Membrane photo-bioreactor coupled with heterogeneous Fenton fluidized bed for high salinity wastewater treatment: Pollutant removal, photosynthetic bacteria harvest and membrane anti-fouling analysis

Science of the Total Environment 696 (2019) 133953

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Membrane photo-bioreactor coupled with heterogeneous Fenton fluidized bed for high salinity wastewater treatment: Pollutant removal, photosynthetic bacteria harvest and membrane anti-fouling analysis Chang Li a,1, Xiong Li a,1, Lei Qin a,1, Wei Wu a, Qin Meng b, Chong Shen b, Guoliang Zhang a,⁎ a Institute of Oceanic and Environmental Chemical Engineering, State Key Lab Breeding Base of Green Chemical Synthesis Technology, Zhejiang University of Technology, Chaowang Road 18#, 310014 Hangzhou, PR China b College of Chemical and Biological Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, Yugu Road 38#, 310027 Hangzhou, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Heterogeneous Fenton/MPBR was successfully built for seafood wastewater treatment. • Novel GO-grafted PVDF membrane was explored for harvesting of photosynthetic bacteria. • Fenton fluidized bed greatly improved biodegradability and reduced sludge discharge. • Modified PVDF membrane held attractive antifouling ability and better water transport.

a r t i c l e

i n f o

Article history: Received 14 June 2019 Received in revised form 15 August 2019 Accepted 15 August 2019 Available online 17 August 2019 Editor: Yifeng Zhang

a b s t r a c t In this study, efficient photosynthetic bacteria (PSB)-GO/PVDF membrane photo-bioreactor (MPBR) combined with heterogeneous Fenton fluidized bed was built and successfully applied for treatment of actual refractory seafood-processing wastewater with extremely high salinity. As effective pre-treatment, heterogeneous Fenton was designed for removing non-biodegradable organics and reducing iron-sludge discharge. In MPBR, GO/PVDF membrane fabricated by chemical grafting GO nanosheets was first used for salt-tolerated PSB harvest. Compared with original PVDF membrane, GO/PVDF membrane exhibited enhanced hydrophilicity, better permeability (4.4 times) and attractive flux recover rate (94%), which was attributed to remarkable reduction in hydrophobic proteins amount of extracellular polymeric substances (EPS). Importantly, COD and NH3-N removal efficiency of

Abbreviations: CA, contact angle (°); COD, chemical oxygen demand (mg/L); EPS, extracellular polymeric substance; FTIR, Fourier transform infrared spectroscopy; F-MPBR, Fentonmembrane phpto-bioreactor; GO, Graphene oxide; HRT, hydraulic retention time (h); Jp, permeate flux (L/m2 h); LB-EPS, loosely bound EPS; Lp, permeability (L/m2 h kPa); NH3-N, ammonia-nitrogen (mg/L); NO3-N, nitrate nitrogen (mg/L); ODp, the optical density of the permeate; PSB, photosynthetic bacteria; PVDF, polyvinylidene fluoride; Rc, resistance of cake layer (m−1); Rf, filtration resistance originated from membrane fouling (m−1); Rif, resistance of inorganic fouling (m−1); Rm, initial resistance of clean membrane (m−1); Rof, resistance of organic fouling (m−1); Rt, total filtration resistance of membrane (m−1); SEM, scanning electron microscope; SRT, solid retention time (day); TB-EPS, tightly bound EPS; TEM, transmission electron microscopy; TMP, transmembrane pressure (kPa); XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction; ΔpTM, transmembrane pressure (bar); μ, permeate viscosity (Pa·s). ⁎ Corresponding author. E-mail address: [email protected] (G. Zhang). 1 These authors contributed equally.

https://doi.org/10.1016/j.scitotenv.2019.133953 0048-9697/© 2019 Elsevier B.V. All rights reserved.

2 Keywords: Membrane photo-bioreactor (MPBR) Heterogeneous Fenton GO/PVDF membrane Biomass productivity Actual seafood-processing wastewater treatment

C. Li et al. / Science of the Total Environment 696 (2019) 133953

MPBR with GO/PVDF membrane were kept about 95 and 98%, respectively, and average biomass productivity reached as high as 105 mg/L·d. This study provides a promising and economical way to build efficient MBR combined with new materials for high salinity hazardous wastewater treatment. © 2019 Elsevier B.V. All rights reserved.

1. Introduction As marine economy including seawater desalination, ocean resource utilization and seafood processing industry grows rapidly, large volume of salinity hazardous wastewater has been discharged into water environment every year (Linares et al., 2014). Recently, the treatment of high salinity wastewater (salt wt% N1.0%) has been regarded as one of the most annoying issues since it is causing more serious hazards to the environment (Tomei et al., 2017). Owing to the presence of abundant soluble salts and toxic organic pollutants, this kind of wastewater is very difficult to be disposed by traditional ways (Ali et al., 2018). Therefore, development of efficient and environmentally friendly method for treatment of high salinity hazardous wastewater has been a major subject of importance. Over the past decade, many wastewater treatment technologies, including physical, chemical, and biological methods have received great attention. (Ratola et al., 2012; Fu et al., 2012; Liang et al., 2014; Sacco et al., 2018). Among various techniques, the biotechnologies are considered as good choice for the treatment of refractory wastewater because of its advantages of simple process and low cost (Zhang et al., 2013a; Zhang et al., 2019). As one of the most fascinating biotechnologies for wastewater treatment, membrane bioreactor (MBR) has shown great potential in dealing with high-concentration organic wastewater efficiently and stably (Qin et al., 2012; Neoh et al., 2015; Krzeminski et al., 2019; Li et al., 2019a). However, the large amounts of soluble inorganic salts often have deleterious effects on the microorganism's growth and their activity by breaking osmotic stress balance across cell wall (Kargi and Dincer, 1996). In order to conquer the problem, more and more attention has been paid to screening and cultivating salt-tolerant microorganism (Fan et al., 2018; Lu et al., 2019). In our previous study, membrane photo-bioreactor integrated with salt-adapted photosynthetic bacteria (PSB) was constructed for efficient treatment of synthetic high salinity organic wastewater (Qin et al., 2017). The system of PSB/MBR was developed for combining organic pollutants removing with biomass harvest. However, although some progresses on salt-tolerated microorganism have been made, there are still some primary problems hindering the wide application of MBRs for the actual high salinity wastewater advanced treatment, for example, much higher salinity and organic loading, the existence of nonbiodegradable contaminants (including natural pigment and polycyclic aromatic hydrocarbons), heavy metals and serious membrane fouling (Meng et al., 2017). In view of the practices up to now, single unit process is rarely found to be available for proper high salinity wastewater treatment. It might be essential to combine multiple wastewater treatment process efficiently for advanced treatment of high salinity wastewater (Wang et al., 2014). Fenton oxidation, as a green, inexpensive, and practical chemical process, is more readily employed for remediation of toxic and recalcitrant organic hazardous (Zhang et al., 2016; Moattar et al., 2019). Since Fenton process can effectively decompose large molecule organic matters to improve the biodegradability of wastewater, from our previous experience, it may be beneficial to be used as the pretreatment of sequence microbial degradation (Qin et al., 2018). However, some bottlenecks still exist in homogeneous Fenton process currently used in industrial application, including very limited operating pH range of 2.0–3.0, forming of vast iron sludge, and high labor and reagent costs. To avoid these drawbacks, development of heterogeneous Fenton

leads to find a promising alternative (Li et al., 2019b). As far as we know, few reports on combining MBR with heterogeneous Fenton for advanced treatment of high salinity wastewater exist at present. Moreover, for the sake of dealing with the aforementioned membrane fouling problem, surface hydrophilic modification of membrane has been a topic of much interest, which includes nanomaterial blending, surface coating and chemical grafting (Liu et al., 2011; Zhang et al., 2013b; Ren et al., 2019; Zarca et al., 2019). As a good choice, surface chemical grafting modification not only makes most of hydrophilic groups be exposed on the outside surface of membrane, but also provides strong interaction between membrane and functionalized species, and therefore exhibits great potential in industrial wastewater treatment (Shin et al., 2017). Recently, many functional materials, such as hydrophilic polymers, amphiphilic polymers and inorganic/metal oxides, have been attempted for membrane hydrophilic modification (Ahmad et al., 2013). Among them, graphene oxide (GO) nanosheets, as novel two-dimensional structural materials, reveal a useful space in membrane modification due to their large surface area, high hydrophilcity of oxygen-containing functional groups, and good mechanical and compatible properties (Hu et al., 2016). Owing to high antimicrobial activity and hydrophilcity, GO nanosheets were often preferred to be intercalated into membranes for improving permeate flux and antifouling performance. Though the incorporation of GO on the surface or into polymer matrix by coating and blending have been tried in fabricating some interesting antifouling membrane, there is no report on the facile preparation of hydrophilic hollow fiber PVDF membrane by surface grafting GO nanosheets, especially in MBR system. In this work, a novel efficient PSB-MPBR system by coupling heterogeneous Fenton oxidation was built for high salinity organic wastewater treatment, in which the anti-pollution property of the membrane can be effectively enhanced by chemical grafting GO nanosheets and the combined system as well as new materials can enhance the biomass productivity in actual refractory seafood-processing wastewater treatment. In the present work, Fe-Ti bimetallic oxides were chosen as heterogeneous Fenton catalysts due to high catalytic activity and stability for activation of H2O2 under the visible light irradiation. The investigations of critical points in the present study are: (1) performance of heterogeneous Fenton fluidized bed as an effective pretreatment, (2) morphology and physicochemical properties of prepared GO/PVDF hollow fiber membrane compatible for bioreactor, (3) enhanced removal performance, permeability and antifouling properties of PSB-MPBR with Fenton oxidation and new materials compared to a control traditional system, (4) analysis of filtration resistance, biomass productivity and EPS content on the membrane surface in different MPBR system. 2. Materials and methods 2.1. Materials All chemical reagents used are of analytical grade, including NaHSO3, KMnO4, KOH, H2SO4, NaOH, K2HPO4, HCI, H2O2, HNO3, bovine serum albumin (BSA) and L-ascorbic acid (L-AA, 99%). Porous PVDF membranes were self-made membranes by a wet-spinning technique with molecular weight cut off 150,000 Da. Photosynthetic bacteria (PSB), bought from Zhejiang Dinglong Co., China, was chosen for high salinity wastewater treatment. The photosynthetic bacteria were cultivated in the flasks with sterile medium,

C. Li et al. / Science of the Total Environment 696 (2019) 133953

which contained 0.1 g/L NH4Cl, 0.5 g/L NaCl, 0.5 g/L NaHCO3, 3.0 g/L CH3COONa·3H2O, 0.1 g/L MgSO4·7 H2O, 0.1 g/L CaCl2·2H2O, 0.2 g/L K2HPO4, 0.5 g/L Yeast extract, 0.5 g/L Peptone. During culture process, the illumination intensity, pH and temperature were adjusted to about 3000 lx, 8.0 and 30 °C, respectively. Actual seafood-processing (edible marine algae) wastewater as the typical high salinity organic wastewater was collected from the workshop of Zhejiang Seafood Plant, which was mainly derived from the boiling, brining and rinsing technology of food processing. Table 1 clearly lists water quality characteristics of edible marine algae processing wastewater.

2.2. Experimental setup The scheme of experimental device is shown in Fig. 1a, which includes heterogeneous Fenton oxidation and membrane photobioreactor (MPBR). The pre-treatment of seafood-processing wastewater by heterogeneous Fenton process was taken in a fluidized bed to increase the biodegradability of high salinity wastewater. The heterogeneous Fenton process in pilot scale with working volume of 5 L (15 cm of diameter, 30 cm of height) were operated under the optimum condition of laboratory scale test. The wastewater and H2O2 oxidant were pumped into fluidized bed from the bottom of column, and heterogeneous Fenton catalysts were filled into the reaction column. The liquid flow was useful to keep solid catalysts uniformly disperse into the reaction column, which improves the utilization rate of catalysts and thus increases the removal efficiency of organic pollutants. The structure of the MPBR system was identical except that different membranes (self-made original PVDF and modified PVDF membrane) were used. The working volume of each MPBR system was 25 L, and effective area of hollow fiber module was 0.2 m2. During the MPBR operation, continuous illumination was used to maintain the growth of PSB, the suction pump operated intermittently (running for 5 min and stopping for 5 min), and the membrane fouling was evaluated by continuously checking the change of transmembrane pressure (TMP). To reduce membrane fouling and increase the dissolved oxygen in the system, air is pumped into MPBR at regular intervals. The detailed parameters includes hydraulic residence time (HRT) of 36 h, solid residence time (SRT) of 30 days, aeration of 1.5 m3/m2 h, temperature of 25–30 °C, and pH of 7.5–8.0. In this study, the operation time of whole experiment was approximately three months, which was divided into three periods for better evaluating the feasibility and stability of membrane photo-bioreactor coupled with heterogeneous Fenton fluidized bed. The influent and effluent quality, including chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD5), nitrate nitrogen (NO3-N) and ammonia nitrogen (NH3-N) were determined by standard analytical methods. The anions and cations in samples were monitored by Metrohm 861 Compact IC and IRIS Intrepid ICP, respectively.

2.3. Synthesis of catalyst and membrane 2.3.1. Preparation of porous bimetallic oxides The porous heterogeneous Fenton catalyst Fe-Ti bimetallic oxides was fabricated by sulfuric acid impregnating strategy using citric acid as templates, and the synthetic route is according to the previous work (Qin et al., 2014). 2.3.2. Preparation of GO/PVDF membrane Graphene oxide (GO) was fabricated from natural graphite powders (500 mesh, Nanjing Xianfeng Nanomaterials Co., LTD) using a modified Hummers method (Hummers and Offeman, 1958). The preparation process of GO/PVDF membrane is shown in Fig. 1b. In order to improve surface hydrophilic property of self-made PVDF hollow-fiber membrane, PVDF membrane was treated under the conditions of alkaline (KOH) and strong oxidizing (KMnO4) solution to remove fluorine. After washing with NaHSO3 solution, the membrane surface was recovered to white. Then the membrane was immersed in GO solution (0.5 wt%), and GO was grafted onto the surface of defluorinated PVDF membrane by hydrogen bond interaction. The modified PVDF membrane was washed to remove residual GO with deionized water, and GO/PVDF hollow fiber membrane was obtained as expected. 2.4. Characterization of samples The structures of surface and cross-sectional of different samples were characterized by JEM-1200EX transmission electron microscopy (TEM, Hitachi, Japan) and scanning electron microscopy (SEM, Hitachi, Japan). The surface functional groups of the original and modified PVDF membranes were analyzed by Fourier transform infrared spectroscopy (FTIR, iS50, Thermo Fisher Nicolet, USA). The crystal morphology of different membranes was investigated by X-ray diffraction (XRD, X'Pert PRO, PNAlytical, Holland). The X-ray photoelectron spectroscopy (XPS, PHI5300, Thermo, USA) was used to assess the elements binding energies of various membranes. The contact angle (CA) measurements were conducted with the sessile drop method using contact angle meter (OCA50AF, Dataphysics, Germany). The organic matters of the seafood-processing wastewater and Fenton-treated effluent were analyzed by gas chromatography (GC) coupled with tandem mass spectrometry (MS, Thermo ISQ). The ferric ions content of heterogeneous Fenton effluent was examined by 55B atomic absorption spectrometer (AAS, Varian, USA). 2.5. Evaluation of membrane properties 2.5.1. Permeability and filtration resistance of hollow fiber membrane The fouling degree of hollow fiber membrane in F-PMBR system is evaluated by measuring the membrane permeability changed with time. Membrane permeability (Lp) is calculated by the following Eq. (1):
Lp L=m2 h
bar ¼

Table 1 Characteristics of marine algae processing wastewater. Parameters

Values

COD (mg/L) BOD5 (mg/L) BOD5/COD NH3-N (mg/L) NO3-N (mg/L) Conductivity (ms/cm) Turbidity (NTU) pH Cu (mg/L) Cd (mg/L) Pb (mg/L) Cr (mg/L)

3335 ± 145 770 ± 100 0.23 ± 0.02 41.91 ± 1.16 8.25 ± 0.43 68.16 ± 0.14 590 ± 20 5.0 ± 0.4 0.07–0.15 0.10–0.21 0.05–0.13 0.06–0.10

3

Jp ΔpTM

ð1Þ

where Jp (L/m2h) is the permeate flux, ΔpTM (bar) is the transmembrane pressure. The filtration resistance (Rt) is obtained according to the Darcy's law based on Eq. (2): Rt ¼ Rm þ R f ¼

ΔpTM μ
Jp

ð2Þ

where Rt (m−1) is the total filtration resistance of membrane, Rm (m−1) is the intrinsic filtration resistance of membrane, Rf (m−1) is the filtration resistance originated from membrane fouling, and μ (Pa·s) is the permeate viscosity.

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Fig. 1. (a) Experimental setup of the heterogeneous Fenton/MPBR combined process, and (b) synthesis schematic diagram of the GO/PVDF modified membrane.

In order to analyze the primary components of membrane fouling more accurately, Rf can be expressed as Eq. (3): R f ¼ Rc þ Rof þ Rif

ð3Þ

where Rf (m−1) is further divided into the resistance of cake layer Rc (m−1), the resistance of organic fouling Rof (m−1) and the resistance of inorganic fouling Rif (m−1). 2.5.2. Characterization of membrane morphology The microstructure of cross-section and external surface of PVDF membrane and GO/PVDF membrane before and after the operation were characterized by SEM to observe the situation of the membrane fouling.

2.5.3. Membrane cleaning The fouled membrane components were cleaned by physical and chemical methods as follows: (1) membrane module was washed to eliminate the cake layer on membrane surface by using deionized water; (2) the membranes were dipped in NaClO solution (1.0%) with aeration to remove the organic pollutants of membrane; (3) the membranes were immersed in citric acid solution (1.0%) to remove inorganic contaminants; (4) the membrane components were backwashed with deionized water for 30 min. The cleaning effect of membrane was evaluated via measuring the membrane permeability.

2.6. Measurement of PSB biomass and extracellular polymeric substances According to previous study (Qin et al., 2017), the biomass of PSB was measured with a 722s spectrophotometer at a wavelength of 805 nm. The biomass is calculated by Eq. (4):   Biomassðg=LÞ ¼ 1:3677  OD805 −2:7  12−2 R2 ¼ 0:9987

ð4Þ

where OD805 is the illumination intensity of photosynthetic bacteria suspension. The concentration of bound EPS on membrane surface was determined by heat extraction method, including loosely bound EPS (LBEPS) and tightly bound EPS (TB-EPS). The protein and DNA concentration were determined through ultraviolet absorption method, and the content of polysaccharide was detected by phenol-sulfuric acid method (Zhao et al., 2014). 3. Results and discussion 3.1. Marine algae wastewater characterization Edible marine algae as one kind of nutritive seafood have been very popular in the market recently due to high nutritive and medicinal value. However, the processing of edible marine algae often generates large numbers of hazardous wastewater, which presented a brown color associated with a mass of soluble salts (conductivity of 68.30 ms/cm), organic contaminants (COD of 3335 mg/L) and ammonia

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5

Fig. 2. (a) SEM images of porous bimetallic oxide catalyst, (b) TEM images of porous bimetallic oxide catalyst.

nitrogen (NH3-N of 42.12 mg/L). In comparison with the synthetic high salinity wastewater reported before (Qin et al., 2017), the actual seafood-processing wastewater held much higher salinity (about 2 times), organic loading (approximately 1.6 times) and turbidity (7.8 times), and contained refractory polycyclic aromatic hydrocarbons and hazardous heavy metal ions including Pb, Cr and Cd. In order to further investigate the organic matters of actual wastewater, Fig. S1 shows the GC–MS chromatograms of seafood-processing wastewater. As shown in Fig. S2 and Table S1, it was found that some ester compounds and polycyclic aromatic hydrocarbons including N-phenyl-2pyridinamine, sulfapyridine and diisooctyl phthalate were existed in the wastewater. All the characteristics of the actual seafoodprocessing wastewater collected from the workshop of seafood plant showed more toxicity and lower biodegradability (BOD5/COD ratio of 0.21) in routine treatment. Based on the above observations, the actual seafood-processing wastewater was categorized as the non-degradable high salinity wastewater, which was hard to be treated by the single MBR unit process. Therefore, the necessary pretreatment of actual high salinity wastewater should be designed and carried out before fed into the membrane bioreactor for further treatment.

As noticed from the figure, the COD of seafood processing wastewater was fluctuated between 3190 and 3480 mg/L throughout the operation (Fig. 3a). After Fenton treatment, COD declined rapidly, and the effluent COD concentration was reduced to 1530–1720 mg/L. The maximum rate of COD removal reached approximately 54.0% in total continuous process, which suggested that heterogeneous Fenton oxidation can efficaciously decompose most non-biodegradable organic matters (such as grease and aromatic hydrocarbon compounds). Heterogeneous Fenton for organic matters degradation was monitored by UV–Vis spectrum and GC/MS. As shown in Fig. S4, the characteristic peaks of original

3.2. Pretreatment of high salinity wastewater by heterogeneous Fenton The prepared Fe-Ti bimetallic oxides used in this study presented the hierarchical porous structure and high surface area (SEM and TEM images of catalyst shown in Fig. 2). On the basis of previous results, the visible-light assisted heterogeneous Fenton operation for high salinity wastewater was carried out at 0.04 M of H2O2, 2.5 g/L of catalysts, and pH of 4.0. The degradation performance of organic matters along with operation time in heterogeneous Fenton process was depicted in Fig. S3. In the initial stage of operation (60 min), a dramatic reduction in COD was observed with the removal efficiency of 51.0%. When the reaction time increased to 90 min, the removal efficiency of organic matters was slowed down and reached 54.4%. To further investigate the degradation dynamics of organic matters, a pseudo-first-order kinetics equation (Eq. (5)) was put forward as follows: C ¼ C 0 expð−ktÞ

ð5Þ

where C represents the dye concentration (mgL−1), C0 is the initial dye concentrations of first-order reaction (mgL−1); k is the reaction rate coefficient (min−1) and t is the time (min). As apparent from figure, the pseudo-first-order model can fit the results of organic matter degradation in Fenton process as the evidenced by high regression coefficient (R2 = 0.9575). By calculation, the rate constant for heterogeneous Fenton oxidation of organic pollutants is 0.0086 min−1. After heterogeneous Fenton process, the color of raw wastewater was decreased obviously.

Fig. 3. (a) Changes of COD concentration and removal rate by heterogeneous Fenton pretreatment, and (b) Changes of NH3-N concentration and removal rate by heterogeneous Fenton oxidation.

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Fig. 4. SEM images of surfaces and cross-section for pristine PVDF membrane (a, b), and GO/PVDF membrane(c, d).

Fig. 5. (a) XRD spectra of pristine PVDF membrane and GO/PVDF membrane, (b) FTIR spectra of pristine PVDF and GO/PVDF membranes, (c) full XPS spectra of pristine PVDF membrane and GO/PVDF membrane, and (d) XPS spectra of pristine PVDF and GO/PVDF membranes.

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contaminants at 272 and 341 nm belong to the benzene ring of polycyclic aromatic hydrocarbons. During the whole reaction, it was clear that the benzene ring of organic matters were decomposed along with the time increasing since the intensity of absorption bands at 272 and 341 nm decreased dramatically. As the generated ·OH radicals during the heterogeneous Fenton process was unable to selectively attack chemical bond of organics pollutants, many intermediates could be produced. By GC–MS analysis, it was clear that the ester compounds and polycyclic aromatic hydrocarbons of raw wastewater were decomposed into the alcohols and alkanes with higher biodegradability after Fenton oxidation (Fig. S5 and Table S2). As shown in the Fig. 3b, the NH3-N content of influent was varied in range of 40.75–43.07 mg/L. Comparatively, the Fenton process showed a poor effect on NH3-N treatment, and the removal rate of which was only maintained around 12.59%, consist with the previous studies (Wu et al., 2009). Despite this, the BOD5/ COD of wastewater increased from 0.21 to 0.43, which indicated the biodegradability of seafood-processing wastewater was evidently enhanced by Fenton pretreatment. Meanwhile, using of solid catalysts also greatly reduced the iron-sludge discharge and secondary environmental pollution. Hence, heterogeneous Fenton is indeed quite suitable for the pretreatment process of subsequent biological treatment. 3.3. Surface chemical grafting of hollow fiber membrane 3.3.1. Characterization of GO/PVDF membrane The presence of free hydroxide radicals on the surface of pre-oxidized PVDF substrates could be anchored with GO nanosheets to form the hydration layer on the surface of PVDF hollow fibers. In order to determine whether GO successfully grafted on PVDF membrane surface, a series of characterizations were performed. The surface morphology of PVDF and GO/PVDF membranes was characterized and shown in Fig. 4. Seen from the surface of membrane, it was found that a layer of grafted GO nanosheets can be observed on the modified PVDF membrane. By contrast, the roughness of the surface significantly increased after GO chemical grafting. The cross-sectional SEM image display that the GO/PVDF membranes also maintained the typical asymmetric structure through chemical modification. After defluorination treatment and modification by chemical grafting, the pore size of prepared GO/PVDF hollow fiber membrane was around 0.116 μm. In XRD pattern, the characteristic peaks at 2θ = 18.3 and 19.8° corresponds to amorphous PVDF polymer (Fig. 5a). However, no obvious indication of bulk GO crystalline phases was found, which may be due to less loading of GO on the surface of PVDF membrane. Fig. 5b depicts the FTIR spectra of PVDF and GO/PVDF hollow fiber membrane. The -CF2 deformation band at 1179 cm−1 and –CH2 stretching vibration band at 1403 cm−1 were obviously shown in curve of original PVDF membrane. Compared with original PVDF membrane, a new peak at 1644 cm−1 appeared in GO/PVDF membrane, which was in accordance with C_O stretching vibrations of GO structure. In the full XPS spectrum, the characteristic peaks of C1s, O1s, F1s and N1s can be ascertained (Fig. 5c). In comparison with original PVDF membrane, the content of C and O elements on the surface of GO/PVDF membrane increased, and the content of the F element was remarkably reduced. To further acknowledge the chemical state of C element, high-resolution C1s peak of different membranes was analyzed (Fig. 5d). For original PVDF membrane, the binding energy of C-F species was observed at 290.7 eV. After grafting of GO nanosheets, the peaks of C-F species disappeared, and the binding energies of C1s at 286.6, 287.5 and 290.7 eV correspond to characteristic peaks of C\\O, C_O and C-OOH oxygen-containing groups were discovered for GO/PVDF membrane. The percentage of oxygen-containing groups were higher than 13.08%, indicating that GO nanosheets were successfully grafted on surface of PVDF hollow fiber substrate, consistent with FTIR results. 3.3.2. Hydrophilicity and filtration performance of GO/PVDF membrane The surface hydrophilicity of original and modified membrane was investigated by the measurement of water contact angle. As shown in

7

Fig. 6a, the contact angle of GO/PVDF membrane decreased from 92.31° to 72.13°. It was evident that the hydrophilicity of membrane surface increased after the GO grafting modification, and consequently significantly improved the flux of membrane. The pure water permeability of the PVDF membrane before and after modification was measured. By analysis, the pure water flux of original PVDF membrane was 110 L/m2·h. After chemical grafting, the pure water flux of modified PVDF hollow fiber membrane increased to 448 L/m2·h, confirming that grafted GO hydrophilic sites promoted water transport rate. Recently, the existing GO based PVDF membranes were mostly synthesized by the blending method which was hard to be used to improve the now available commercial membranes (Zhao et al., 2017; Ko et al., 2018). Comparatively, our prepared GO/PVDF hollow fiber membrane exhibited higher water flux revealing that surface chemical grafting strategy takes the great potential for the enhancement of membrane flux. Fig. 6b presents the permeability of original PVDF and GO/PVDF membranes for BSA filtration. Besides membrane permeability, the flux recovery rate is another key parameter to evaluate the antifouling ability and stability of membrane in practical application. In the first 30 min, the water flux of both membranes changed little, after that it began to decrease significantly as the adhesion of hydrophobic protein foulants on the surface of membrane caused pore blocking. In comparison with original PVDF membrane, GO/PVDF membrane maintained much higher flux, and the flux recovery rate of which reached 94% of initial value after washing. The high antifouling performance of GO/

Fig. 6. (a) Pure water flux of pristine PVDF and GO/PVDF membranes: a) GO/PVDF membrane contact angle, b) pristine PVDF membrane contact angle, (b) BSA flux of pristine PVDF and GO/PVDF membranes.

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Fig. 8. (a) Time-variation of membrane flux for pristine PVDF and GO/PVDF membranes during system operation, and (b) filtration resistance of pristine PVDF and GO/PVDF membranes during system operation.

PVDF membrane was also maintained during the further operation. The results indicated that well-dispersed GO nanosheets obviously improved the antifouling property and permeability stability of modified PVDF membrane. 3.4. Biological treatment of high salinity wastewater by PSB-MPBR system

Fig. 7. (a) Changes of PSB biomass production with operation time in MPBR systems, (b) variation of the concentration of COD and corresponding removal efficiency in MPBR systems, and (c) variation of the NH3-N content and removal efficiency in MPBR systems.

3.4.1. Cultivation of PSB biomass In present study, both original PVDF and GO/PVDF modified membranes were applied into the PSB-MPBR system to deal with high salinity seafood-processing wastewater. To evaluate the application feasibility of GO/PVDF membranes for PSB harvest, the biomass production changed with the operation time was measured by profiling in term of suspended solids. The relevant results on volumetric biomass concentration for MPBR systems with different membranes are shown

Table 2 Qualities of influents and effluents in the MPBR system coupling with heterogeneous Fenton oxidation. Items

COD (mg/L) NH3-N (mg/L) NO3-N (mg/L) Turbidity (NTU) a

Influent

3335 ± 145 41.91 ± 1.16 8.25 ± 0.43 619

Integrated wastewater discharge standard, China, GB 8978-1996.

Pre-treated (Fenton)

1625 ± 95 36.82 ± 1.77 0.19 ± 0.051 14.8

a

Std

Effluent PVDF

GO-PVDF

103.04 ± 11.24 2.47 ± 0.42 0.06 ± 0.014 b0.5

81.3 ± 5.12 1.01 ± 0.33 0.05 ± 0.007 b0.5

100 15 – –

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in Fig. 7a. The initial concentration of PSB added in MPBR system was set at 0.1 g/L. Seen from the figure, the PSB biomass concentration of MPBR system with GO/PVDF membranes exhibited an obvious increase from 0.1 g/L to 3.17 g/L in the initial stages of 16 d operation. After 16 days, the PSB biomass concentration was maintained around 3.29 g/L and the average biomass daily productivity was as high as 105 mg/L·d, indicating that the improved rejection of modified membrane promoted the growth of PSB in high salinity wastewater through alleviating the adhesion of cells on membrane surface. In comparison with MPBR with original PVDF membrane (93 mg/L·d), the increase in biomass daily productivity was observed for GO/PVDF membrane system, which demonstrated that the introduction of modified membrane had positive effect on the PSB cultivation. Moreover, to explore the influence of Fenton effluent on the growth of PSB, the bacteria was inoculated into the MPBR system without the pretreatment. It was obvious that the biomass concentration and productivity of PSB in the mixed liquid was significantly reduced without heterogeneous Fenton treatment. After stabilization, the PSB biomass only maintained at 2.75 g/L, which well confirmed that trace ferric ions (varied from 0.6 to 1 mg/L) contained in heterogeneous Fenton effluent promoted the growth of bacteria. 3.4.2. COD and NH3-N removal The performance of MPBR systems with different membranes by way of influent and effluent characteristics (COD and NH3-N) and related removal rate are shown in Fig. 7b and c. Compared with sole MPBR process, the heterogeneous Fenton/MPBR combined technology represented high organic contaminants removal and effluent COD kept constant at about 80 mg/L although influent COD varies greatly. After about a week, the organic contaminants removal rates gained in the MPBR were rapidly enhanced and then kept stable after bacterial adaptation. Meanwhile, the removal rate of PSB-MPBR remained as high as 90%. Compared with the original PVDF membrane, the effluent COD after GO/PVDF membrane filtration was lower than 100 mg/L

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and the average COD removal rate was higher than 95%. By analysis, the effluent qualities of GO/PVDF membrane higher than original PVDF membrane may be due to the fact that surface grafted GO layer prevented the living cells adhering on the membrane surface and thus enhanced biomass concentration. Although the low removal efficiency in NH3-N was observed in the pretreatment, it was shown from Fig. 7c that a significant decline in NH3-N concentration from 36.82 to 1.01 mg/L happened after MPBR treatment during the coupling process. Similar to COD removal, the removal rate of NH3-N in MPBR system was up to 98%, displaying that the novel combined process provided high quality effluent meeting the local discharge standards (Table 2). Based on the excellent performance for wastewater treatment, it was expected that the new system with GO/PVDF modified membrane presented the promising potential in practical applications. 3.4.3. Membrane permeability and fouling Fig. 8a shows the variation of membrane permeability for original PVDF and GO/PVDF membranes versus operation time during the MPBR system. It is obvious that the initial water flux of GO/PVDF membranes (265 L/m2·h·bar) were much higher than original PVDF membranes (90 L/m2·h·bar). Subsequently, an evident decrease in membrane flux was observed in the initial stage and a cake layer appeared on the membrane surface. The permeability descending tendency was in agreement with the previous studies, which was attributed to the deposition of PSB cells and extracellular polymeric substances on the membrane surface. On the fifth day of the operation, the membrane flux was remained constant. By comparison, the flux of GO/ PVDF membrane was maintained around 75 L/m2·h·bar, while the PVDF membrane flux was only fluctuated above and below 14 L/m2·h·bar. The significantly enhanced membrane flux (N5 times) revealed that GO nanosheets grafting effectually mitigated the membrane fouling and guaranteed GO/PVDF membrane to keep high flux for a long time. Moreover, the critical flux has significant effects on

Fig. 9. SEM images of surfaces for fouled PVDF membrane (a), fouled GO/PVDF membrane (b), cleaned PVDF membrane (c), and cleaned GO/PVDF membrane (d).

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cleaned membranes surface (Fig. 9), it was also found that most of surface pollutants was effectively wiped off, besides a thin cake layer adhered on the of GO/PVDF membrane surface. For the sake of further investigating the primary factor of membrane fouling in various MPBR systems, the filtration resistances of original PVDF and GO/PVDF membranes were calculated for comparison. As apparent from figure, it was found that the original membrane resistance (Rm) only accounted for a small part of the total resistance (12.73% for PVDF and 19.27% for GO/PVDF). During the MPBR operation, the filtration resistance derived from the deposition of cake layer was significantly increased (Abass et al., 2018). As depicted in Fig. 8b, an obvious reduction in filtration resistance (Rc) is observed for GO/PVDF membrane compared to original PVDF membrane, which indicated that the presence of hydrophilic layer of modified PVDF can effectively reduce the membrane pollution caused by cake layer and increase the service life of MPBR system. 3.5. Analysis of extracellular polymeric substances on membrane surface

Fig. 10. (a) EPS content of mixed liquid in the different MPBR systems, and (b) analysis of EPS concentration on the pristine PVDF and GO/PVDF membranes surface.

membrane fouling in the MBR operation (Zhang et al., 2013). In this study, the critical flux of different membranes was determined by using a flux-step method. As shown in Fig. S6, it was found that the critical flux of the GO/PVDF membrane was 8.5–11.5 L/m2·h in the membrane photo-bioreactor, approximately 2 times higher than that of original PVDF membrane, which demonstrated that the introduction of GO effectively enhanced the anti-fouling ability of membrane. After 15 days of operation, the fouled membranes were cleaned. As apparent from figure, the permeability of GO/PVDF membrane can be recovered to as high as 252 L/m2·h·bar, and the flux recover rate of which achieved the 95% of the initial permeability, much higher than original PVDF membrane (86%). The results demonstrated that surface grafted hydrophilic GO nanosheets were capable to exhibit better permeability and antifouling property via hindering some foulants from adhered on membrane surface. As illustrated from SEM images of the fouled and Table 3 Components and contents of EPS on membrane surface during MPBR operation. Component LB-EPS (mg/g MLSS)

TB-EPS (mg/g MLSS)

Protein DNA Polysaccharide Total LB-EPS Protein DNA Polysaccharide Total TB-EPS

PVDF membrane

GO-PVDF membrane

42.76 2.40 5.80 50.96 18.71 2.85 5.42 26.98

11.60 4.80 21.82 38.22 5.49 3.45 7.98 16.92

Extracellular polymeric substances (EPS) are secreted by the bacteria in the mixed liquor (including polysaccharides, proteins, DNA, etc.), which are often regarded as the major foulants to induce the membrane fouling in MPBR system (Qin et al., 2015; Lin et al., 2014). To further illustrate the role of EPS concentration and composition, EPS is divided into tightly bound EPS (TB-EPS) and loosely bound EPS (LB-EPS). During the whole operation in MPBRs, the concentration of LB-EPS in mixed liquor exhibited the more obvious variation with time compared with TB-EPS (Fig. 10a). In the first 18 days, the amount of LB-EPS increased from 421.9 to 848.5 mg/g-MLSS in the photobioreactor. The sudden rise was probably due to the rapid growth of PSB cells, and the metabolic EPS was beneficial to the decomposing of organic matters in the wastewater. However, after 18 days, when the growth of PSB biomass was kept steady, the LB-EPS concentration tended to decrease from 848.5 to 551 mg g-MLSS. The finding was in consistent with the previously reported studies (Ji et al., 2010). By comparison, the LB-EPS concentration of the mixed liquor in GO/PVDF membrane system was much bigger than that in original PVDF membrane system, suggesting that fewer extracellular polymeric foulants were adsorbed on membrane surface. To better acknowledge the distribution of EPS composition and content on the membrane surface, Fig. 10b clearly shows that the proteins, polysaccharides and DNA concentration of both original and modified PVDF membrane in terms of LB-EPS and TB-EPS during the MPBR operation. By analysis, the DNA component only accounted for a small part of total EPS compared with the protein and polysaccharide, and the content of LB-EPS on the membrane surface was higher than LB-EPS (Table 3). Compared with the original PVDF membrane, it was found that the sum of TB- proteins and LB- proteins for GO/PVDF membrane was much lower. In sharp comparison, the protein amount of LB-EPS on the GO/PVDF membrane surface (11.6 mg/g-MLSS) was only 27% of that on original PVDF membrane surface (42.76 mg/g-MLSS) and the protein amount of TB-EPS on the modified membrane (5.49 mg/gMLSS) was less than half of that on unmodified membrane surface (18.71 mg/g-MLSS). However, the polysaccharide concentration of both LB-EPS and TB-EPS on GO/PVDF membrane (21.82 and 7.98 mg/ g-MLSS) was higher than that on original PVDF membranes (5.8 and 5.42 mg/g-MLSS). This reveals that the improvement of anti-fouling property for GO/PVDF membrane was mainly owing to the remarkable reduction of hydrophobic protein deposited on the surface of membrane. 4. Conclusions In conclusion, novel MPBR technology combined with heterogeneous Fenton process offers an attractive alternative for advanced treatment of high salinity actual seafood-processing wastewater, which

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provided high quality effluent meeting the wastewater discharge standards. Development of heterogeneous Fenton fluidized bed greatly improved biodegradability (increased from 0.21 to 0.43) and reduced ironsludge discharge. In MPBR system, GO/PVDF membrane with enhanced hydrophilicity, better permeability (4.4 times) and attractive flux recover rate (94%) was successfully applied for high-active PSB harvest. By EPS analysis, it was found that grafted GO nanosheets effectively hindered the adhesion of hydrophobic protein on the surface of membrane since the sum of TB- and LB- proteins for GO/PVDF membrane was 3.6 times lower than original PVDF membrane. Importantly, COD and NH3-N removal efficiency of MPBR with GO/PVDF membrane were kept about 95 and 98%, respectively, and average biomass productivity reached as high as 105 mg/L·d. As expected, the progress may highlight the promising potential of the combined MBR process with new materials for the efficient treatment and valuable re-utilization of high salinity refractory organic wastewater. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank for financial support from the National Natural Science Foundation of China (Grant Nos. 21736009 and 21506193), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY18B060010) and the Minjiang Scholarship from Fujian Provincial Government. A patent application related to this work has been filed. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.133953. References Abass, O.K., Fang, F., Zhuo, M., Zhang, K., 2018. Integrated interrogation of causes of membrane fouling in a pilot-scale anoxic-oxic membrane bioreactor treating oil refinery wastewater. Sci. Total Environ. 642, 77–89. Ahmad, A.L., Abdulkarim, A.A., Ooi, B.S., Ismail, S., 2013. Recent development in additives modifications of polyethersulfone membrane for flux enhancement. Chem. Eng. J. 223, 246–267. Ali, A., Tufa, R.A., Macedonio, F., Curcio, E., Drioli, E., 2018. Membrane technology in renewable-energy-driven desalination. Renew. Sust. Energ. Rev. 81, 1–21. Fan, Z., Qin, L., Zheng, W., Meng, Q., Shen, C., Zhang, G., 2018. Oscillating membrane photoreactor combined with salt-tolerated Chlorella pyrenoidosa for landfill leachates treatment. Bioresour. Technol. 269, 134–142. Fu, F.L., Xie, L.P., Tang, B., Wang, Q., Jiang, S.X., 2012. Application of a novel strategyadvanced Fenton-chemical precipitation to the treatment of strong stability chelated heavy metal containing wastewater. Chem. Eng. J. 189, 283–287. Hu, M., Zheng, S.X., Mi, B.X., 2016. Organic fouling of graphene oxide membranes and its implications for membrane fouling control in engineered osmosis. Environ. Sci. Technol. 50, 685–693. Hummers, W.S., Offeman, R.E., 1958. Preparation of graphitic oxide. J. Am. Chem. Soc. 80, 1339. Ji, J., Qiu, J.P., Wai, N., Wong, F.S., Li, Y.Z., 2010. Influence of organic and inorganic flocculants on physical–chemical properties of biomass and membrane-fouling rate. Water Res. 44, 1627–1635. Kargi, F., Dincer, A.R., 1996. Effect of salt concentration on biological treatment of saline wastewater by fed-batch operation. Enzym. Microb. Technol. 19, 529–537. Ko, K., Yu, Y., Kim, M.-J., Kweon, J., Chung, H., 2018. Improvement in fouling resistance of silver-graphene oxide coated polyvinylidene fluoride membrane prepared by pressurized filtration. Sep. Purif. Technol. 194, 161–169. Krzeminski, P., Tomei, M.C., Karaolia, P., Langenhoff, A., Almeida, C.M.R., Felis, E., Gritten, F., Andersen, H.R., Fernandes, T., Manaia, C.M., Rizzo, L., Fatta-Kassinos, D., 2019. Performance of secondary wastewater treatment methods for the removal of contaminants of emerging concern implicated in crop uptake and antibiotic resistance spread: a review. Sci. Total Environ. 648, 1052–1081. Li, P., Liu, L., Wu, J., Cheng, R., Shi, L., Zheng, X., Zhang, Z., 2019a. Identify driving forces of MBR applications in China. Sci. Total Environ. 647, 627–638. Li, X., Qin, L., Zhang, Y.F., Xu, Z.H., Tian, L., Guo, X.W., Zhang, G., 2019b. Self-assembly of Mn(II)-amidoximated PAN polymeric beads complex as reusable catalysts for

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