Woon Chan Chong*,Abdul Wahab Mohammad,Ebrahim Mahmoudi,Ying Tao Chung,Kamrul Fakir Kamarudin,Mohd Sobri Takriff

1Chemical Engineering Programme,Faculty of Engineering and Built Environment,Universiti Kebangsaan Malaysia,43600 Bangi,Selangor,Malaysia

2 Department of Chemical Engineering,Lee Kong Chian Faculty of Engineering and Science,Universiti Tunku Abdul Rahman,Jalan Sungai Long,Bandar Sungai Long,Cheras,43000 Kajang,Selangor,Malaysia

3 Research Center for Sustainable Process Technology(CESPRO),Faculty of Engineering and Built Environment,Universiti Kebangsaan Malaysia,43600 Bangi,Selangor,Malaysia

4 Department of Chemical Engineering and Petroleum Engineering,Faculty of Engineering,Technology and Built Environment,UCSI University,No.1,Jalan Menara Gading,UCSI Heights,Cheras,56000 Kuala Lumpur,Malaysia

Keywords:Algal-membrane photoreactor Nanohybrid membrane Wastewater polishing Microalgal cultivation Nutrient recovery

ABSTRACT Microalgae cultivation has gained tremendous attention in recent years due to its great potential in green biofuel production and wastewater treatment application.Membrane technology is a great solution in separating the microalgae biomass while producing high quality of permeate for recycling.The main objective of this study was to investigate the filtration performance of Ag/GO-PVDF(silver/graphene oxide-polyvinylidene fluoride)membrane in an algalmembrane photoreactor(A-MPR)by benchmarking with a commercial PVDF (com-PVDF)membrane.In this study,Chlorella vulgaris microalgae was cultivated in synthetic wastewater in an A-MPR for ammoniacal-nitrogen and phosphorus recovery and the wastewater was further filtered using Ag/GO-PVDF and com-PVDF membranes to obtain high quality water.Spectrophotometer was used to analyze the chemical oxidation demand(COD),ammoniacal nitrogen(NH3-N)and phosphate(PO43?).The concentration of proteins and carbohydrates was measured using Bradford method and phenol-sulfuric acid method,respectively.The COD of the synthetic wastewater was reduced from(180.5±5.6)ppm to(82±2.6)ppm due to nutrient uptake by microalgae.Then,the Ag/GO-PVDF membrane was used to further purify the microalgae cultivated wastewater,resulting in a low COD permeate of(31± 4.6)ppm.The high removal rate of proteins(100%)and carbohydrates (86.6%)as the major foulant in microalgae filtration,with low membrane fouling propensity of Ag/GO-PVDF membrane is advantageous for the sustainable development of the microalgae production.Hence,the integrated A-MPR system is highly recommended as a promising approach for microalgae cultivation and wastewater polishing treatment.

1.Introduction

Nowadays,various advance wastewater treatment methods have been applied in wastewater treatment plants to meet stringent regulations.Tertiary and quaternary treatments are implemented in wastewater polishing for further recycle and reuse purpose as well as nutrient recovery from the wastewater.Municipal wastewater has been identified to contain plenty of nutrients such as nitrogen and phosphorus which are essential resources in agricultural sector for plant growth[1,2].It is a known fact that the content of nitrogen and phosphorus in the soil is limited.Majority of the nutrients were lost in the water or air pathway instead of accumulating in the crops.Therefore,the anthropogenic productions of nitrogenous and phosphorus fertilizer are increasing in order to cope with the increment of world population[3].However,the discharge of these compounds into the environment has imposed severe impacts on human health and biodiversity.Hence,it is a crucial need to establish an effective,economical and feasible method to recover these nutrients from wastewater in order to achieve sustainable development.

To date,there have been a few studies suggesting that cultivating microalgae as biological nutrient removal method in recovering nutrient like phosphorus (P),carbon (C),nitrogen (N)and trace elements in wastewater could be one of the effective solutions to the effluent quality and nutrient deficit issues[4,5].In addition,this method could reap the benefits of producing biofuels and other valuable materials via extraction of lipids,proteins and carbohydrates from the microalgae[6].According to Pittman et al.[7],the employment of solely microalgal cultivation was not economically feasible and would provide negative energy return if it did not couple with wastewater treatment.Generally,the main advantages of coupling wastewater treatment with microalgal cultivation include:i)production of economical biomass for biofuel or commercially valuable products such as health food and pharmaceutical products,ii)recovery of nutrients with low-cost process compared with chemical nutrient removal method,iii)high quality of water discharge into water bodies and iv)reduction of greenhouse gases emission[7,8].However,harvesting microalgae biomass is an energy consuming process.It contributed over 20%of the total biomass production cost[9].In recent years,the development of algal-membrane photoreactor (A-MPR)appears to be a great solution for microalgal cultivation and biomass harvesting due to the lower cost of membrane technology.Incorporating membrane technology in the microalgal cultivation process not only could produce high quality of permeates,but also retain valuable products such as proteins and carbohydrates,in which proteins are widely used as a safe source of dietary supplement while carbohydrates could produce bioethanol by fermentation[10].

According to the study by Baerdemaeker et al.[11],the findings indicated that polyvinylidene fluoride(PVDF)membrane exhibited greater permeability and fouling resistance compared with polyvinyl chloride(PVC)and polyethersulfone-polyvinylpyrolidone (PES-PVP)membranes in microalgae filtration.However,PVDF membrane was rapidly fouled due to the hydrogen bonding between hydroxyl groups of biopolymers (mainly proteins and carbohydrates)and fluoride of PVDF membrane.Therefore,membrane fouling remains an unsolved issue in microalgae harvesting or dewatering process.In order to minimize the fouling problems,modifying membrane into highly negative charge was vital due to the existence of electrostatic repulsive forces between negatively-charged microalgae and the membranes [12].Recently,graphene oxide (GO)incorporated membrane has been identified as a promising material in producing membrane with high permeability,rejection and anti-fouling properties.A study conducted by Lee et al.[13]found that the operational time of polysulfone(PSF)membrane embedded with GO increased five folds compared with pure PSF membrane in a membrane bioreactor(MBR)system.Moreover,a study by Mahmoudi et al.[14]in blending the Ag/GO nanohybrids into the PSF polymer had demonstrated significant enhancement in membrane biofouling mitigation due to the antimicrobial property originated from Ag.

The objectives of this study were to conduct further investigations of the PVDF nanohybrid membranes in an A-MPR cultivated with C.vulgaris and the evaluation study of rejections of biopolymer(proteins and carbohydrates),COD and turbidity.The workability and reliability of the PVDF nanohybrid membranes in microalgae filtration was further examined by comparing with the performance of a commercial PVDF membrane followed by leaching test.

2.Experimental

2.1.Materials

Fine graphite,potassium permanganate (KMnO4),sulfuric acid(H2SO4),hydrogen peroxide(H2O2),silver nitrate(AgNO3)and sodium borohydride(NaBH4)were used for the synthesis of Ag/GO nanohybrids.Fine graphite was purchased from Merck Co.,KMnO4and H2SO4were from Accot,and the rest were from Sigma Aldrich.

Besides,N,N-Dimethylacetamide (DMAc,≥99.5%,Sigma Aldrich)and PVDF powder(Solef 6010,Solvay)were used for membrane preparation.Sodium hypochlorite (NaOCl,John Kollin)was used to clean fouled membranes.The chemical used to produce synthetic wastewater is presented in the Supplementary data.Humic acid(HA,Aldrich)and bovine serum albumin (BSA,nacalai tesque)were used as foulant model in evaluating membrane performance.Ultrapure water was used throughout the experiment.

2.2.Membrane fabrication and characterization

Neat PVDF and Ag/GO-PVDF membranes were fabricated using phase inversion method.The Ag/GO-PVDF membrane was made from PVDF polymer blended with Ag/GO nanohybrids as described elsewhere[15].The quantity of the Ag/GO nanohybrids was 0.4 wt%with respect to PVDF polymer.Commercial PVDF(Com-PVDF)membrane purchased from Ande Membrane Inc.was used for performance benchmarking with the self-fabricated membranes.The ultrafiltration membrane was commonly used for water and wastewater treatment.

The pore size of the membranes was calculated using gravimetric method and Guerout-Elford-Ferry equation[15].The hydrophilicity of the membranes was determined with sessile-drop method (KRüSS GmbH,FM40Mk2,Germany).Membrane surface charge was measured using zetasizer(Malvern Instruments,Zetasizer Nano ZS,UK).The surface and sectional view of the membranes were observed via field emission scanning electron microscope(FESEM)(Zeiss,MERLIN,Germany).AFM study was performed using scanning probe microscopy (NTEGRA PRIMA,NT-MDT,Russia)to observe the membranes'surface morphology.

2.3.Membrane performance study

Membrane permeation flux was obtained using stirred cell(Sterlitech,HP4750)and calculated using Eq.(1).

where Jwois the permeability of pure water(L·m?2·h?1),V is the volume of permeate(L),A is the membrane effective area(m2)and t is the filtration period(h).Then,membrane permeability was obtained by dividing the permeation flux with pressure applied in MPa.

The foulant models,bovine serum albumin(BSA,1000 ppm)and humic acid(HA,10 ppm)were used as proteins and organic humic substances in the stirred cell filtration,operating at 0.01 MPa in order to investigate the membrane fouling characteristics.The rejections of BSA and HA were calculated using Eq.2:

where Coand C1are the concentration of feed and permeate of substances(in ppm),respectively.

The fouled membranes were cleaned with 1000 ppm sodium hypochlorite(NaOCl)solution prior to the pure water filtration again.

2.4.Microalgae culture and characterization

C.vulgaris was selected for microalgae culture owing to its high photosynthetic efficiency and capability in taking up inorganic substances like N and P for their growth.Besides,Bold's Basal Medium containing nitrogen,phosphorus and trace metal elements(pH 7)was prepared and sterilized using autoclave at 121 °C for 20 min.Microalgae was cultivated for around 10 days in the medium.The microalgae was cultivated with the illumination of 6500 Lux and 3 L·min?1of air sparging.The growth of microalgae was monitored using spectrophotometer at wavelength of 650 nm (Hach,DR3900,USA).Standard cell density counting method was employed to identify the concentration of the microalgae using 0.100 mm Tiefe Depth Neubauer Hemacytometer via microscope (Cole-Parmer,National DC5-163,USA).The microalgal biomass was determined using gravitational method in microalgae dry weight/mixed liquid suspended solids.The microalgae were then transferred to the A-MPR when the media reached a cell concentration of around 40×106cell·ml?1.

2.5.A-MPR system setup and operation

The A-MPR system was operated in room temperature.Peristaltic pump (Cole-Parmer,Masterflex L/S,USA)was used to draw water from the microalgal suspension via a flat sheet membrane with an effective filtration area of 5.67×10?3m2.Digital pressure gauge(Fisher Scientific,Traceable Pressure Meter 3166,USA)was installed at the permeate line and the permeate mass was continuously recorded using a digital balance (AND,GF-3000,Japan).Continuous light intensity of 1000 Lux and aeration of 1 L·min?1were supplied to the system for fouling control and microalgae mixing.

2.6.Membrane performance in A-MPR

The evaluation of the A-MPR performance was based on the rejection capability of each membrane in synthetic municipal wastewater(Table S1).After cultivating the microalgae in the batch A-MPR for 5 days,the membranes(PVDF,AG/GO-PVDF and Com-PVDF)were submerged into the A-MPR for filtration at an operating pressure of?0.02 MPa for 2 h.The rejection tests for each membrane were performed in triplicate.The concentration of proteins and carbohydrates was measured using Bradford method and phenol-sulfuric acid method,respectively[16].The feed and permeate turbidity was measured using turbidity meter(Hach,2100AN,USA).Other water quality parameters such as chemical oxygen demand(COD),ammoniacal nitrogen(NH3-N)and phosphate(PO43?)were analyzed based on standard methods using spectrophotometer[17].The removal of proteins,carbohydrates,COD,NH3-N,PO43?and turbidity was calculated using Eq.(2).

Next,the Ag/GO-PVDF and Com-PVDF membranes were selected for membrane fouling test with and without the presence of bacteria(Escherichia coli)in the A-MPR at an operating pressure of ?0.02 MPa for 150 h.The flux decline was observed and the membranes were cleaned with NaOCl once the permeation flux declined to 5 L·m?2·h?1.The hydraulic retention time of the system was around 5 days.

2.7.Stability test of Ag/GO membrane

The stability of Ag/GO nanohybrids embedded in the membranes was examined via 6 h pure water filtration by using a stirred cell at 0.2 MPa.Any leaching of Ag into permeate could be detected with inductively coupled plasma optical emission spectrometry (ICP-OES)(Perkin Elmer,Optima 7000,USA).

3.Results and Discussion

3.1.Membrane permeability

The pure water permeability and contact angle of the membranes are presented in Table 1.It was noticeable that the pure water permeability of the Ag/GO-PVDF and Com-PVDF membranes were higher than the PVDF membrane,whereby the difference was about 23%and 132%,respectively.It was a known fact that membrane permeability correlated proportionately with membrane hydrophilicity[18].Similar trend was observed in this study,where the permeability of the Ag/GO-PVDF membranes was further improved by the addition of hydrophilic Ag/GO nanohybrids.This could be proven by the contact angle measurement on the membranes where the contact angle values reduced by 11.87% (81.7° ± 1.9°),representing the improved membrane's hydrophilicity.Both of the membranes(Ag/GO-PVDF and Com-PVDF)showed large membrane pore structure as presented in Fig.1b and c.This was because the hydrophilic membrane casting solution had stimulated and enhanced the phase inversion rate during themembrane formation process,resulted in the formation of larger membrane pore size.Although the ingredient of the Com-PVDF membrane fabrication was unknown,the addition of hydrophilic substances in membrane solution blending is common in production of commercial membranes.Therefore,the Com-PVDF membrane showed dense and large pore size with contact angle of (78.6° ± 3.7°).On the other hand,the PVDF membrane exhibited small and short layer of membrane pore structure(Fig.1a).The pore size of the PVDF,Ag/GO-PVDF and Com-PVDF membranes was(24 ±1)nm,(27±1)nm and(30± 2)nm,respectively.According to the gravimetric method and Guerout-Elford-Ferry equations[15],the membrane pore size is a function of the membrane thickness,porosity and volume of permeate.The enlargement of pore size had enhanced the water molecule passage across the Ag/GO-PVDF and Com-PVDF membranes in comparison to the PVDF membrane with the smallest pore size.Besides,the detailed physical and chemical characteristics of the Ag/GO nanohybrids could be referred in our previous study[14].The presence of Ag nanoparticles was detected at 38°,44°,64°and 77°,which are assigned to the Miller indices of(111),(200),(220)and(311)in the XRD analysis in Fig.S1.The Ag nanoparticles are evenly distributed on GO nanosheets as could be observed via TEM as shown in Fig.S2.

Table 1 Membrane permeability,contact angle,and rejections of HA and BSA of the membranes

Fig.1.Cross sectional morphology of (a)PVDF,(b)Ag/GO-PVDF and (c)Com-PVDF membranes at magnification of 500×.

3.2.Humic acid and bovine serum albumin rejections

Humic acid(HA)is the major component of dissolved organic matter in wastewater.Therefore,the rejection of HA using the membranes was studied and presented in Table 1.The size of HA was in a high range,from 18%less than 104,20%of ?104?3×104,30%of 3×104?106to 25%larger than 106[19].The rejection of the Ag/GO-PVDF membrane towards HA was the highest,which was 74.6%higher than the Com-PVDF membrane.The rejection of the PVDF membrane was relatively high,probably due to the relatively smaller pore size of the membrane compared to the Ag/GO-PVDF and Com-PVDF membranes as discussed in the previous section,which worked as a barrier prohibiting the passage of HA molecules.After cleaning with NaOCl,the membranes were filtered with pure water again.The flux recovery rate of the PVDF membrane was only 28.3%,which revealed that the membrane was severely fouled by HA and the fouling was irreversible.The irreversible fouling was probably due to the deposition of HA particles into the membrane pores which was hardly washed away during membrane cleaning.On the other hand,the flux recovery rates of the Ag/GO-PVDF and Com-PVDF membranes were 99% and 100%,respectively.The excellent flux recovery rate of these membranes indicated that the foulant deposition on the membrane surface could be easily washed off by chemical cleaning.Therefore,these findings depicted that the Ag/GO-PVDF and Com-PVDF membranes possessed greater anti-fouling ability compared to the PVDF membrane.

On the other hand,BSA was used as a protein foulant model to determine the fouling behavior of the membranes[20,21].The flux decline profile of the membranes using BSA was conducted in three filtration cycles.As seen from Fig.2,the Com-PVDF membrane experienced the greatest flux decline in the three filtration cycles.Apparently,the flux has reduced by approximately 50% in the second filtration cycle and 59%in the third cycle.The rapid flux decline on the Com-PVDF membrane might be due to the larger membrane pore size where BSA(～6.6× 104)was prone to deposit into the membrane pores.On the other hand,the flux decline of the Ag/GO-PVDF membrane was lower than the PVDF membrane although it had bigger pore size.Thanks to the negative charge of the Ag/GO nanohybrids which greatly enhanced the repulsive force existed between the membrane and BSA.Besides,the Ag/GO-PVDF membrane always had the highest recovery after each cleaning as can be observed in the second and third filtration cycles.The average BSA flux of the PVDF,Ag/GO-PVDF and Com-PVDF membranes over the 90 min filtration was 7.3,11.6 and 17.0 L·m?2·h?1,respectively.However,when the BSA rejection of these membranes were further analyzed,the average BSA removal percentage was shown to be the highest for the Ag/GO-PVDF membrane (28.5%),followed by Com-PVDF(19.1%)and PVDF(14.0%)membranes(Table 1).

The physical and chemical properties that commonly affect membrane's performance are membrane pore size,surface morphology,hydrophilicity and surface charge.The rejection of HA was always higher than BSA as it contains high amount of larger particles[19,22].It is well established that particles with the similar electrostatic charge would repel each other and tend to have higher cake compressibility and specific cake resistance[23].Hence,the higher negative surface charge of the membrane and foulants would result in lower fouling potential.The zeta potential of HA and BSA measured in this study was ?43.3 mV and ?9.8 mV,respectively.Meanwhile,the zeta potential of the PVDF,Ag/GO-PVDF and Com-PVDF membranes in pH 10 was?35.6,?44.2 and ?40.5 mV,respectively.Therefore,the greatest repulsive forces exist between foulant models(HA and BSA)and the Ag/GO-PVDF membrane.From the results,although the Com-PVDF membrane exhibited the greatest flux decline in BSA filtration due to larger pore size,its high negative charge repelled the BSA and hence showing good rejection performance compared with the PVDF membrane.Therefore,negative surface charge was a dominant factor in the separation mechanism of BSA filtration.

The membrane surface topography was observed and the images were presented in Fig.3.In a projected area of 10 μm × 10 μm,the roughness of the membranes' surface was in the following order:PVDF membrane ＞Ag/GO-PVDF ＞Com-PVDF membranes.Numerous deep valleys were observed on PVDF membrane,showing potential spots for foulant deposition which could lead to severe fouling[24].As a result,the PVDF membrane showed the lowest flux recovery ability.The surface roughness of the PVDF membrane was smoothened by the addition of the Ag/GO nanohybrids as shown in Fig.3b.On the other hand,the Com-PVDF membrane had the smoothest surface morphology which could prevent the particle attachment on the valleys and therefore showing strong anti-fouling ability.Besides surface roughness,the high flux recovery of the modified membrane(Ag/GO-PVDF)and the Com-PVDF membrane was possibly due to the alteration of membrane wettability.The modified membranes with hydrophilic functional groups are prone to form hydrogen bonds with water molecules on the membrane surface.Hence,the enhancement in membrane hydrophilicity would result in lower hydraulic resistance and higher retention of foulants[18].

Fig.2.BSA flux decline profile of membranes.

Fig.3.AFM 3D-images and surface roughness of membranes:(a)PVDF(b)Ag/GO-PVDF(c)Com-PVDF(d)RMS value.

3.3.Application of membranes in algal-membrane photoreactor

3.3.1.Protein,carbohydrate and COD rejections

After the preliminary evaluation using stirred cell system,the membranes were immersed into the A-MPR and microalgae filtration was performed at ?0.02 MPa for 2 h.Firstly,the average rejection of suspended solid in terms of turbidity for the PVDF,Ag/GO-PVDF and Com-PVDF membranes were determined,in which the rejection observed were 97.8%,98.9%and 99.3%,respectively.Besides,the Ag/GOPVDF and Com-PVDF membranes produced clear permeate in the range of 0.162 NTU to 0.292 NTU.

Fig.4.Rejections of proteins,carbohydrates and COD in A-MPR.

Additionally,the rejections of COD,proteins and carbohydrates using the PVDF,Ag/GO-PVDF and Com-PVDF membranes in the A-MPR were evaluated and shown in Fig.4.For COD removal,the Ag/GO-PVDF and Com-PVDF membranes showed quite similar rejection ability of(83.9±1.3)and (86.5±0.3),respectively.On the other hand,the PVDF membrane removed only(76.6±0.7)of COD.Previous study by Kamarudin et al.[25]showed that C.vulgaris had successfully reduced 50.5% of COD concentration with palm oil mill effluent (POME)as medium.Meanwhile,Ding et al.[17]removed 8.59%to 29.13%of COD in different POME dilutions by microalgae Chlamydomonas sp.UKM 6.There was another study which revealed that Ankistrodesmus falcatus,Scenedesmus obliquus and Chlorella sorokiniana could remove 61%,42%and 69% of COD in an aquaculture wastewater with an initial COD concentration of 96×10?6[5].In this study,microalgae would consume the organic matter present in the wastewater,which simultaneously contributed dissolved oxygen through photosynthesis process [26].Meanwhile,the utilization of membranes could further purify the wastewater,resulting in a low COD permeate.The concentration of feed COD reduced from(180.5±5.6)ppm to(82±2.6)ppm by microalgae in the A-MPR tank and eventually achieved a range of 23 to 45 ppm by membrane filtration(membrane permeate).Hence,it was a promising approach by integrating membrane filtration process with microalgae system,in which high quality of water could be produced for recycle and reuse purpose.

Apart from that,complete removal of proteins was noticed using the Ag/GO-PVDF and Com-PVDF membranes(100%)while the PVDF membrane showed only 84.0% rejection.The rejection of carbohydrates also exhibited comparable trends with the Ag/GO-PVDF(86.6%)and Com-PVDF (80.8%)membranes while the PVDF membrane could only remove 76.9% of carbohydrates.Besides,the zeta potential of microalgal suspension in the A-MPR was (?11.2 ±0.6)mV at pH 7.7 during the filtration.The negative charge could lead to larger repulsion forces existing between the surface of modified membranes and the microalgae.Hence,foulants could be easily repelled and detached from the membrane surface,which could further explain that the permeate quality did not compromise with enlarged membrane pore size.

Generally,microalgae always contains larger amount of carbohydrates than the proteins(ratio of 0.4 mg mg?1of protein:carbohydrate)[27].The carbohydrate contains in microalgal biomass are a major component in biofuel production while proteins are widely used as health care products [10].Therefore,the efforts of retaining proteins and carbohydrates in microalgae are relatively important for maximizing fermentative bio-product and biofuel yield[10,28].However,proteins and carbohydrates have been known as the dominant foulants in membrane filtration process[29].In the previous study,C.vulgaris was found to contain large amount of carbohydrates with 95%of transphilic and hydrophilic carbohydrate [27].In this study,the highly hydrophilic modified PVDF membranes(Ag/GO-PVDF)and Com-PVDF membrane were capable to repel these transphilic and hydrophilic carbohydrates with better rejection tendency than the PVDF membrane.This encouraging phenomenon was specifically contributed by the high negative surface charge existed between the foulants and membrane surface,which dominated the separation mechanism in the A-MPR filtration.

3.3.2.Filtration of microalgae

Owing to the excellent performance of the Ag/GO-PVDF and Com-PVDF membranes as per discussed previously,both membranes were selected for microalgae filtration in a longer period.The membranes were cleaned once the permeation flux reached 5 L·m?2·h?1.The flux decline of the membranes was observed and compared as shown in Fig.5.Referring to the graph of Fig.5a,the permeation flux of the Com-PVDF membrane was high in the beginning but drop rapidly afterwards.The Ag/GO-PVDF membrane showed a better anti-fouling ability in a long filtration period.The rapid flux decline of the Com-PVDF might be caused by the deposition of smaller size foulant into the membrane pore,hence further restricted the flow of water molecules passing through the membrane.The deposition also acted as a barrier,diminishing the effect of membrane charge towards the foulants.On the other hand,the Ag/GO-PVDF membrane's permeation flux decreased gradually and remained stable afterwards,probably due to high negative surface charge which had high repulsive effect towards microalgae.Besides,the most common bacteria(E.coli)that was present in the wastewater was added into the A-MPR to evaluate the biofouling resistance of the membranes.The zeta potential of the suspension in A-MPR(contain E.coli)was(?21.0±1.8)mV.In the first cycle,it took around 50 h for the Ag/GO-PVDF membrane to reduce to 5 L·m?2·h?1,which was one fold longer than the Com-PVDF membrane (Fig.5b).Overall,there were 3 and 5 filtration cycles for the Ag/GO-PVDF and Com-PVDF membranes,respectively within the 150 h filtration process.The membranes were then examined using FESEM analysis to determine the main fouling compound on the membranes.It was noticed that the foulant layer formed on the Com-PVDF membrane was relatively thicker than the Ag/GO-PVDF membrane as observed in Fig.6.E.coli grew in abundance(as indicated by arrows)and also formed a layer of biofilm(as circled)on the membrane surface.Furthermore,the uneven surface resulted from bacteria attachment increased the deposition of microalgae on the membrane.On the other hand,Ag/GO-PVDF membrane showed excellent anti-biofouling ability in the A-MPR as little E.coli was observed on the membrane surface.E.coli cells which tried to attach on the membrane were repelled by the negatively charge membrane and some were being destroyed by ROS generated by the nanohybrids.The adsorption of surface oxygen by GO and rapid electron transfer between the Ag and GO had increased the production of ROS[30].Previous study by Vatsha et al.[31]also reported that Ag/GO nanohybrid had greater antimicrobial properties compared to GO nanosheets.Therefore,it is anticipated that the operational cost for the A-MPR could be reduced in long-term operation.

Fig.5.Membrane flux decline profile:(a)without and(b)with E.coli.

Fig.6.FESEM images for membranes after 150 h filtration in the A-MPR at magnifications of(1)500×and(2)5000×:(a)Ag/GO-PVDF and(b)Com-PVDF.

3.3.3.Nutrient recovery

The nutrient recovery from the microalgal cultivation was evaluated in terms of the removal percentage ofand NH3-N elements.The removal ofand NH3-N in the system was mainly due to nutrients up-take by the microalgae instead of membrane separation.Therefore,the removals ofand NH3-N for both membranes were likely similar during the filtration period.The removal ofranged from 58.8% to 66.1%,givingconcentration of 7.0 mg·L?1to 8.5 mg·L?1in the permeate.Meanwhile the concentration of NH3-N had reduced to the range of 2.9 mg·L?1to 5.3 mg·L?1with high removal efficiency of 91.9% to 92.3%.The general chemical formula of microalgae is C106H181O45N16P,which showed an abundance of carbon,nitrogen and phosphorus as the main elemental block for microalgae structural[32].Microalgae survived in the wastewater environment through assimilation of nutrients by accumulating carbon,nitrogen and phosphorus from the surrounding environment.Nitrification is the main mechanism of nitrogen acquisition of green microalgae.Microalgae commonly used(the nitrogen sources)to react with oxygen,converting toand further intoin the biological oxidation process.Besides,nitrogen stripping from the volatilization process also contributed to the reduction of nitrogen concentration[32].The pH of the medium increased due to the contribution of hydroxide ions from the photosynthetic process.The increment of pH would promote the transformation process ofto NH3and stripping into the atmosphere.According to Martínez et al.[33],there are several factors contributing to the phosphorus intake of microalgae,and one of it is pH value.At high pH condition,phosphate precipitated and therefore reduce the concentration of total phosphorus.Xu et al.[34]also evidenced that the main mechanism of P removal (66%)in an A-MPR was due to algae-induced precipitation of calcium phosphate.The amount of phosphorus content in the wastewater could affect the lipid metabolism and biosynthesis of carbohydrates in the microalgae biomass[35].On the other hand,nitrogen contributed to the generations of lipids,proteins and carbohydrates[36].Hence,cultivating the microalgae in the existing wastewater ponds in the industry could be a potential approach to greatly reduce the use of anthropogenic fertilizer.However,microalgal cultivation could be challenging and extensive studies should be performed in order to optimize the system's performance.

In terms of biomass productivity,the microalgae dry weight in A-MPR using Ag/GO-PVDF membrane and commercial PVDF membrane increased from average initial concentration of 40.67 mg·L?1to 389 mg·L?1and 38.33 mg·L?1to 380.67 mg·L?1,respectively(Table S2).The amounts of biomass generated from the A-MPRs with different sets of membranes have no significant differences.This is because the cultivation condition was similar and the types of membranes did not affect the growth of the microalgae in the A-MPR.

3.4.Stability test of Ag/GO-PVDF membrane

The stability of the composite membranes is rather important to ensure consistent membrane performance for a long-term filtration.Apart from that,the leaching of nano-metals into the environment could impose negative impact to aquatic life and human health.Previous studies stated that the presence of nano-metals in water would damage microalgae's chloroplasts,resulting in granulation and contraction in algal cells,which directly inhibited the population growth of microalgae in the water bodies [37,38].Another study reported that the degree of damage was influenced by the nano-material concentration[39].For instance,chloroplast only became fragmented when Ag nanoparticle concentration in water reached 1.5 mmol·L?1.On the other hand,Kulacki and Cardinale[40]found that the amount of TiO2nanoparticles had no significant effect on the growth rate of algae.In this study,no leaching of Ag nanoparticle from the Ag/GO-PVDF membrane was detected in the permeate across the 6 h filtration process.This was probably due to the strong bonding of the Ag/GO nanohybrids'oxygen functional groups with the C-F groups originated from PVDF polymer molecular chains.Therefore,the stability of the Ag/GO-PVDF membrane was proven and could be safely utilized in various applications.

4.Conclusions

The integration of microalgal cultivation in wastewater bioremediation had reduced the dependency of microalgae on fresh water,produced high quality of recycle water,and simultaneously produced biomass for valuable bio-products.The Ag/GO-PVDF membrane showed greater performance towards HA and BSA,and similar trends of rejections on COD,proteins and carbohydrates when benchmarking with a commercial membrane.With the high anti-fouling and anti-biofouling propensities of the membrane,less chemical cleaning was required during the operation where the operating cost could be reduced.Hence,the integrated A-MPR system is highly recommended as a promising approach for microalgal cultivation and wastewater polishing treatment,owing to its high stability and enhanced performance.

Acknowledgements

This study was supported by Universiti Kebangsaan Malaysia[Grant No.DIP-2016-031].The authors would like to acknowledge CRIM(Center for Research and Instrumentation Management,UKM)for sponsoring the postgraduate study of W.C.Chong via Research University Zamalah Scheme and the technical supports in this work.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.05.015.