Effects of membrane property and hydrostatic pressure on the performance of gravity-driven membrane for shale gas flowback and produced water treatment

Journal of Water Process Engineering 33 (2020) 101117

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Effects of membrane property and hydrostatic pressure on the performance of gravity-driven membrane for shale gas flowback and produced water treatment

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Jialin Lia,1, Haiqing Changa,1, Peng Tanga, Wei Shanga, Qiping Heb, Baicang Liua,* a Key Laboratory of Deep Earth Science and Engineering (Ministry of Education), College of Architecture and Environment, Institute of New Energy and Low-Carbon Technology, Institute for Disaster Management and Reconstruction, Sichuan University, Chengdu 610207, PR China b Chuanqing Drilling Engineering Company Limited, Chinese National Petroleum Corporation, Chengdu 610081, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Shale gas flowback and produced water Gravity-driven membrane filtration Membrane fouling Membrane property Hydrostatic pressure

Hydraulic fracturing of shale gas extraction generates numerous flowback and produced water (FPW), which will cause huge pollution if not properly treated. Gravity-driven membrane with economic advantages was applied as a pretreatment for desalinating this wastewater. The effects of membrane materials (polyvinylidene fluoride (PVDF) and polyvinylchloride (PVC)) with different mean pore sizes, porosities, contact angles, and pure water permeabilities and hydrostatic pressures (40 and 120 mbar) were investigated. The setups were operated for 90 days and the fluxes stabilized at about 0.87–1.00 L/(m2 h). PVDF membranes with higher price, had 6 % higher stable fluxes than PVC membranes, and the extracellular polymeric substances (EPS) contents in fouling layer of PVDF membranes were 10 %–20 % lower than those of PVC membranes. At higher pressures, the stable fluxes increased by only 8 %, but the total resistances increased by nearly 180 %, and there were more EPS, dissolved organic carbon, Na+, Ca2+, Mg2+, Cl− and NO3− on the fouling layer at 120 mbar. A denser cake layer was formed at a higher hydrostatic pressure, as observed by a scanning electron microscope and energy dispersive spectroscopy. Membrane properties and pressures had no significant effect on permeate quality (p > 0.05).

1. Introduction Shale gas is an unconventional natural gas resource with a wide distribution and huge reserves, which can meet the growing demand for energy. However, the rapid development of shale gas industry has brought many environment issues [1,2]. A combined technology of horizontal drilling and hydraulic fracturing is widely applied in extraction of shale gas. In the process of hydraulic fracturing a substantial volume of shale gas flowback and produced water (SGFPW) is generated. The SGFPW contains a large number of chemical additives, as well as oil, mineral salts, heavy metals, and radioactive substances [3–5]. The direct discharge of SGFPW will bring great damages to the ecological environment and human health. Up to date, recycling the SGFPW for beneficial reuse is a promising method for wastewater management. In fact, the SGFPW should be considered as the resources of water and valuable components rather than a waste [6]. Thus, finding a way appropriate treating SGFPW for reuse is crucial for shale gas industry to achieve sustainable development [4,7].

Membrane-based technologies, including ultrafiltration (UF), microfiltration (MF), reverse osmosis (RO), nanofiltration (NF) and forward osmosis, are considered as an effective way in facilitating treatment of SGFPW [8–10]. RO and NF have been evaluated as effective desalination treating SGFPW in Sichuan Basin for external or internal reuse [11–13]. To decrease the oil, particulate matter and suspended solids in the water before desalination unit, UF or MF is adopted as an effective desalination pretreatment [10]. The performance of UF/MF membrane has been evaluated in previous literature for treating SGFPW [14,15]. However, for traditional UF/MF, it is considered indispensable to effectively control membrane fouling by physical and chemical cleaning, which increased the operational cost [16–18]. In recent years, the continuous decline in the price of membranes means that it is feasible to save energy consumption and cost of membrane fouling control by sacrificing the flux level [18]. Compared with traditional UF, gravity-driven membrane (GDM) filtration uses the gravity as the driving force without any backwash or chemical cleaning. Therefore, GDM process has the advantages of low cost, simple

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Corresponding author. E-mail address: [email protected] (B. Liu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jwpe.2019.101117 Received 18 November 2019; Received in revised form 17 December 2019; Accepted 22 December 2019 Available online 30 December 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 33 (2020) 101117

J. Li, et al.

feed water was periodically (every 3 days) added to an opaque storage tank, which was connected to an acrylic transparent feed tank. The level of water in the feed tank was kept constant with a float valve. The feed tank was connected to four separate filtration units and the permeate of the filtration was then collected in four corresponding plastic permeate tanks. The tubes of the permeates were always in a full flow state from the membrane to the overflow tank. Therefore, the liquid level difference between the feed tank and the overflow tank (i.e., 0.4 and 1.2 m) were the hydrostatic pressures acting on the membranes (i.e., 40 and 120 mbar), as reported in published studies [45,46]. Four parallel GDM systems, in a dead-end mode without backflushing or cleaning, were operated with two different membrane materials (PVDF and PVC) under two different hydrostatic pressures (40 and 120 mbar) by gravity. All the membranes were provided by Litree Purifying Technology Co., Ltd. (Haikou, Hainan, China) and the module type of PVDF and PVC membrane was both hollow-fiber out-in membranes. Both the nominal pore sizes of PVDF and PVC membranes were 10 nm given by provider. The effective area of each membrane was 15 cm2. The specific parameters of these two types of membrane were presented in Table 1.

operation, and convenient maintenance [19]. Different from the continuous decline of permeate flux in traditional UF, a stabilization of flux occurred in GDM system [19–24]. This is because open, porous and heterogeneous biofilm layers are formed on the membrane surface under the activities of microorganisms during the long-term operation [22,20–24]. In addition, GDM can remove some of organic matter in raw water through the biological activities leading to water quality improved [25,26]. GDM was used to treat surface water [19,27–29], rainwater [30–34], grey water [35,36], decentralized sewage [37] and pretreat seawater [38–42]. The performance of GDM system was affected by many working conditions. Peter-Varbanets et al. [19] demonstrated that the stable flux of GDM did not depend on the working pressure for treating surface water and diluted wastewater. However, the GDM flux in treating seawater depended on and the hydrostatic pressure [38], membrane materials and module types [40]. Tang et al. [43] confirmed that a higher working pressure slightly increased the stable flux. Lee et al. [44] verified that membrane properties had no significant effect on the formation and characteristics of the biofilm layer for treating lake water. Chang et al. [45] pointed GDMs with different membrane configurations and pressures had similar contaminant removals and stable fluxes. This study focused the performance of GDM filtration as desalination pretreatment for SGFPW of a long-term filtration (612 days), especially for the enrichment of eukaryotic biodiversity on the fouling layer and the improvement of subsequent RO/NF processes. Further, the influence of hydrostatic pressures on fouled membrane and the effect of membrane properties in GDM need to be investigated. The inconsistent conclusions of membrane property and hydrostatic pressure in previous literature were mainly because of the difference in quality of the treated water, and future researches are needed to determine the effect of membrane property and operation pressure on GDM performance for SGFPW. Thus, the purposes of this research are (a) to evaluate the composite influence of polyvinylidene fluoride (PVDF) and polyvinylchloride (PVC) membranes with different properties (i.e., materials, pure water permeabilities, contact angles, mean pore sizes and porosities) and the hydrostatic pressures (i.e., 40 mbar and 120 mbar) on GDM flux and fouling resistance; (b) to investigate the removals of contaminants in SGFPW by GDM under different hydrostatic pressures using PVDF and PVC membranes; and (c) to evaluate the morphology of fouling layer and corresponding pollutants on the membrane surface. Specifically, the permeate flux, the quality of permeate water, and the morphology and pollutant compositions of the fouling layer have been investigated.

2.3. Analytical methods 2.3.1. Filtration flux and resistance analysis The permeate flux is calculated according to Eq. (1),

J=

V A×t

(1)

where J (L/(m h) (LMH)) is the permeate flux, V (L) is the volume of permeate measured by a balance, A (m2) is the effective area of the membrane (0.0015 m2) and t (h) is filtration time. The total fouling resistance Rtotal (m−1) was calculated based on Darcy’s law, 2

Rtotal =

TMP μ×J

(2)

where TMP (Pa) is the transmembrane pressure, and μ (Pa·s) is the permeate viscosity.

μ = 1.784 − (0.0575×T) + (0.0011 × T 2) − (10−5 × T3)

(3)

Where, T (℃) is the temperature. The distribution of total fouling resistance was estimated according to the resistance-in-series model,

Rtotal = Rmembrane + Rreversible + Rirreversible

(4)

where Rmembrane is the intrinsic membrane resistance determined by measuring the flux of the virgin membrane with pure water, Rreversible is the resistance of hydraulically reversible and Rirreversible is hydraulically irreversible resistance. Rirreversible and Rreversible are calculated by testing the flux before and after sample physical cleaning the membrane surface using pure water.

2. Materials and methods 2.1. SGFPW The feed water in this experiment was collected from No.204H5 drilling platform in Weiyuan County, Sichuan Province, China. Water samples were taken from a storage pool, which was mixed with fresh and recycled flowback and produced water from six wells on site and kept in sealed containers and placed in dark environments to avoid changes in water quality. Generally, the raw water was turbid and tawny, with lots of suspended matters and particulate matters. The detailed water quality parameters are different from those of SGFPW collected in previous studies and summarized in Table S1 (Supporting Information).

2.3.2. Water quality analysis We employed a turbidimeter (TL2310, Hach Company, Loveland, USA) and a PB-10 pH meter (Sartorius, Germany) to determine the turbidity and pH. An Ultrameter II 6PFC (Myron L Company, Carlsbad, California, USA) portable multifunctional meter was used to measure the TDS and conductivity. UV absorbance at the wavelength of 254 nm (UV254) and the dissolved organic carbon (DOC) were measured using the UV/ vis spectrophotometer (Thermo Orion Aquamate 8000, USA) and a TOC-L CPH CN200 inspector (Shimadzu, Japan). We took the method of acid-base titration to measure the alkalinity. We used a Dionex ICS-1100 Ion Chromatography (Thermo Fisher Scientific Inc., MA, USA) to measure the concentrations of Na+, K+, NH4+, Ca2+, Mg2+, Ba2+, Sr2+, Cl−, Br−, F−, NO2− and NO3−. The RO fouling potentials of SGFPW and permeate water were evaluated by silt density index (SDI) (Akhondi et al., 2015). The Determination of the SDI5 is according to the ratio of two filtration times of the same volume of raw

2.2. GDM system In this experiment which was shown in Fig. 1, the GDM systems with PVDF and PVC membranes were located in a room condition without isolating air and sunlight deliberately because proper light and dissolved oxygen are beneficial to the operation of the GDM and operated continuously for 90 days under 40 and 120 mbar. The SGFPW as 2

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Fig. 1. Schematic diagram of four parallel GDM systems. Table 1 Specific parameters of the two membranes. Membrane material

Average pore size (nm)

Maximum pore size (nm)

Minimum pore size (nm)

Porosity (%)

Pure water permeability at 20℃ (L/m2 h kPa)

Contact angle (°)

PVDF PVC

10.15 8.41

64.12 34.87

4.91 4.13

90.10 83.64

4.53-4.86 1.54-1.77

84.68 95.75

water with a 0.45 μm membrane filter at 2.07 bar. SDI5 is calculated from Eq. (5),

SDI5 =

the fouling layer. The fouled membrane was washed with pure water, and the sludge (i.e., fouling layer) was collected. The washed membrane was added to a tube and centrifuged at 3000 r/min for 5 min with pure water. The supernatant was collected, and then it was mixed with the washed sludge before. Then, the ion concentrations and DOC were measured. As for EPS, the sludge on the fouled membrane was carefully scraped off with a plastic plate, and the residual sludge on the membrane was washed with pure water and collected together with the previously scraped sludge. A heating extraction method was employed to extract EPS, including the loosely bound EPS (L-EPS) and tightly bound EPS (T-EPS) from the fouling layer as described previously and after extraction of EPS, the concentration of protein was measured by the Lowry method [48].

(1 − ) × 100 ti tf

5

(5)

where ti (s) is the time to filter the first 500 mL feed water and tf (s) is the time to filter the same volume (500 mL) feed water after 5 min filtration. 2.3.3. Analysis of the membrane The contact angles of virgin membranes, fouled membranes, and physically cleaned membranes in this experiment were measured by KRÜSS DSA 25S measuring apparatus (KRÜSS GmbH, Germany). The membrane samples were air dried for 48 h before measurement. Carefully spreading the hollow membrane sample on the glass plate to prevent the droplet from sliding down the surface of the membrane during measurement. Pure water was used to measure the contact angle and the volume of droplet was controlled at 2 μL each time. A scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) (Regulus 8230, Hitachi, Tokyo, Japan) were used to detect the morphology and elemental ratio of the membrane surface. The membrane samples before SEM-EDS measurement were air dried for 48 h and then membrane surface was covered by ∼2 nm of gold using a magnetron sputter (MSP-2S, IXRF Systems, USA). The acceleration voltage during measurement was 5 kV. Based on the surface SEM images of virgin PVC and PVDF membranes, the distributions of the pore size were statistically obtained via Image Pro Plus V.7.0 software (Media Cybernetics, USA), while membrane porosity was measured by the dry-wet weight method, as presented in previous literature [47]. We tested the concentrations of some ions (i.e., Na+, Ca2+, Mg2+, Cl−, NO3−), DOC and extracellular polymeric substances (EPS) per unit membrane area on

3. Results and discussion 3.1. Variation of permeate flux and resistance during GDM operation Fig. 2a and b reveal the normalized permeate flux development profiles and corresponding resistances over the filtration time for GDM under different hydrostatic pressures with different membrane materials, respectively. As observed in Fig. 2a, the permeate flux of PVDF membrane under 120 mbar experienced a decreasing trend during the whole process. The flux decreased sharply from 33.20 LMH in the initial to about 4.00 LMH on the 5th day. After this period, the flux slowly decreased for about 40–50 days, and the flux finally stabilized at about 1.00 LMH. Similarly, the flux using PVC membrane under a pressure of 120 mbar underwent a rapid decrease in the first several days and stabilized at 0.94 LMH. In comparison, a similar flux development was observed for the GDMs under a pressure of 40 mbar, and the stable flux were 0.92 and 0.87 LMH for PVDF and PVC membrane, respectively. Moreover, the flux fell continuously during the whole filtration in this 3

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Fig. 2. The variation of (a) permeate flux (normalized at 20℃), (b) total fouling resistance under different conditions and (c) the comparison of permeability with the DOC concentration and filtration time summarized from previous studies.

study, and there was not a slight recovery in the process of flux decline, similar to published studies [42,45]. As demonstrated in Fig. 2b, for the PVDF membrane under 40 mbar, the total fouling resistance significantly increased from about 1.07 × 1012 to about 5.00 × 1012 m−1 in 5 days. Then the resistance climbed slowly as the flux reached stable, with the final resistance of 17.91 × 1012 m−1. A similar result was observed for the variation of the total resistance of PVC membrane under 40 mbar. However, for the GDMs under 120 mbar (both PVDF and PVC membrane), the fouling resistances increased sharply from less than 2.50 × 1012 to about 15.00 × 1012 m−1 in about the first 5 days. Then they still increased to about 50.00 × 1012 m−1 at a high rate. The decline in fluxes and climb in resistances were a result of the formation of a dense fouling layer attributed to the blocking of membrane pores, as reported in a previous report [38]. And the flux stabilized when the reduction of the resistance due to the formation of an open and heterogeneous structure in the fouling layer under the biological activities counteracted the increment of resistance because of membrane fouling [49,50]. In addition, the stable flux level in this research (no more than 1.00 LMH) is lower than those in our previous work (1.1–1.5 LMH) for treating SGFPW on Day 90 [45] and previous study (1–20 LMH) [18]. The final resistances (17.91 × 1012-52.60 × 1012 m−1) in this work were also higher than the total resistances reported (7.7 × 1011-157.7 × 1011 m−1) during filtration of surface water, sewage or seawater [18]. The difference in permeate flux and total resistance may be attributed to membrane properties and feed water properties. On the one hand, the intrinsic membrane resistances were 0.63 × 1012 -0.70 × 1012 and 1.81 × 1012 -2.10 × 1012 m−1 for PVDF and PVC membranes, respectively. These values were much higher than those employed in most studies (0.8 × 1011 -33.1 × 1011 m−1) [18], probably due to the smaller membrane pore size (about 10 nm) and higher contact angles (84.68–95.75° in this work vs 74 ± 2°-81 ± 3°) [18]. On the other hand, the complexity of SGFPW (e.g., complex components, high turbidity, salinity, organic content) greatly influenced the stable flux. Previous studies pointed that the high concentrations of turbidity and DOC resulted in a low flux and high resistance [19,20]. The effect of the DOC concentration of influent on stabilized permeability of membrane

in GDM process with different operation times in previous studies is summarized in Fig. 2c. The details are described in Table S1 (Supporting Information). As presented in the Fig. 2c, the permeability of the membrane decreases significantly as the concentration of DOC increases. The stable permeabilities in this study (i.e., 0.08-0.23 L/(m2 h kPa) were comparable to those obtained on the 90th day when treating SGFPW with less organics in our previous study (i.e., 0.10-0.32 L/(m2 h kPa)) [45]. In addition, the stable permeabilities at the end of the test (on Day 512 or 612) in our previous study were 0.06-0.21 L/(m2 h kPa). The lower permeabilities in this study were mainly due to the higher DOC concentration (40 mg/L vs 13.5 mg/L) of the SGFPW. In addition to DOC content, the specific conditions of the experiment design such as the types of membrane module configuration (e.g. flat sheet membrane and hollow fiber membrane) and the scale of the experiment (e.g. labscale and pilot-scale) also have a certain impact on the flux level at the same DOC level. According to previous reports, the fluxes of flat sheet membranes were higher than those of hollow fiber membranes, while the fluxes at pilot-scale were larger than those at lab-scale [39–41]. On the whole, higher fluxes were observed at higher hydrostatic pressure in the initial, while the difference got smaller till the flux stabilized. However, 3 times the hydrostatic pressure only brought about 8 % flux enlargement, whereas the total resistance raised by near 180 %. Therefore, it is not necessary to use excessive hydrostatic pressure in the selection of this working parameter, as mentioned in previous literature [18,45]. As for the membrane types, the fluxes of PVDF membranes were always larger than those of PVC membranes with higher total resistances for the latter membranes, irrespective of hydrostatic pressure. Not only the flux was stable, but also the permeability of the virgin PVDF membrane was higher which was mainly due to the larger average pore size and higher porosity of PVDF membrane. PVDF membranes had about 6 % higher stable fluxes than PVC membranes. However, the price of PVDF membrane was higher than that of PVC membrane meant higher flux levels required higher economic costs.

4

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Fig. 3. Development of water quality parameters during filtration at different conditions: (a) DOC, (b) UV254, (c) turbidity and (d) SDI5.

considered that the RO system can treat such water without prefiltration, and the degree of membrane fouling caused should be relatively low. When SDI5 is larger than 5, a media filter is required before RO [52]. The great difference of SDI between feed water and the permeate water indicates the GDM process can effectively reduce the membrane fouling potential of the subsequent process by removing some pollutants (particulate matter, colloid, organic matter) in the raw water [38,45] which was the main purpose of GDM filtration applying for SGFPW pretreatment. Moreover, it can also be observed in Fig. 3c and d that the turbidity and SDI5 also had a tendency to rise over time. In fact, in the submerged filtration mode without backwashing and discharge, the non-biodegradable membrane-rejected substances in the raw water were accumulated in the GDM system continuously as mentioned above. This problem can be resolved in the practice operation by a) employing combination of other technologies (coagulation and preprecipitation, cross-flow mode and backwash) to remove the part of pollutant which may cause irreversible fouling; b) optimizing operating conditions to reduce membrane fouling (a more open fouling layer at lower hydrostatic pressure in this study) or other conditions to enhance the ability of microorganisms to degrade pollutants throughout the filtration and lead a heterogeneous fouling layer with lower resistance. The TDS, conductivity, pH and inorganic ions (including Cl−, Br−, F−, NO2−, NO3−, Na+, K+, NH4+, Ca2+, Mg2+, Ba2+, and Sr2+) of permeate water in Day 90 and the corresponding one-way analysis of variance is shown in Table S3 and S4 (Supporting Information). As expected, GDM has almost no effect for removing various ions and TDS, and most of the ions had a removal of less than 10 %. In addition, there was no regular distinction in pH, TDS, and conductivity with different membranes and hydrostatic pressures (p > 0.05) which meant that the membrane property and operation pressure had no significant effect on the removal of contaminants during the operation. Altogether, although PVDF membrane had smaller pore size (average pore size, maximum pore size, and minimum pore size), and the operating pressure is different, due to the deterioration of organic matter and the enrichment of the non-biodegradable membrane-rejected substances in the raw water during the experiment, the effects of

3.2. Variation of permeate water quality Fig. 3 presents the primary parameters of permeate water quality during the filtration. The mean DOC concentration of the GDM permeate using PVDF membrane under 120 mbar in the first 10 days was 34.10 mg/L, with DOC removals of about 15.1 %. However, the mean DOC concentrations of the permeate water increased to 37.45–37.81 mg/L from 11 st day, and the removals dropped to about 4.1 %-0.5 %. Further, the permeate DOC (41.73 mg/L, PVC-120 mbar in Day 36 to Day 55) was even higher than that of raw water. Similar trends of DOC concentration changes for PVDF membrane under 40 mbar for PVC membranes, with the DOC concentrations of the permeate water climbing gradually. As shown in Fig. 3b, UV254 also gradually increased from about 0.165 cm−1 to about 0.180 cm−1 under different conditions, and the mean removals of UV254 ranged from 11.3 % to -3.4 %. The phenomenon of deteriorating water quality has been seen in published studies and these negative removals of organic matter mainly because the hydrolysis of organic matter was more pronounced than the degradation of organic matter and physical interception in some periods [38,40,46]. The removals of organics in GDM systems in this study (0.5 %–15.1 % for DOC and -3.4 %–11.3 % for UV254) were slightly lower than a traditional UF unit (DOC, 10 %–20 %, and UV254, 10 %–20 %) during filtration of shale gas wastewater [11,51]. However, similar results have been reported during filtration of surface water or municipal wastewater [20,22,38]. The organic removals can be enhanced by improving the biological activity in GDM systems by applying appropriate approaches, such as coagulation, aeration and adsorption [34–37]. As shown in Fig. 3c, the turbidity of the permeate water was no more than 0.01 NTU, which was significantly lower than that of SGFPW indicating that most particulates and colloids in the raw water were removed by the GDM process as expected. Compared to the SDI5 of permeate water from Fig. 3d with SGFPW in Table S1 (Supporting Information), the SDI5 of the water after the filtration membrane was greatly reduced from about 15 in feed water to about 5 in permeate. In general, when the SDI5 in the permeate water samples is less than 5, it is 5

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Fig. 4. The fouling characteristics of membrane surface in GDM systems: (a) distribution of fouling resistance including membrane resistance, the reversible resistance and irreversible resistance of the membrane at the end of GDM and (b) the contact angles of virgin membrane, fouled membrane, and membrane after physical cleaning.

formations of the depressions with more uneven distribution in the case of 40 mbar. There were many flaky crystalline structures were formed on the upper layer and the deep layer was exposed under 40 mbar where can be observed at 10,000 magnification. At the 1000 magnification, these structures were more obviously on PVDF membrane than PVC membrane. According to EDS analysis shown in Fig. 5, the upper scaly crystal mainly consisted of NaCl because the distribution of sodium and chlorine on the surface of the fouling layer was consistent with the shape of the crystal in the SEM micrography in Fig. 5c. In sharp contrast, the surface distribution of oxygen and silicon on the fouling layer was exactly the same as the shape of the deep layer below the crystalline layer. As for carbon, it was distributed throughout the whole surface of the fouling layer and there was no obvious shape. It showed that the layered structure had an effect on the surface distribution of chlorine, sodium (i.e., NaCl) and oxygen, silicon (probably SiO2), but had little effect on carbon (may be the organic substrate of fouling layer and the carbon of virgin membrane). Inorganic particles such as SiO2 can improve the heterogeneity of fouling layer, which enhanced the permeate flux [25]. The layered and open structures under 40 mbar in this study can improve the roughness and the non-uniformity of the fouling layer. Thus, the fouling resistance can achieve dynamic balance and the stable flux can be obtained, as reported in previous studies [22–24]. By contrast, a higher hydrostatic pressure resulted in more foulants accumulated on the membrane and formed a more compact fouling layer which led a higher resistance of the membrane. These results can explain the phenomenon why high hydrostatic pressure cannot significantly promote the stable flux. The organic and inorganic substances accumulated on the membrane were measured and shown in Fig. 6. These substances influence the flux level during the GDM process [18,25,26]. As shown in Fig. 6a, the concentrations of each ion under higher hydrostatic pressure were higher than that under lower hydrostatic pressure. Similarly, in Fig. 6b, this pattern can also be observed from the concentrations of EPS and DOC. The L-EPS concentrations of each conditions (PVDF-120 mbar, PVC-120 mbar, PVDF-40 mbar and PVC-40 mbar) were 67.32, 73.41, 56.81 and 69.74 mg/m2, respectively. The corresponding T-EPS concentrations were 90.03, 103.13, 73.51 and 92.03 mg/m2, respectively. The concentrations of DOC under 120 mbar (202.9 and 233.42 mg/m2) were higher than those under 40 mbar (151.22 and 130.87 mg/m2). On the one hand, the divergence of the organic and inorganic substances on the fouling layer under different pressures was mainly resulted from different fouling layer structure. A more compact fouling layer was formed under the higher hydrostatic pressure, and this was supported by the surface morphology of fouling layer based on SEM analysis in Fig. 5. On the other hand, the larger flux under higher hydrostatic pressure resulted in more pollutants accumulated on the cake layer during the same operating time. Therefore, the higher concentrations of organic and inorganic substances on the fouling layer under higher hydrostatic pressure caused higher fouling resistance. The EPS fouling has been considered to be an important factor in the increase in hydraulic resistance [29,35,36,53]. The total EPS concentrations on fouling layer of PVDF membranes were 10.9 % and 19.5

different membranes and operating pressures on the removal of the pollutants cannot be significantly reflected. 3.3. The distribution of fouling resistance and the reversibility of membrane fouling At the end of the experiment, the distribution of fouling resistance was tested and shown in Fig. 4a. The final total membrane resistance for GDM using PVDF under 40 mbar was 17.91 × 1012 m−1, and the reversible and irreversible parts were 14.83 × 1012 and 2.45 × 1012 m−1, sharing 82.8 % and 13.6 % of the total resistance, respectively. Moreover, the final total resistance of PVC-40 mbar was 18.94 × 1012 m-1, and the reversible and irreversible resistances were 14.52 × 1012 and 2.61 × 1012 m−1, with the corresponding percentages of 76.7 % and 13.7 %, respectively. Proportion of reversible resistance implied that a large part of membrane fouling could be removed by simple pure water cleaning. The virgin membrane resistances in this study of PVC membrane (1.81 × 1012-2.11 × 1012 m-1) were nearly three times that of PVDF membrane (about 0.63 × 1012-0.70 × 1012 m-1). It was deemed when the hydrostatic pressure rose from 40 mbar to 120 mbar, the total resistances of both PVDF membrane and PVC membrane increased significantly near 180 % (from 17.91 × 1012 and 18.94 × 1012 m-1 to about 49.44 × 1012 and 52.60 × 1012 m-1 for PVDF and PVC membrane, respectively). The proportions of reversible resistances increased from about 80 % to about 90 % with both PVDF membrane and PVC membrane. Further, we measured the contact angles of membrane samples to investigate the reversibility of hydrophilicity. As shown in Fig. 4b, the lower contact angle of virgin PVDF membrane (84.68°) than that (95.75°) of PVC membrane showed that PVDF membrane had better hydrophilicity. This result could also explain the difference in permeability between different membranes. The contact angles of virgin PVDF membrane and fouled membrane differed greatly, decreased from about 84.68° to below 35° after GDM operation. The change trend of the contact angle of the PVC membrane was similar. It was most likely because there were many hydrophilic foulants accumulated on the fouling layer which enhanced the hydrophilicity of the membrane surface significantly [53]. After physical cleaning, the contact angles of each membranes rose back to the level close to the virgin membranes. The recovery trends of contact angles also can indirectly verify that the reversible fouling accounted for a large part of total fouling as mentioned. Overall, the fouling on the membrane caused by GDM operation was highly reversible. 3.4. Morphology of fouling layer and foulants characteristics The surface morphologies of fouling layer at different conditions are presented in Fig. 5. On the one hand, there was no significant difference in the surface morphologies with PVC membrane and PVDF membrane at 120 mbar where all the surfaces of the fouling layers were composed of many particles and depressions. On the other hand, different hydrostatic pressures had certain influence on the surfaces of fouling layer. Compared to the GDMs under 120 mbar, there were fewer 6

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Fig. 5. SEM micrographs of fouling layers on membrane surface in top view at high (×10,000) and low (×1000) magnification: (a) PVDF-120 mbar (b) PVC-120 mbar (c) PVDF-40 mbar (d) PVC-40 mbar and distribution of several elements (Na, Cl, Si, O, C) on fouling layer in GDM surface at ×10,000 magnification under 40 mbar using PVDF membrane by EDS.

subsequent process. The quality of permeate was not significantly influenced by membrane materials and hydrostatic pressure. The effective removals of organic matters were proposed to integrate with other processes to further improve the permeate quality of GDM filtration. (3) Lower hydrostatic pressure results in less foulants accumulated on the membrane. There were more open structures and delamination with higher heterogeneity obviously under 40 mbar. The degree of EPS fouling on the PVDF membrane was lighter than that of the PVC membrane.

% lower than those of PVC membranes, implying the role of membrane materials in the accumulation of EPS. Therefore, this also likely to be one of reasons for the difference in membrane flux levels of two different materials. The PVDF membranes can be observed that they had a stronger resistance to EPS foulants might be due to the difference in membrane properties like higher hydrophilicity. The control of protein (a main component of EPS) fouling using UF membrane higher hydrophilicity has been reported in previous study [54], because the different interaction between the protein and membrane. Thus, PVDF membranes with lower EPS fouling had lower fouling resistances and higher stable fluxes than PVC membranes.

Declaration of Competing Interest

4. Conclusion

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

The feasibility and potential of gravity-driven membrane (GDM) filtration as a low-cost pretreatment for the SGFPW of Sichuan Basin was investigated in this research. Experiments were performed for 90 days with membrane materials of PVDF and PVC, under hydrostatic pressures of 40 and 120 mbar. The following conclusions can be drawn: (1) The fluxes dropped rapidly and finally stabilized in all tests, and the stable fluxes ordered from large to small with PVDF-120 mbar (1.00 LMH), PVC-120 mbar (0.94 LMH), PVDF- 40 mbar (0.92 LMH) and PVC-40 mbar (0.87 LMH). Compared with PVC membrane, PVDF membrane can improve slightly stable flux about 6 % due to their different intrinsic properties. Higher hydrostatic pressure can increase the stable flux by 8 %, but caused a sharp increase (near 180 %) in fouling resistance, mainly reversible resistance. (2) GDM process can effectively reduce the SDI5 and turbidity from the SGFPW thus decrease membrane fouling potential of the

Acknowledgments The work was supported by the National Natural Science Foundation of China (51678377, 51708371), China Postdoctoral Science Foundation (2018T110973, 2017M612965), Full-time Postdoctoral Foundation of Sichuan University (2017SCU12019), the State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University) (M2-201809) and the Fundamental Research Funds for the Central Universities. We would like to thank the Institute of New Energy and Low-Carbon Technology,

Fig. 6. The concentrations of (a) Na+, Ca2+, Mg2+, Cl−, NO3−, and (b) L-EPS, T-EPS and DOC accumulated on the membrane per unit membrane area which were also the components of fouling layer. 7

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Sichuan University, for SEM-EDS measurement. The authors thank Qidong Wu and Wancen Xie for SEM measurements and Yu Sun and Qidan Hu for EPS measurements.

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