 min), slight changes were observed in EPS from an OLR of 0.7–1.4 gCOD/L/d, whereas there was a noticeable jump in the number of large molecules when the OLR changed to 2.1 gCOD/L/d (Fig. 5(f): 7.5–12 min). This implies that substances with a greater MW appeared in the EPS in this stage. The remarkable increase of substances with large MW can also be attributed to the greater amount of biomass decay compared to biomass growth at a relatively long SRT. This result was consistent with several observations in earlier studies on aerobic activated sludge, which inferred that SRT had also a significant effect on SMP and EPS production [29,30]. Furthermore, as the number of large molecules within EPS increased and EPS production itself increased, membrane fouling behaviour tended to be dominated by cake formation because of the hydrophobic and colloidal properties of EPS [31].

brane interception due to its colloidal state, while SMP tended to be membrane-permeable due to its soluble state. 3.4. SMP and EPS molecular weight Besides the specific production of protein and carbohydrate, organic compositions in SMP and EPS also displayed remarkable differences at various OLRs, which can be reflected by MW distribution. The GFC analyser is widely used to identify organic substances based on a differential permeation process to obtain information regarding MW distribution [17,24]. The correlation of logarithmic MW versus the peak time was shown to be negatively linear [25], hence the peak appearance time can be utilized to estimate the MW distributions of SMP and EPS. Fig. 5 shows the results for MW at various OLRs. Since the peak appearance time was from 5 to 20 min, the MW distribution of SMP and EPS extended over a broad range, possibly covering large, moderate and small MW. This was consistent with many other research results, indicating that SMP/EPS has a wide range of MW from less than 0.5 kDa to more than 300 kDa [26]. The small SMP molecules which appeared at an OLR of 0.7 gCOD/ L/d with relatively high intensity (Fig. 5(a): 16–19 min) could be humic-like materials [27,28]. When the OLR changed to 1.4 gCOD/L/ 5

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120

SMP/EPS, which is in agreement with the results of related research indicating that foulant–membrane forces are always higher than foulant–foulant forces [9,32]. As shown in Fig. 6(a), the SMP-membrane were 2.90 and 3.53 mN/m at the OLR 0.7 and 2.1 COD/L/d respectively, slightly increasing with increased OLR, whereas the EPSmembrane were 0.10 and 5.22 mN/m at the OLR 0.7 and 2.1 COD/L/d respectively, which was a marked increase. Similar results were obtained from SMP–SMP and EPS–EPS as shown in Fig. 6(b), indicating a gradual increase in the SMP–SMP, and a sharp increase in the EPS–EPS with the increase of OLR. From the forces transformation at various OLRs shown in Fig. 6(c) and (d), the following results can be found: (1) a strong SMP-membrane force at all OLRs enabled the SMP to be an important foulant over the whole experimental process; and (2) EPS related forces including EPS–membrane and EPS–EPS, kept increasing remarkably, which implied that the potential contribution of EPS to fouling became increasingly more important as the OLR increased. Among the main components in SMP/EPS, the carbohydrates had hydrophilic properties, while many proteins can be considered as hydrophobic [33]. Hydrophobicity has been reported to have a close correlation to adhesion forces [34]. Adhesion forces between hydrophobic organics were shown to be much higher than those between both hydrophilic and transphilic organics [9]. Furthermore, hydrophobic foulants can also cause greater adhesion to hydrophobic membranes [35]. Referring to Figs. 4 and 5, the specific protein and macromolecules in EPS showed obvious increases with increasing OLR. Therefore, the increase in the number of macromolecules and protein was likely responsible for the increasing adhesion forces, and especially the EPS–membrane adhesion forces. Although the results of Fig. 6 can only reflect the average interforces between SMP/EPS and membrane materials on the molecular level, the composition transformation of SMP and EPS and their microscopic influence on membrane fouling was clearly revealed. The observed tendencies conform to the macroscopic fouling performance, including TMP and membrane resistance. With the increase in SMP and EPS production, and the increase in the content of macromolecules, the adhesion strength of molecules in the mixed liquor was enhanced. The macromolecules in EPS may become coiled and spherical in shape, forming a more compact cake layer [36], more resistant to shear stress by gas scouring by the returned biogas.

(a) Specific protein (mg/gMLVSS)

Specific SMP protein 90

Specific EPS protein

60

30

0 Specific carbohydrate (mg/gMLVSS)

(b) 20

Specific SMP carbohydrate Specific EPS carbohydrate

10

0 0.70

1.05

1.40

2.10

OLR (gCOD/L/d) Fig. 4. Specific production of SMP and EPS at various OLRs: (a) specific protein of SMP and EPS; and (b) specific carbohydrate of SMP and EPS.

3.5. Adhesion forces of SMP and EPS The interaction forces between SMP/EPS fractions and a membrane probe were expressed as SMP-Membrane and EPS-Membrane. In the same way, the interaction forces between SMP/EPS fractions and a SMP/EPS coated membrane probe were expressed as SMP-SMP and EPS-EPS. Because the adhesion force of SMP–EPS was believed to fall between SMP–SMP and EPS–EPS [9], it was not measured. The results of all the forces at various OLRs are shown in Fig. 6. The forces for SMP/EPS-membrane were much higher than those for SMP/EPS-

(a)

3.0 2.0

OLR=0.7 gCOD/L/d

10.0

OLR=0.7 gCOD/L/d

1.0

5.0

0.0 3.0

EPS intensity (mv)

SMP intensity (mv)

(d)

15.0

(b)

2.0

OLR=1.4 gCOD/L/d

1.0 0.0 3.0

(c)

0.0 15.0

(e)

10.0

OLR=1.4 gCOD/L/d

5.0 0.0 15.0

OLR=2.1 gCOD/L/d

2.0

10.0

1.0

5.0

0.0

(f) OLR=2.1 gCOD/L/d

0.0 0

5

10 15 Minutes

20

25

0

5

10

15 Minutes

20

25

Fig. 5. Distribution of molecular weight of SMP and EPS at various OLRs: SMP molecular weight at the OLR of (a) 0.7 gCOD/L/d, (b) 1.4 gCOD/L/d and (c) 2.1 gCOD/L/d; and EPS molecular weight at the OLR of (d) 0.7 gCOD/L/d, (e) 1.4 gCOD/L/d and (f) 2.1 gCOD/L/d.

6

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1.0

(a) SMP-Membrane at OLR 0.7 SMP-Membrane at OLR 2.1 EPS-Membrane at OLR 0.7 EPS-Membrane at OLR 2.1

3.0

Adhesion force (mN/m)

Adhesion force (mN/m)

6.0

0.0 -3.0 -6.0

(b) SMP-SMP at OLR 0.7 SMP-SMP at OLR 2.1 EPS-EPS at OLR 0.7 EPS-EPS at OLR 2.1

0.5 0.0 -0.5 -1.0

0

50 Separation distance (nm)

100

0

50 Separation distance (nm)

100

Average adhesion forces (mN/m)

8.0 (c) 6.0

SMP-membrane

EPS-membrane

SMP-SMP

EPS-EPS

4.0 2.0 0.0 0.7

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1.4 OLR (gCOD/L/d)

2.1

Average adhesion forces (mN/m)

1.4 (d)

1.2

SMP-SMP

1

EPS-EPS

0.8 0.6 0.4 0.2 0 0.7

1.05

1.4 OLR (gCOD/L/d)

2.1

Fig. 6. Adhesion forces for SMP, EPS and membranes at various OLRs: (a) SMP–Membrane and EPS–Membrane at OLR 0.7 and 2.1 gCOD/L/d; (b) SMP–SMP and EPS–EPS at OLR 0.7 and 2.1 gCOD/L/d; (c) all the average forces at various OLRs; and (d) average SMP-SMP and EPS-EPS at various OLRs.

severe pore blockages. This resulted in a decrease in the intermolecular electrostatic repulsion while intermolecular adhesion forces increased significantly because of the increasing amount of substances with moderate/large MW within SMP/EPS. Cake formation developed to some degree in this phase. These can be shown by the change of fouling rate from the OLR 0.7–1.05 gCOD/L/d in Table 4 and MW changes from the OLR 0.7–1.4 gCOD/L/d in Fig. 5(a) and (b). (3) Finally, substances with a large MW increased significantly in the EPS. In this phase, the adhesion forces became much higher than the electrostatic repulsion between macromolecules, and severe EPS adsorption and flocculation to the membrane surface resulted in the fast growth of the cake layer. This is reflected in Fig. 3, Figs. 5(f) and 6. Except for the above mentioned static forces, the rate of molecular movement towards the membrane is clearly accelerated with increasing OLR due to the enlarged filtration rate, because it involves more hits against the membrane surface. The specific fouling history of SMP and EPS in this study is shown in Fig. 7. It was found that the fouling process was dominated by SMP fractions at a low OLR (≤0.7 gCOD/L/ d). In this phase UAP and BAP with moderate MW were the main substances within SMP. High pore resistance caused by SMP blockages led to the deterioration of permeability. In contrast, BAP with large MW and EPS took a leading role in the organic substances within the

Furthermore, it has been reported that non-biodegradable and slowbiodegradable substances inside macromolecules tended to increase with prolonging SRT, and those substances largely contain hydrophobic functional groups which result in greater adhesion forces [37]. 4. Discussion Based on the above results, the fouling behaviour of SMP and EPS is supposed to transform under different conditions, and it can be possibly divided into three phases under the effect of OLR in the AnMBR: (1) Firstly, when the total amount of SMP and EPS was small at a low OLR, small molecules dominated in SMP and EPS, and most of them tended to be hydrophilic. The effect of intermolecular electrostatic repulsion played a more important role than adhesion forces. In this phase, small molecules passed through the membrane pores and the slight pore narrowing and blockages resulted in only a slight increase in the fouling rate, and the required flux was easy to maintain. This is evident by the fouling rate at the OLR of 0.35 and 0.7 gCOD/L/d shown in Table 4. (2) Secondly, UAP increased significantly with increasing OLR. The amounts of total SMP/EPS rose with biomass multiplication due to sufficient substrate utilization in this phase. Large amounts of UAP passed through the membrane pores and caused 7

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High OLR (≥ 1.4 gCOD/L/d)

Low OLR (≤ 0.7 gCOD/L/d)

High cake resistance

Low pore resistance

Large BAP

SMP

Low cake High pore resistance resistance

Low flux

High flux

UAP

Moderate BAP

Sludge floc Moderate EPS

Large EPS

Non-degradable EPS

EPS

Fig. 7. Fouling behaviour of SMP and EPS at low and high OLRs.

mitigating SMP/EPS fouling in the AnMBR in further studies.

mixed liquor when the OLR was quite high (≥1.4 gCOD/L/d). The high flux drives the complex of large BAP, EPS and sludge flocs to adhere to the membrane surface at a high rate. Accordingly, cake formation is significantly accelerated and covers the membrane surface in a short period of time, even when the pores are unsaturated. Hence, cake resistance instead of pore resistance was the main reason for the decrease of the permeate capacity in this condition.

Acknowledgements This work was supported by the KAKENHI Grant-in-Aid for Scientific Research (No. 26289179), the Research Fund of Tianjin Key Laboratory of Aquatic Science and Technology(No. TJKLAST-ZD2016-07), JSPS (Japan Society for the Promotion of Science) KAKENHI Grant-in-Aid for JSPS Fellows (No. 15F15353) and the Shaanxi Program for Innovative Research Team (No. 2013KCT-13).

5. Conclusions The transformation of SMP and EPS properties in an AnMBR treating low-strength wastewater at room temperature were investigated and their role in membrane fouling was synthetically studied. Our main findings are as follows.

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