polyethersulfone ultrafiltration membrane: Application in MBR for dairy wastewater treatment

Journal of Water Process Engineering 7 (2015) 280–294

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Preparation and characterization of antifouling graphene oxide/polyethersulfone ultrafiltration membrane: Application in MBR for dairy wastewater treatment Sirus Zinadini a , Vahid Vatanpour b,∗ , Ali Akbar Zinatizadeh a , Masoud Rahimi c , Zahra Rahimi a , Mohsen Kian b a

Water and Wastewater Research Center (WWRC), Department of Applied Chemistry, Razi University, Kermanshah, Iran Faculty of Chemistry, Kharazmi University, Tehran, Iran c CFD Research Center, Department of Chemical Engineering, Razi University, Kermanshah, Iran b

a r t i c l e

i n f o

Article history: Received 22 March 2015 Received in revised form 21 July 2015 Accepted 26 July 2015 Keywords: Membrane bioreactor Mixed matrix membrane Graphene oxide Antifouling

a b s t r a c t This study was performed to investigate effect of different concentrations of graphene oxide (GO) nanoplates on fouling mitigation of polyethersulfone (PES) membranes applied in membrane bioreactor (MBR) to treatment milk processing wastewater. The GO was prepared from graphite and characterized by FTIR, SEM and XRD. The mixed matrix membranes were prepared in three concentrations of 13, 15 and 17 wt% of PES polymer. Static contact angle of the membranes were decreased significantly with addition of the GO nanosheets caused to increasing of pure water flux and MWCO. Cross sectional SEM images showed that the finger-like pores for all of the GO embedded membranes were slightly wider than that of the unfilled PES membrane. Ultrafiltration performance and fouling resistance of the membranes were tested by filtration of activated sludge. With addition of GO nanoplates, fouling resistance ratio (FRR) of the nanocomposite membranes was improved. AFM images and FRR results presented that a membrane with smoother surface has greater fouling resistance ability. Based on antifouling and water flux results, the PES/GO membrane with 15 wt% of PES and GO content of 0.5 wt%, was selected as an optimal membrane and tested in MBR system. The MBR showed an increased capacity for removal of organic matter, both in terms of COD and BOD5 of milk processing wastewater. With increasing of MLSS concentration, flux of the membrane was increased due to a decrease in soluble microbial products and extracellular polymeric substance from the bacterial cells in the lower food to microorganism ratio (F/M). © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Food industries are generally considered as the largest source of strong wastewater generation which is characterized by high biological oxygen demand (BOD5 ) and chemical oxygen demand (COD) [1]. Among these industries, the dairy sector is the most polluting in terms of volume (i.e., large water consumption) and characteristics of generated effluent [2]. Generally, the dairy wastewater produces from various sources due to large water consumption during processing and cleaning (sanitization, heating, cooling, and floor washing) and from the drying of dairy materials [3]. It generates about 1–10 L of effluents/L of processed milk [3] and comprises a high concentration of organic material such as proteins,

∗ Corresponding author. Fax: +98 26 34551023. E-mail address: [email protected] (V. Vatanpour). http://dx.doi.org/10.1016/j.jwpe.2015.07.005 2214-7144/© 2015 Elsevier Ltd. All rights reserved.

carbohydrates and lipids, high BOD5 and COD, and high concentrations of suspended solids, oil and grease along with milk solids, detergents, sanitizers, milk wastes and etc. [4]. The dairy wastewaters are generally treated using aerobic and anaerobic processes such as activated sludge (AS) processes, aerated lagoons, trickling filters, upflow anaerobic sludge blanket (UASB) reactor, sequencing batch reactor (SBR) and anaerobic filters [5–8]. However, each of these systems has its own disadvantages caused by either high energy requirement or strong operational difficulty [9]. Recently, because of significant advances of membrane technology in reliability and cost effectiveness, researchers have investigated the dairy industry effluent treatment through the membrane process [2,10]. There is a growing interest in combining the membrane processes with biological wastewater treatment because of reduction in water availability and increase in water treatment costs [4]. The main advantages of membrane bioreactors (MBRs) are the membrane’s capacity for complete

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 ω1 ω2 t A AS AFM BOD BSA Cf Cp COD dw DMAc EPS F/M FRR FTIR GO HRT IMBR Jp Jw,1 Jw,2 JMBR l M MBR MLSS MPW MWCO PAOs PEG PES PSf PVDF PVP PWF RAS Rir Rr Sa Sq Sz SBR SEM SMPs SND SR% SRT SS TN TP UF UASB XRD

Overall porosity Weight of the wet membrane Weight of the dry membrane Permeation time (h) Membrane effective area (m2 ) Activated sludge Atomic force microscopy Biological oxygen demand Bovine serum albumin Feed concentration Permeate concentration Chemical oxygen demand Water density (0.998 g/cm3 ) Dimethylacetamide Extracellular polymeric substance Food to microorganism ratio Flux recovery ratio Fourier transform infrared Graphene oxide Hydraulic retention time Immersed membrane bioreactor Flux of activated sludge solution (kg/m2 h) Water flux (kg/m2 h) Flux of washed membrane (kg/m2 h) Jet loop membrane bioreactor Membrane thickness (m) Weight of the permeate (kg) Membrane bioreactor Mixed liquor suspended solids Milk processing wastewater Molecular weight cut-off Phosphate accumulating microorganisms polyethylene glycol Polyethersulfone polysulfone Polyvinylidene fluoride Polyvinyl pyrrolidone Pure water flux Recycle activated sludge Irreversible fouling ratio Reversible fouling ratio Average roughness (nm) Root mean square of the Z data (nm) Mean difference between highest peaks and lowest valleys (nm) Sequencing batch reactor Scanning electron microscopy Soluble microbial products Simultaneous nitrification and denitrification Percentage of solute rejection Solids retention time Suspended solid Total nitrogen Total phosphorus Ultrafiltration Upflow anaerobic sludge blanket X-ray diffraction

retention of all microorganisms and viruses, higher biodegradation efficiency, smaller footprint, better quality of treated water, higher solids retention time (SRT), independence of hydraulic retention time (HRT), and easy control [11]. MBRs also present better removal efficiency of micropollutants, recalcitrant organic pollutants and

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slowly biodegradable pollutants due to the absolute rejection of these compounds by the membrane [4] and the high biomass concentration. However, membrane fouling, which raises operating costs related to membrane material/replacement, performance maintenance and also diminishes the membrane module life time, is still a major problem that restricts the wide application of MBR [12,13]. Complicated interactions between membrane surface and various components exist in activated sludge mixed liquor lead to biofouling of the membrane. This type of fouling is often considered irreversible and is very difficult to detach from the membrane surface due to the self-replicating nature of microbes [14]. Therefore, many efforts have been done in order to prevent fouling happening such as optimization of membrane characteristics, adjustment of operating conditions, and modification of biomass characteristics [12]. Among these methods, modification of membrane materials (or blending modification) and surface modification can significantly alleviate the membrane fouling [15]. Blending modification with inorganic materials, especially nanoparticles, has been widely studied due to convenient operation and mild conditions as well as one step membrane preparation and hydrophilic modification process. The most blended inorganic nanoparticles are TiO2 [16,17], ZrO2 [18], Al2 O3 [19], ZnO [20] and SiO2 [21], which can significantly improve the hydrophilic property and reduce biofouling. Graphene oxide (GO) with oxygen-containing groups (e.g., hydroxyl, carboxyl, carbonyl, and epoxy groups) attached on its edge and basal plane is a highly dispersible derivative and strongly hydrophilic [15,22]. GO nanosheets due to the extraordinary properties, such as strong hydrophilicity, superior chemical stability, innocuity, and high surface area can be used as an additive in preparation of polymeric membranes. As well as, their functional groups would ensure a large negative zeta potential, which it can impede biofouling process as a result of bio-foulants attachment and their accumulation on the membrane surface [23,24]. Ionita et al. [25] have reported that the GO nanosheets could improve the mechanical properties of the prepared polysulfone (PSf) membranes. Zhao et al. [26] found that GO blending into polymer matrix resulted in enhanced hydrophilicity, water flux, and antifouling property of the polyvinylidene fluoride (PVDF) membrane. Xu et al. [27] reported preparation of functionalized GO (f-GO) dispersed PVDF mixed matrix membranes via the phase inversion induced by immersion precipitation technique. The PVDF/f-GO membranes exhibited superior hydrophilicity, water and BSA flux, and rejection rather than nascent PVDF and PVDF/GO membranes. Graphene derivatives had the high application in the preparation of different membranes [28–32]. There are two articles published in scientific journals about application of this type of membranes in MBR in order to the reduction of biofouling. Zhao et al. [15] described fabrication of PVDF/GO microfiltration membrane in order to use in the MBR. The PVDF/GO composite membrane demonstrated a sustained permeability, lower cleaning frequency, and filtration time that there were three times longer than that of the unfilled PVDF membrane. In terms of anti-extracellular polymeric substance (EPS) accumulation, the PVDF/GO composite membrane showed lower membrane resistance, particularly, lower pore plugging resistance than the PVDF membrane. Lee et al. [33] demonstrated that the nanosheets of graphene oxide embedded in the preparation of PSf membranes suppressed the fouling to such an extent that a fivefold lengthening is achieved of the time between chemical cleanings. In this work, the GO modified polyethersulfone (PES) membranes were prepared via the phase inversion induced immersion precipitation process, and successfully used in MBR system. The characterization of the prepared GO nanoplates was determined by FTIR, SEM and XRD. Also, the membranes structures and properties were characterized using SEM, AFM and water contact angle measurements. Ultrafiltration performance and fouling resistance of the prepared mixed matrix membranes were tested by filtration

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Table 1 The compositions of casting solutions. Membrane type

PES (wt%)

PVP (wt%)

GO nanoplate (wt%)

13 (0) 13 (0.5) 13 (1) 15 (0) 15 (0.5) 15 (1) 17 (0) 17 (0.5) 17 (1)

13 13 13 15 15 15 17 17 17

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

– 0.5 1.0 – 0.5 1.0 – 0.5 1.0

of activated sludge. Finally, the optimal membrane with the high antifouling properties was applied in the MBR system and its performance was examined in term of the process and quality parameters. 2. Materials and methods 2.1. Materials Polyethersulfone (Ultrason E 6020P) as a polymer and N,Ndimethylacetamide (DMAc) as a solvent were purchased from BASF Co., Germany. Polyvinylpyrrolidone (PVP) with a molecular weight of 25,000 g/mol was purchased from Merck. Extra pure fine graphite with a particle size less than 50 ␮m was obtained from Merck Co. and used to manufacture graphene oxide. Potassium permanganate (KMnO4 ) and sulfuric acid (H2 SO4 ) (98 wt%) were supplied from Merck and used to oxidize graphite to graphene oxide for exfoliation. Distilled water was used throughout this study. 2.2. Synthesis of GO nanosheets The graphene oxide was prepared from natural graphite according to the Hummers method as reported in the literature [34,35]. Graphite powder (5 g) was added to the concentrated H2 SO4 in an ice-bath. Then, 7 g of potassium permanganate was gradually added while maintaining the temperature below 20 ◦ C. The mixture was stirred at 35 ◦ C for 30 min and slowly added into 250 mL of the deionized water, followed by stirring the mixture at 98 ◦ C for 15 min in order to improve the oxidation degree of graphite oxide. The treatment of graphene was finished by adding 750 mL of H2 O2 (2 wt%) with stirring at 10 ◦ C. The mixture was washed with deionized water and centrifuged several times to clean out the remained salt until a neutral pH was reached. Finally, the GO slurry was sonicated for 1 h followed by filtering and drying in a vacuum oven at 40 ◦ C for 24 h. 2.3. Preparation of membranes The asymmetric flat sheet PES membranes blended with graphene oxide nanoplates were fabricated via the phase inversion induced by immersion precipitation method. The components of casting solutions were PES, PVP (1 wt%) and proper amounts of GO nanoplates in DMAc as solvent. The compositions of casting solutions for all membranes are listed in Table 1. The measured amounts of GO nanoplates were dispersed into DMAc and sonicated for 30 min to prepare homogenous solutions using DT 102H Bandelin ultrasonic (Germany). After dispersing nanoplates in the solvent, PES was dissolved in the dope solution by continuous stirring for 24 h. A sonication-assisted method was again used for 10 min to remove air bubbles. Afterward, the dope solutions were scattered on glass plates and casted using a casting knife with 200 ␮m thickness. The glass plates were immediately dipped in a non-solvent bath (distilled water at 25 ◦ C) without any

evaporation time. After primary phase separation and membrane formation, the membranes were stored in fresh distilled water for 24 h to guarantee the complete phase separation. Finally, the membranes were sandwiched between two sheets of filter papers in order to dry for 24 h at room temperature. 2.4. Characterization of GO nanoplates and membranes The measurements of the FTIR spectra of the GO nanoplates were performed using Bruker spectrometer (TENSOR 27). XRD analysis was carried out on graphene oxide by a Siemens X-ray diffraction D5000 diffractometer (Munich, Germany), in the scanning range of 2 between 4◦ and 70◦ using Cu K␣ as a source of radiation with the X-ray generator operating at accelerating voltage of 40 kV and emission current of 30 mA. Membranes structure and morphology, and GO nanoplates structure were observed using Philips-X130 and Cambridge scanning electron microscopes (SEM). The cross-sections morphology was prepared by cutting the membranes into small species. Before the membranes being observed using SEM, the membrane pieces were immersed in nitrogen liquid for 60–90 s and then were frozen. The frozen pieces of the membranes were broken and kept in air for drying. The dried samples were sputtered with gold to create electrical conductivity. After sputtering with gold, photomicrographs of the membranes were taken in very high vacuum conditions at 26 kV. The molecular weight cut off (MWCO) of the membranes was determined by detecting an inert molecule with lowest molecular weight that has a solute rejection of 80–90% in steady-state UF experiments. The MWCO of the unfilled PES and PES/GO blend membranes were calculated by ultrafiltration of polyethylene glycol (PEG) with different molecular weights. A series of different molecular weight PEGs such as 15, 20, 35 and 100 kDa were employed for estimation of MWCO and solute rejection studies. The PEG solutions were prepared individually at a concentration of 500 mg/L using deionized water and utilized as a standard for the rejection studies. The UF cell was filled with PEG solutions and pressurized at a constant pressure of 3 bar throughout the experiments. During ultrafiltration, the permeate solutions of corresponding membranes were collected in a graduated tube and were analyzed for the PEG concentration using modified Dragendorff reagent method [36]. The percentage of solute rejection (SR%) was calculated by the following equation: SR(%) =

C − C  p f Cf

× 100

(1)

where Cf and Cp are the concentration of the feed and the concentrate of the permeate, respectively. The surface wetting characteristics (hydrophilicity) of the membranes was characterized by contact angle measurement on the flat sheet membranes surface. The contact angle measurement was conducted using a contact angle goniometer (G10, KRUSS, Germany) equipped with video capture at 25 ◦ C and a relative humidity of 50%. The measurements were carried out using 2 ␮L of deionized water as a probe liquid in all measurements and average value of at least five random locations was obtained for each sample to minimize the experimental errors. The roughness and surface morphology of the prepared membranes were evaluated by atomic force microscopy (AFM). The AFM device was Nanosurf® Mobile S scanning probe-optical microscope (Switzerland) equipped with Nanosurf® MobileS software (version 1.8). The membrane samples (approximately 1 cm2 ) were fixed on a specimen holder and 5 ␮m × 5 ␮m areas were scanned in the tapping mode in air. The surface roughness parameters were reflected in terms of the average roughness (Sa ), the root mean square of the

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Z data (Sq ) and the mean difference between the highest peaks and lowest valleys (Sz ). The overall porosity () was determined by gravimetric method, as defined in the following equation [37]: ω − ω2 = 1 A × l × dw

(2)

where ω1 is the weight of the wet membrane; ω2 is the weight of the dry membrane; dw is the water density (0.998 g/cm3 ) and l is the membrane thickness (m). The thickness of membranes was measured by a digital micrometer (Mitotoyo, Japan). 2.5. Permeate flux measurements and permeability The performance of GO nanoplates blended ultrafiltration membranes was characterized by measuring the pure water flux (PWF) and activated sludge fouling tests. A dead-end stirred cell system was used to study the membrane filtration performance. The system consisted of a membrane with surface area of 12.56 cm2 and a filtration cell with a total volume of 200 mL. The membrane was left on rigid sponge and placed in the cell and then the cell fitted with a pressure gauge. Pressurized nitrogen gas was employed to force the liquid through the membrane. The feed solution was stirred at a rate of 400 rpm. In initial, each membrane hydraulically compacted at a pressure of 0.4 MPa with distilled water for about 30 min to avoid the compaction effect of the membrane. Then, the pressure was reduced to the operating pressure of 0.3 MPa. The pure water flux Jw,1 (kg/m2 h) was calculated using the following equation: Jw,1 =

M At

(3)

where M is the weight of the permeate sample (kg), A is the membrane effective area (m2 ) and t is the permeation time (h). The experiments were carried out at 20 ± 1 ◦ C and average of three replicates was reported. 2.6. Antibiofouling performance In this study, a biological suspension was used in order to investigate antibiofouling performance of the prepared nanocomposite membranes. Some activated sludge was cultivated in our laboratory and was fed with milk processing wastewater (MPW) for more than 3 months. The MPW composition is given in Table 2. After water flux tests, the biological suspensions with mixed liquor suspended solids (MLSS) of 1000 mg/L (as foulant solutions) were

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Table 2 Milk processing wastewater characteristics. Parameter

Unit

Range

Average

TCOD BOD5 TN TP TKN N-NO3 − pH

mg/L mg/L mg/L mg/L mg/L mg/L –

1906–2513 1372–1809 245–297 55.3–72.9 218–241 21.7–47.7 5–6

2131 1535 273 60.2 233 38.1 5.5

quickly replaced in the stirred cell. The flux for activated sludge, Jp (kg/m2 h), was measured based on the water quantity permeated through the membranes at 0.3 MPa for 90 min. After filtration of activated sludge, the fouled membranes were washed with distilled water for 15 min. Then, the water flux of washed membranes, Jw,2 (kg/m2 h) was measured again. The flux recovery ratio (FRR) can be described as the following equation:



FRR =

jw,2 jw,1



× 100

(4)

Generally, higher FRR indicated better antifouling property of the membranes. In order to analyze the fouling process in details, several ratios were defined to describe the fouling resistance of the prepared membrane. Reversible fouling ratio (Rr ) and irreversible fouling ratio (Rir ) were calculated using the following equations [38]:



Rr (%) =

Rir (%) = (

jw,2 − jp jw,1



× 100

jw,1 − jw,2 ) × 100 = Rt − Rr jw,1

(6)

2.7. MBR configuration and performance A laboratory-scale membrane bioreactor (MBR) was used in this study. The schematic diagram of the experimental setup as shown in Fig. 1. The system consisted of an aeration tank and a settling tank with total volume of 14.74 and 6.45 L, respectively. The working volume of aeration tank was determined 5.46 L and completely mixed. Air was supplied through the air diffusers (with rate of 2.5–5 L/min) placed at the bottom of the reactor to: (i) allow complete sludge mixing in the reactor, (ii) supply adequate dissolved oxygen for biological processes.

Fig. 1. Schematic of the MBR system.

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Fig. 2. The FTIR spectrum of synthesized graphene oxide.

Fig. 3. XRD patterns for the GO nanoplates.

The dissolved oxygen (DO) concentration in the bioreactor was 3–4 mg/L. A sedimentation tank was placed after the aeration tank to provide the required biomass concentration in the system that a recycle activated sludge (RAS) pump (Sisdoz-PRS6), controlled by

a timer, and periodically returned the sludge from settling chamber bottom to the aeration tank. Adjustable speed peristaltic pump (PD5201, Heidolph, Germany) with analogue tuner was used to feed. Milk processing wastewater (MPW) was used as a substrate

Fig. 4. SEM images of the prepared graphene oxide.

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285

Fig. 5. The FTIR spectrum of 1 wt% graphene oxide embedded polyethersulfone membrane.

in the MBR process with COD:N:P ratio of 100:15:3. The characteristics of MPW are summarized in Table 2. The seeding source of the reactor was taken from a working wastewater treatment plant (Bisotoon, Kermanshah, Iran) and fed with MPW wastewater for 3 months and there was no sludge discharge and the system operated as wise batch. The operating conditions were: HRT of 6 h and MLSS of 6000, 10,000 and 14,000 mg/L (the defined values were based on existing literature and previous tests as explained elsewhere [4,39]). Variations within ±5% of effluent COD concentration at each condition were considered as the criterion for steady state conditions. The permeates were drawn from the membrane and measured by an electronic balance (BP2215, Sartorius, Germany)

for reporting the membrane flux and then were collected in the vessel in order to determination of the process parameters (BOD5 , total nitrogen (TN), total phosphorus (TP) and turbidity). The permeation flux was determined from the amount of permeate collected through the membranes per unit time using Eq. (3). All processing analyses were done according to the recommendations of the standard methods for the examination of water and wastewater [40]. The pH adjustment of the bioreactor was not necessary as it remained relatively constant and suitable for microorganism’s growth (pH 7–8) throughout the experiments. The dissolved oxygen concentration in the wastewater was determined using a DO probe. The DO meter was supplied by WTW DO Cell OX 330, elec-

Fig. 6. Pure water flux results for the prepared membranes after 90 of test at 0.3 MPa.

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tro DO probe, Germany. The pH meter model HANNA-pH 211 was used to measure the pH. Turbidity was measured by a turbiditmeter model 2100p (Hach Co.). 3. Results and discussion 3.1. Characterization of graphene oxide nanosheets For characterization of the graphene oxide functional groups, the Fourier transform infrared (FTIR) test was done. Fig. 2 shows FTIR spectra of GO, the band at 1720 cm−1 is attributed to the C O stretching vibration of the COOH group. While two bands at 1078 cm−1 and 1234 cm−1 originated from the C O stretching vibrations of alkoxy [41]. This means that the carboxylic acid groups were formed on the surface of the graphene. The peaks at 3420 and 1390 cm−1 corresponded to the stretching vibration and deformation vibration of O H [29]. The peak at 1627 cm−1 is assigned to the vibrations of the adsorbed water molecules and the contributions from the vibration of aromatic due to C C stretching as part of the phenol ring in the GO skeleton. However, IR spectrum of graphite is featureless [29], it can be concluded that graphene oxide is effectively functionalized. Fig. 3 shows the XRD spectra of GO nanoplates. The peak at 10–20◦ attributed to formation of GO nanoplates. The characteristic reflection of GO was the peak centered at around 12◦ and appointed to (0 0 2) interplanar spacing and two weak peaks at about 26 and 43◦ [42]. It revealed that the plates of graphite away from and laminated, consequently graphene oxide are formed. Fig. 4 displays the SEM images of the synthesized graphene oxide nanoplates. The image of GO shows the wrinkled surface of graphene sheet with a wormlike structure randomly aggregated. This confirms that the GO sheet had been successfully exfoliated from graphite containing ordered stacking graphene layers. 3.2. Characterization of graphene oxide embedded PES membranes 3.2.1. FTIR of the GO/PES membrane Fig. 5 shows the surface FTIR spectra of the 1 wt% GO embedded PES membrane. The peaks at 1242 and 1142 cm−1 can be attributed to the stretching vibrations of asymmetric and symmetric S O, respectively [38]. The strong absorptions in 1480–1600 cm−1 region are associated with the benzene ring skeletal stretching mode. The peaks at 1715 and 3409 cm−1 , which observed in the GO FTIR, were also appeared in the FTIR of GO/PES membrane. Appearance of these peaks confirms the existence of GO on the surface of the nanocomposite membranes. 3.2.2. Pure water flux and hydrophilicity Fig. 6 indicates the pure water flux results for all the fabricated membranes. When the polymer concentration increased from 13 to 17 wt%, the PWF of the membranes decreased. It’s due to the increase in polymer concentration leading to increase of solution viscosities [43], which this reduces the porosity (Table 3) and MWCO of the membranes and causes a decrease in the PWF of the membranes. These results suggest that increasing polymer concentration form a denser and thicker skin layer, resulting in higher pressure resistance, but less productive asymmetric ultrafiltration membranes for liquid separation [43]. Thus, the separation capability of the membrane will increase. The membranes with 17 wt% polymer concentration have a dense and porous structure as well as thick skin, as shown in SEM images (Fig. 7). Asymmetric membranes from the dilute polymer solution (13 wt%) produce a thin and porous skin layer, leading to a high flux. Addition of

Fig. 7. Cross-section SEM images of the prepared membranes.

S. Zinadini et al. / Journal of Water Process Engineering 7 (2015) 280–294 Table 3 Porosity and MWCO of the nanocomposite membranes. Membranes

Porosity (%)

MWCO

13 (0%) 13 (0.5%) 13 (1%) 15 (0%) 15 (0.5%) 15 (1%) 17 (0%) 17 (0.5%) 17 (1%)

79.5 80.0 80.3 76.2 77.7 80.2 75.5 77.2 84.4

± ± ± ± ± ± ± ± ±

95000 70000 75000 31000 37000 38000 28000 30000 32000

4.6 3.7 2.8 2.6 4.9 2.1 1.9 4.2 2.5

GO nanosheets to the polymer casting solution increases the pure water flux of the nanocomposite membranes. The cross-sectional SEM images of the prepared membranes with different polymer concentrations and graphene oxide contents are exhibited in Fig. 7 with two resolutions. As can be seen from the SEM images, all the membranes exhibit a dense skin layer and a porous sub-layer with finger-like pores. With increasing of polymer concentration from 13 to 17 wt%, finger-like structure becomes narrow. After being blended with nanoparticles, the membranes show slight structure changes. The finger-like pores for all of the GO embedded membranes are slightly wider than that of the unfilled PES membranes. It is well proven that improving of hydrophilicity has an influence on the pure water flux. The hydrophilicity of the prepared membrane surface can be investigated by water contact angle measurements, which can reflect the natural wettability of the material. Lower contact angle shows that the membrane surface is more hydrophilic in nature. As shown in Fig. 8, the static contact angle decreased significantly with the addition of the graphene oxide nanosheets into the polymer matrix. These results suggest that during phase inversion process in water, the GO nanosheets with their hydrophilic nature migrate to the membrane surface causing to higher hydrophilicity of the membrane surface [41]. This behavior also was observed for the GO/PES blended membranes in this study. However, adding of the 1 wt% GO did not result significant decrease in the contact angle; this is probably due to the agglomeration of the nanoplates on the membrane surface. As becomes apparent from the Fig. 6, 0.5 wt% GO/PES membranes had the highest pure water flux for 13, 15 and 17 wt% membranes, although the hydrophilicity was improved for 1 wt% membranes. This might be due to either the reduced pore radius or blockage of some pores of the membrane induced from the agglomeration of the nanoplates [44]. Similar results were also reported for the boehmite nanoparticles in polyethersulfone matrix [45]. Table 3 presents the porosity and MWCO of the prepared mixed matrix membranes. The porosity of the unfilled PES membranes is reduced with increasing of polymer concentrations. However, in the presence of hydrophilic GO nanosheets in polymer matrix, reduction of porosity is diminished due to interaction among GO, PES and DMAc in the phase separation process. To measure the molecular weight cut-off of the membranes, different molecular weights of polyethylene glycol are used. As presented in Table 3, the membranes with 13 wt% PES concentration have highest MWCO about 80,000 Da. With increasing of polymer concentration, the MWCO is decreased. The addition of GO nanosheets in 13 wt% polymer solution is reduced the MWCO. However, in 15 wt% and 17 wt% membranes, GO addition increase the MWCO. Addition of the GO nanosheets to polymer solution can be two effects. One is increasing of polymer solution viscosity and another is introducing of functional groups to the membrane surface. Probably in 13 wt%, viscosity increasing is dominant effect and reduces the pore size and MWCO. Nevertheless, in 15 wt% and 17 wt% membranes, due

287

Table 4 Surface roughness parameters of the mixed matrix PES membranes with different concentrations of polymer and GO nanoplates resulted from analyzing three randomly chosen AFM images. Membranes

13 (0%) 13 (0.5%) 13 (1%) 15 (0%) 15 (0.5%) 15 (1%) 17 (0%) 17 (0.5%) 17 (1%)

Roughness parameters Sa (nm)

Sz (nm)

Sq (nm)

24.2 ± 2.1 12.9 ± 2.2 20.2 ± 3.1 16.4 ± 3.4 12.6 ± 2.9 14.9 ± 2.8 15.7 ± 3.2 12.9 ± 1.9 13.1 ± 1.3

277.5 ± 24.4 146.1 ± 32.1 313.0 ± 18.9 194.5 ± 35.9 131.0 ± 18.8 145.3 ± 33.9 209.7 ± 34.5 100.5 ± 25.7 117.5 ± 13.2

31.4 ± 6.5 17.3 ± 3.3 32.5 ± 4.3 23.8 ± 4.5 16.3 ± 2.1 19.3 ± 3.8 22.8 ± 3.6 16.1 ± 2.7 15.6 ± 2.4

to presence of functional groups, interactions among GO, PES and DMAc causes to increasing water flux and MWCO.

3.2.3. Antifouling performance The membrane fouling severely affected the performance of membrane filtration, in terms of fluid separations and usage life time. The membrane flux is reduced with the adsorption and deposition of foulants on the membrane surface and entrapment of fouling agents in the membrane pores as well as the concentration polarization. Therefore, the antibiofouling performance of the unfilled PES and modified GO/PES ultrafiltration membranes was characterized by means of measuring water flux recovery ratio (FRR) after the membrane was fouled by 1000 mg/L suspension activated sludge as a foulant. The FRR results of the prepared membranes are shown in Fig. 9. With addition of GO nanoplates, the FRR is improved. In the best case, related to 0.5 wt% GO membranes with polymer percentage of 13, 15 and 17 wt% PES, the flux recovery ratios of the membranes are 90, 92.8 and 94%, respectively. The trend of change in FRR is consistent with the membrane surface roughness observed by AFM images are presented in Fig. 10. The roughness parameters of the surfaces of the GO/PES blended membranes are given in Table 4, which are calculated in an AFM scanning area of 5 ␮m × 5 ␮m. The surface roughness of the unfilled PES membranes is obviously higher than that of the GO mixed membranes. It is well recognized that a membrane with smoother surfaces has greater fouling resistance ability [46]. The roughness parameters of the prepared membranes are decreased with increase of polymer concentration; this may be due to the slow exchange of solvent and non-solvent occurring during the phase inversion process because of the high viscosity. After blending of graphene oxide into polymer matrix, the roughness parameters are decreased leading to a smooth membrane surface and increase in flux recovery ratio. The AFM images show that when the GO content increases from 0.5 wt%, the surface roughness also increases. This may be due to accumulation of the hydrophilic GO nanoplates on the membrane surface in high amounts of the nanoplate [46]. This indicate although the presence of the GO nanoplate on the surface of the modified membranes significantly improve hydrophilicity of the membrane surface (Fig. 8), and decrease the interaction between the microorganisms and the membrane surface, the smoother surface has more significant effect rather than the hydrophilicity in the fouling characteristic. The high roughness of 1 wt% GO membranes causes to decline in flux recovery ratio. Because of foulants can adsorb in the valleys of membrane with coarser surfaces and result in clogging of the valleys and fouling [47]. The observed FRR is comparable with GO nanoplate employed for the preparation of polyethersulfone nanofiltration membranes [41].

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Fig. 8. Static contact angle of the prepared membranes.

Membrane fouling is always consists of reversible and irreversible fouling. Reversible fouling that could be removed by simple hydraulic cleaning such as backwashing is brought by loose fouling agent’s adsorption, while irreversible fouling is caused by tight adsorption of foulants on the surface or entrapment of foulant in pores [48]. To more understand about the fouling phenomenon, resistance parameters such as reversible fouling resistance (Rr ) and irreversible fouling resistance (Rir ) were measured for the fabricated mixed matrix membranes and results are presented in Fig. 11. The results proved that the resistance factor in the modified membranes was reduced whereas the flux recovery ratio of this membrane wa increased. The results are presented in Fig. 11 revealed that the irreversible fouling resistance of the UF membranes prepared with the embedded GO nanoplates was lower in comparison with that of the unfilled PES membranes. Also, the Rir of the UF membranes prepared with the high polymer

concentration (15 and 17 wt%) was lower compared with the low polymer concentration (13 wt%). The unfilled PES membrane (13 wt%) had highest irreversible fouling resistance (30%) due to the lower surface hydrophilicity and the higher surface roughness. These results demonstrated that the blended membranes showed outstanding antifouling properties. In summary, the FRR, reversible resistances, and irreversible resistances of GO embedded membranes were improved i.e., the surface properties of the membrane were modified. 3.3. MBR performance The PES/GO membrane with 15 wt% of PES and GO content of 0.5 wt%, which selected as optimal membrane were tested to ascertain the ability of GO to mitigate membrane biofouling and investigation of processes and quality parameters in MBR system.

Fig. 9. Water flux recovery ratio of the GO nanoplates blended PES membranes with the different concentrations of GO and polymer after activated sludge fouling (average of three replicates was reported).

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The qualities of the permeate and the respective removal of the MBR system with different mixed liquor suspended solids (MLSS) are shown in Table 5. It can be observed that the MBR showed an increased capacity for the removal of organic matter, both in terms of COD and BOD5 , which may be attributed to the elevated biodegradability of the wastewater and high concentration of biomass in the reactor [39]. The reduction of the concentration of organic matter and the increase in removal efficiency with the increase of MLSS concentration is related to the reduction of the food to microorganism ratio (F/M). Removal of nutrients is also noted with increase of MLSS, this may be the following reasons. Initially, once the reactor was fully aerated and had no anoxic zones, significant removal of total nitrogen, which exhibits denitri-

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fication process, was not expected. Nevertheless, this phenomenon might have occurred due to reduced efficiency of oxygen transfer provided by incomplete air distributions in a high concentration of biomass and high viscosity of the medium. Therefore, the inner regions of the biological flocs probably did not receive oxygen and transformed to anoxic zones, thus providing favorable conditions for denitrification [49,50]. In such cases, nitrification occurs on the surface of the flocs, whereas denitrification occurs in the internal layers of the flocs due to a dissolved oxygen gradient, therefore simultaneous nitrification and denitrification (SND) take place in the reactor even during aeration [51,52]. Due to sludge concentration was high; part of the total nitrogen removal may be result from a higher nutrient uptake.

Fig. 10. AFM images of the mixed matrix PES membranes with different concentrations of polymer and GO nanoplates.

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Fig. 10. AFM images of the mixed matrix PES membranes with different concentrations of polymer and GO nanoplates.

There were also phosphorus removals of 38.5, 58.4 and 76.1% for MLSS of 6000, 10,000 and 14,000 mg/L, respectively. Conventionally, systems that are projected to remove phosphorus should have aerobic and anaerobic reactors in series in order to the selection and growth of phosphate accumulating microorgan-

isms (PAOs) [53]. However, MBRs present a potentially suitable environment for PAOs proliferation because the high biomass concentrations in MBRs might lead to microzone of anoxic or anaerobic within the sludge flocs [54]. In the case of conventional biological treatment systems, partial removal of phosphorus

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291

Fig. 11. Fouling resistance ratio of GO blended PES membranes.

Table 5 Values of the main physicochemical parameters of permeate and removal efficiencies of the MBR process. Parameters

COD BOD5 TN TP

MLSS 6000 mg/L

MLSS 10,000 mg/L

MLSS 14,000 mg/L

Permeate (mg/L)

Removal (%)

Permeate (mg/L)

Removal (%)

Permeate (mg/L)

Removal (%)

182.3 131.2 185.4 37.1

91.4 91.5 28.6 38.5

112.3 80.8 111.9 23.0

94.7 94.5 55.0 58.4

98.0 70.6 81.6 13.4

95.4 95.1 66.8 76.1

occurs through its assimilation by the biomass for cellular synthesis [4,39]. Also, it is possible that a part of phosphorous removal found in the existing study was related to biological assimilation by PAOs and other microorganisms [39]. The results obtained in this study were similar to the scientific literature as presented in Table 6. However, the development of membrane bioreactors has been restricted by problems of membrane fouling during filtration of the activated sludge. Fouling of the membrane decreases the filtration fluxes and thus the treated water flow, and increases the operating costs. In this research, the ability of GO imbedded membrane to mitigate biofouling, or antifouling capability, in the different MLSSs was examined. The modified hydrophilic composite membrane with GO nanosheets showed the lower thickness of biofilm, which is formed by the microorganism, and better antifouling property than the no GO membranes [32,33]. Fig. 12 shows SEM images from fouled surface of the membranes after MBR filtration. As shown, the membranes containing GO nanosheets have less biofilm accumulation on the surface. It is reported that most microorganisms and microbial products in aquatic system such as extracellular

polymeric substance, a major fouling component, have negatively charged surface or characteristics [55]. These microbial products are well known to accelerate deposition of microorganism on the membrane surface in the initial stage of MBR operation [56]. The GO embedded membranes exhibited negative zeta potential which induces electrostatic repulsion between the microorganism and the membrane surface, thus hindering the surface attachment of the microorganism [33]. As shown in Fig. 13, with increasing of MLSS concentration the flux was increased, which might be related to a decrease in soluble microbial products (SMPs) and extracellular polymeric substance from the bacterial cells in the lower F/M ratios [57]. Zhao et al. [32] proves that fouling in PVDF/GO membrane used in MBR was reduced and less EPS, specifically polysaccharides, was adsorbed on PVDF/GO membrane compared with that of the PVDF membrane without GO. Effluent turbidity of composite membrane was under 1 NTU during the test and the suspended solids (SS) concentrations in the effluent were nearly zero (data not shown).

Table 6 The MBR results of other researchers in treatment of dairy wastewater. Type of reactor

Type of wastewater

HRT, h

MLSS, mg/L

BOD removal, %

COD removal, %

TP removal, %

TN removal, %

Ref.

MBR

Dairy Dairy Dairy Cheese whey Dairy Combined domestic and dairy wastewater

17251 (MLVSS) 22371 (MLVSS) 19500 (MLVSS) 2312–38684 5800–14000 – –

100 99 99.5 – – – 47.6

99 99 97.9 96–99 97 98 50.4

53 86 89 – – – 䊐10

98 89 86.1 – – – –

[4]

MBR JLMBR JLMBR MBR IMBR

8 6 8 1.9–7.7 0.82–2.8 days – –

JMBR: jet loop membrane bioreactor, IMBR: immersed membrane bioreactor.

[39] [11] [58] [9] [59]

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Fig. 12. Surface SEM images of the fouled membranes after MBR filtration.

used in membrane bioreactor for treatment of milk processing wastewater. From obtained results can be concluded that:

Fig. 13. The changes of the flux with MLSS concentration in 3 bar during 2 h filtration of suspension activated sludge (15 (0.5) membrane was used).

4. Conclusions The mixed matrix polyethersulfone membranes were prepared by blending various amounts of graphene oxide nanoplates using the phase inversion method with purpose of fouling reduction and

1 Increasing hydrophilicity (decreasing contact angle) by addition of GO nanosheets increased pure water flux of the prepared membranes. 2 Presence of GO nanosheets in casting solution caused to wider finger-like pores in cross sectional structure of the GO embedded membranes related to the unfilled PES membranes. 3 By addition of GO nanoplates, fouling resistance ratios of the membranes (tested by filtration of activated sludge) were improved. 4 AFM images and FRR results proved that a membrane with smoother surfaces has greater fouling resistance ability. 5 PES/GO membrane with 15 wt% of PES and GO content of 0.5 wt% showed the best performances and selected as optimal membrane for testing in MBR system. 6 The designed MBR process showed good performance for treatment of milk processing wastewater.

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