Desalination 353 (2014) 48–56

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Desalination

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Investigation of submerged membrane photocatalytic reactor (sMPR)operating parameters during oily wastewater treatment process

C.S. Ong, W.J. Lau ⁎, P.S. Goh, B.C. Ng, A.F. IsmailAdvanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, 81310 Johor, MalaysiaFaculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, Skudai, 81310 Johor, Malaysia

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

• sMPR could achieve high TOC degrada-tion and oil rejection under optimizedconditions.

• Excessive TiO2 loading and modulepacking densitywould deteriorate sMPRperformance.

• Higher ABFR could reduce the severefouling impacts during high feedconcentration.

⁎ Corresponding author. Tel.: +60 75535926.E-mail addresses: lwoeijye@utm.my, lau_woeijye@yah

http://dx.doi.org/10.1016/j.desal.2014.09.0080011-9164/© 2014 Elsevier B.V. All rights reserved.

Schematic diagram of oily wastewater separation and degradation using PVDF-TiO2 hollow fiber membraneunder UV irradiation.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 August 2014Received in revised form 4 September 2014Accepted 6 September 2014Available online xxxx

Keywords:TiO2

Submerged membrane photocatalytic reactorAir bubble flow rateOil concentrationModule packing densityWastewater

The performance of a submergedmembrane photocatalytic reactor (sMPR) consisted of polyvinylidene fluoride-titanium dioxide (PVDF-TiO2) hollow fiber membranes was evaluated for the separation and degradation ofsynthetic oily wastewater under UV irradiation. The effects of operating parameters such as TiO2 catalyst loading,membrane module packing density, feed oil concentration and air bubble flow rates (ABFR) on the permeateflux, oil rejection and total organic carbon (TOC) degradation (in the bulk feed solution) were studied. Thecomparison of TOC degradation based on direct photolysis, neat PVDF membrane and PVDF-TiO2 membranewere determined. It was clearly observed that TOC degradation using PVDF-TiO2 membrane was remarkablyhigher compared to neat PVDF membrane. Gas chromatography–mass spectrometry (GC-MS) analyses showedthat oil components in thewastewater could be efficiently degraded in the presence of TiO2 underUV irradiation.The average flux of membrane was reported to be around 73.04 L/m2 h using PVDF membrane embeddedwith 2 wt.% TiO2 at 250 ppm oil concentration with module packing density of 35.3% and ABFR of 5 L/min.A remarkable TOC degradation and oil rejection as high as 80% and N90%, respectively, could be reachedunder these optimized conditions. The findings shown in this work provide useful information for the re-search of simultaneous separation and degradation of oily wastewater and facilitate the development of hybridsMPR.

© 2014 Elsevier B.V. All rights reserved.

oo.com (W.J. Lau).

49C.S. Ong et al. / Desalination 353 (2014) 48–56

1. Introduction

Emulsions of cutting fluid are always used in the machining processto aid the cutting process, to prevent corrosion, and to improve lubrica-tion, cooling, surface cleaning and tool life [1]. A considerable amount ofoil-contaminated wastewater which contains high concentration ofaliphatic and aromatic compounds can be generated from the abovementioned processes, causing detrimental and harmful effect to envi-ronment and aquatic life as well as groundwater sources [2]. Despitetheir well acceptance in industry, those conventionally used treatmentmethods such as electrocoagulation [3], membrane filtration [4,5], andflotation process [6,7] have been associated with high operating costand lack of capability in removing oil molecules below 10 μm in size[8,9]. Additionally, some harmful by-products are likely to be generated,requiring additional treatment process to eliminate them. In view ofthis, various kinds of advanced treatmentmethods have been proposedby researchers to overcome the drawbacks of the existing treatmentprocesses. Among the methods proposed, advanced oxidation process(AOP) has received great attention over the past decade, mainlydue to its high efficiency to remove recalcitrant, toxic and non-biodegradable organic micropollutants from wastewater [10–12]. Nev-ertheless, most of the classical AOPs are limited by the large amountof residual pollutants and the catalysts retained in the processwhich re-quire additional treatment to eliminate them [13–15]. To overcome thisproblem, a heterogeneous photocatalyst integrated with membraneprocess is proposed as this hybrid process could deal with a broadrange of organic pollutants via degradation followed by separation. Ofthe photocatalysts available in the market, titanium dioxide (TiO2)nanoparticle is the most popular one owing to its good thermal andphysical stability, cheapness, excellent photocatalytic, antibacterialand antifouling properties [16–20]. Many research efforts have been de-voted to design a sustainable photocatalytic membrane reactor (PMR)with high efficiency and low energy consumption. In general, twomain configurations for PMR are being pursued, namely i) reactorwith catalysts suspended in the feed solution and ii) reactor withcatalysts immobilized in/on themembrane. The latter option ismore fa-vorable as the recovery process of TiO2 photocatalysts can be greatlysimplified, hence reduces the operational complexity and cost in practi-cal application [10,21–23]. Worth saying that the presence of unrecov-ered TiO2 nanoparticles might raise another concern to environmentalsafety. Despite the large number of experimental studies showing thatthe suspended-TiO2 photocatalysis process holds significant advan-tages, this mode of photocatalytic water treatment is constrainedby some technical challenges, such as ineffective catalysts recoveryand difficult reuse of the catalyst particles to allow continuous watertreatment [10]. Therefore, a hybrid submerged membrane photocata-lytic reactor (sMPR) is proposed by immobilizing TiO2 photocatalystsin membrane matrix with vacuum pressure driving through permeateside under UV irradiation. In this integrated treatment process,membrane not only functions as the support for TiO2 photocatalystbut also acts as a physical selective barrier for thedegraded products. Al-though the performance of the membrane-photocatalysts integratedprocess has been examined in various industrial wastewater treatmentprocesses [10], only little attention has been paid to the synthetic cut-ting oil wastewater. To the best of our knowledge, this is the first at-tempt to investigate the influence of operating parameters on thesimultaneous separation and degradation of synthetic cutting oil waste-water using TiO2 incorporated PVDF ultrafiltration (UF) membranes insMPR.

Themajor focus of thiswork is to assess the performance of sMPR forsynthetic cutting oil wastewater treatment based on several key operat-ing parameters, i.e. TiO2 catalyst loading (embedded in membrane ma-trix), module packing density, initial feed concentration and air bubbleflow rate (ABFR). The identification of the chemical compounds beforeand after photocatalytic degradation was studied by GC-MS. The com-parison between direct photolysis, neat PVDF UF membrane and PVDF

membrane immobilized with TiO2 catalysts were also evaluated basedon TOC degradation.

2. Experimental

2.1. Materials

PVDF (Kynar®760) pellets purchased fromArkema Inc., Philadelphia,USA were used as the main membrane forming material. N,N-dimethylacetamide (DMAc) (Merck, N99%) was used as solvent to dis-solve polymer without further purification. Polyvinylpyrrolidone (PVP)(Molecular weight: 40,000 g/mol) purchased from Sigma Aldrich andtitanium dioxide (TiO2) (Degussa P25, a mixture of 75% anatase and25% rutile with BET surface area 50 m2/g, average particle size ~21 nm,energy band gap 3.18 eV) from Evonik were used as the photocatalyst.The cutting oil obtained from RIDGID, Ridge Tool Company, Ohio, USAwas used to synthesize oily wastewater of various concentration.

2.2. Preparation of membrane and membrane module

PVDF (18 wt.%) was added into pre-weighed DMAc solvent afterbeing dried for 24 h in oven at 50 °C. The solutionwas thenmechanical-ly stirred at 600 rpmuntil all the polymeric pellets were completely dis-solved. It was followed by the addition of 5 wt.% PVP and TiO2 catalystloading varying from 0 to 4 wt.%. Lastly, the dope solution wasultrasonicated to remove any air bubbles trapped within the solutionprior to spinning process.

PVDF hollow fiber membranes were fabricated using dry-jet wetspinning method as described elsewhere [24]. The as-spun hollow fi-bers were immersed into water bath for 2 days to remove residual sol-vent. Prior to air drying, the fiberswere post-treated by 10wt.% glycerolaqueous solution for 1 day to minimize fiber shrinkage and pore col-lapse. At last, the hollow fibers were dried at room temperature for3 days before module fabrication.

A various number of fibers (30, 60 and 90 fibers) with the length ofapproximately 28 cmwere then potted into each PVC tube using epoxyresin (E-30CL Loctite® Corporation, USA). The membrane module wasthen left for hardening at room temperature before its protrudingparts were cut and fixed into a PVC adaptor to complete the modulepreparation.

2.3. Preparation of synthetic wastewater

The synthetic cutting oil wastewater was prepared by mixingdistilled water with commercial cutting oil (RIDGID Nu-Clear CuttingOil, #70835, Ridge Tool Company). The emulsion was prepared bymixing cutting oil in the range of 250–10,000 ppm and sodiumdodecylbenzenesulfonate (SDS) at the ratio of 9:1. The solution wasthen blended by a high speed blender (Model: BL 310AW, Khind) for2 min with an agitation speed of 50 Hz at room temperature.

2.4. sMPR configuration and experimental procedures

A schematic diagram of sMPR is shown in Fig. 1. The dimension ofthe sMPR is 18 cm(W)×20 cm(L)× 40 cm(H). Twohollowfibermem-brane modules were placed at the bottom of sMPR containing approxi-mately 14 L of synthetic cutting oil solution. An air compressor (Model:2 HP single cylinder 24 L tank, Orimas)was used to generate air bubbleswithin the submerged tank through air diffuser installed underneaththe membrane modules. To investigate the effect of ABFR, an air flowmeter was used to control the flow rate in the range of 1–5 L/min. An8W black light blue UV-A lampwith a maximum light intensity outputat 365 nm (Model: FL8BLB, Sankyo Denki Co., Ltd., Japan) was placed inbetween twomembranemodules. The UV light intensity wasmeasuredusing a UVX radiometer (UVP Inc., Upland, CA) with an UV-A sensor(UVX-36, UVP Inc., Upland, CA) at the side wall of sMPR without feed

Fig. 1. Schematic diagram of the sMPR system: (a) connection to UV lamp control panel,(b) feed solution tank, (c) UV-A lamp, (d1, d2) membrane modules of different packingdensity, (e) connection to peristaltic pump and permeate collection tank, (f) air diffuser,(g) air flowmeter and (h) air compressor.

Fig. 2. Calibration graph between absorbance and oil concentration.

50 C.S. Ong et al. / Desalination 353 (2014) 48–56

solution in the tank and an intensity of approximately 0.333 mW/cm2

was recorded. Water permeate was then produced using peristalticpump (Model: 77200-60, Masterflex L/S, Cole Parmer) by creatingvacuum pressure on permeate side.

The performance of sMPR was determined based on direct photoly-sis, neat PVDF UF membrane and PVDF-TiO2 UF membrane under UVirradiation. The effect of different catalyst loading, module packing den-sity, feed concentration and ABFR were studied. Membrane modulepacking density (φ) could be calculated according to Eq. (1):

φ ¼ n� ODð Þ2fiberIDð Þ2module

ð1Þ

where n is the number of fibers for each module, OD is the outer diam-eter of the hollow fiber membrane (1.15 mm) and ID is the inner diam-eter of PVC module (15 mm).

2.5. Analytical methods and measurements

For each permeate flux and its quality analysis, three samples (with10ml each) were taken and the remaining permeate was recycled backto the tank. Three measurements were made for each sample and thenthe average value was reported together with its standard deviation(based on 95% confidence level). To determine membrane water flux,J (L/m2 h), the following equation was employed.

J ¼ QAt

ð2Þ

where Q is the quantity of permeate (L), A is the effective membranearea (m2), and t is time to obtain the quantity of Q (h). The membraneoil rejection, R (%), was evaluated using the Eq. (3).

R ¼ 1−Cp

C F

� �� 100 ð3Þ

where Cp and CF are the concentration of oil in the permeate (ppm) andthe feed (ppm), respectively.

The oil concentrations in permeate and feed were determined usinga UV–vis spectrophotometer (Model: DR5000, Hach) with absorbancemeasured at 294 nm which the maximum absorption occurs. The rela-tion between absorbance and oil concentration is found to be linear asshown in Fig. 2. The same relation has been used for measuringunknown oil concentration in each permeate.

To measure zeta potentials of the membrane surface, streaming po-tential measurements were performed using a SurPASS electrokineticanalyzer (Anton Paar, Graz, Austria). As only the outer surface of hollowfiber membrane was analyzed, adjustable gap cell was used. Duringanalysis, potassium chloride (1 mM) was used as electrolyte, whereas0.1 M hydrochloric acid and 0.1 M sodium hydroxide were used forpH titration. The detailed characterization of streaming potential mea-surements can be found elsewhere [25].

To determine the photocatalytic degradation efficiency, TOC degra-dation (%) of feed was determined according to Eq. (4).

TOC degradation ¼ 1− TOCt

TOC0

� �� 100 ð4Þ

where TOCt and TOC0 are the TOC concentration of the permeate (ppm)at reaction time t and initial feed (ppm), respectively and are measuredby TOC analyzer (Model: TOC-LCPN, Shimadzu Co.).

To describe the kinetics of photocatalytic reactions of aquatic or-ganics, the Langmuir–Hinshelwood model as expressed in Eq. (5) wasemployed [26,27]. This model basically relates the degradation rate (r)(mg/L min) with the concentration of organic compound (C) (mg/L).

r ¼ − dCdt

¼ krKadC1þ KadC

ð5Þ

where kr is the intrinsic rate constant (mg/L min) and Kad is the adsorp-tion equilibrium constant (L/mg). When the adsorption is relativelyweak and/or the concentration of organic compound is low, Eq. (6)can be simplified to the first-order kinetics with an apparent rate con-stant kapp (min−1):

lnCC0

� �¼ −krKadt ¼ −kappt: ð6Þ

A plot of − ln CC0

� �versus reaction time t yields a straight line, and

the slope is the kapp.

Table 1Apparent rate constant (kapp) and correlation coefficient (R2) for TOCdegradation at differ-ent UV operating conditions.

UV operating condition kapp (×10−3 min−1) R2

UV (photolysis) 1.50 0.9987UV with neat PVDF membrane 3.10 0.9351UV with PVDF-TiO2 membrane 5.20 0.8958

51C.S. Ong et al. / Desalination 353 (2014) 48–56

2.6. Solid phase extraction (SPE) and gas chromatography–massspectrometry (GC-MS) analysis

The determination of oil in permeate sample involved a pre-concentration by solid-phase extraction (SPE). Specifically, oil extrac-tion was performed with HyperSep C18 cartridges (500 mg resin/3 mlcartridge volume). All samples were acidified to pH 2 with HCl and anappropriate volume of methanol was added, equal to 2% of the samplevolume. Subsequent to acid and methanol addition, samples werepassed twice through C18 cartridges followed by HPLC-grade water,using vacuum pump at a flow rate of 2–4 ml/min. The sample wasloaded directly on top of the cartridge and the same flow rate as the car-tridge volume was used (equal to 3 ml/min). After sample loading, thecartridges were washed with HPLC-grade water twice before pouring10 ml of dichloromethane at flow rate of 1 ml/min. The eluted samplewas analyzed with HP 7890B/5977A GC-MSD (Agilent Technologies,Palo Alto, CA, USA) operated at an electron impact (70 eV) and in afull-scan mode. The HP Model 7890B GC was equipped with a HP-5MS coated capillary column (30m× 0.25mm ID × 0.25 μm film thick-ness). Samples were injected in splitless mode with He as carrier gas.The GC oven was programmed at 75 °C and held 2 min, heated to275 °C at 6 °C/min, and held at the maximum temperature for 35 min.System control and data acquisition were achieved with the AgilentMassHunter GC-MS Acquisition B.07.00.SP2 and MSD ChemStationEnhanced Data Analysis F.01.00.1903. The identification of the photo-catalysis degradation productswasdoneby comparing theGC-MS spec-tra patterns with those of standard mass spectra in the NationalInstitute of Standards and Technology (NIST) library.

3. Results and discussion

3.1. Role of TiO2 photocatalyst

Fig. 3 compares the TOC degradation performance of direct photoly-sis, neat PVDF UFmembrane and PVDF-TiO2 UFmembrane under UV ir-radiation. It is found that oil particles were barely photodegraded after120 min of UV-A light exposure in the case of direct photolysis. In con-trast, it is observed that the TOC degradation using PVDF-TiO2 mem-brane was remarkably higher compared to neat PVDF membrane,confirming the important role of TiO2 catalyst in assisting oil degrada-tion. The significant oil degradation in the case of PVDF-TiO2membranecan be further explained by the generation of strong oxidants from theTiO2 nanoparticles under UV irradiation that have consequently oxi-dized most of the organic compounds found in the feed solution. The

Fig. 3. Comparison of TOC degradation between photolysis, neat PVDF membrane andPVDF membrane in the presence of TiO2 catalyst (Operating condition: 8 W UV-A lamp,temperature = 25 °C and initial feed concentration = 1000 ppm).

explanation is further supported by the higher value of apparent rateconstant (kapp) in the case of PVDF-TiO2 membrane as shown inTable 1. To further confirm the degradation of the organic compounds,the chromatograms of solution samples before and after photocatalysisusing PVDF-TiO2 membrane were compared and the results are shownin Fig. 4. It can be clearly seen that significant peak as shown in sample Awas decreased in sample B. The identification of possible organic com-pounds is summarized in Table 2. It is found that majority of identifiedhydrocarbons can be associated to chemical compounds in the rangeof C12–C39. Among these compounds, C27 fractions had approximately23.8% while C16 and C14 fractions were 18.7% and 14.7%, respectively.This observation was analogous to that reported by Li et al. [28] inwhich approximately 90% of hydrocarbon groups detected within oilywastewater are C10–C30 straight chain alkanes. In our study, the previ-ous most abundant hydrocarbons (C27–C39) were disappeared in sam-ple B, most probably due to photocatalytic activity that breaks thelong-chained organic compounds into relatively smaller organic com-pounds (C12–C16).

3.2. Effect of TiO2 catalyst loading

Fig. 5(a) shows the permeate flux and oil rejection with PVDFmem-brane made of different TiO2 loadings under UV irradiation. Owing tothe photoinduced-hydrophilicity effect of TiO2 catalyst embeddedwith-in membrane matrix, the permeate flux was reported to increase withincreasing TiO2 loading from 0 to 2 wt.%. Similar results have alsobeen reported by Vernardou et al. [29] and Risse et al. [30], wherethey found that membrane hydrophilicity was remarkably increased(i.e. lowerwater contact angle)with increasing TiO2 loading uponUV il-lumination. Excessive use of TiO2 (N2 wt.%) however was found to neg-atively affect membrane permeate flux, mainly because of the TiO2

particle agglomeration which increases the water transport resistanceon membrane surface and results in lower flux [31]. Despite the fluxwas declined with increasing TiO2 loading N2 wt.%, promising oil rejec-tion (N90%) was able to achieve throughout the entire experimental

(A)

(B)

Fig. 4. Comparison of chromatograms before (A) and after (B) photocatalytic degradationin 240min of UV irradiation (operating conditions: 8WUV-A lamp, feed concentration=1000 ppm and PVDF membrane immobilized with 2 wt.% TiO2).

Table 2Identification of possible organic compounds before and after photocatalysis.

Photocatalyticconditions

No. Name Chemicalformula

Chemical structure Probability(%)

Before photocatalysis 1 1-Monolinoleoylglycerol trimethylsilyl ether C27H54O4Si2 23.8

2 Octasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13,15,15-hexadecamethyl-

C16H50O7Si8 18.7

3 Heptasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13-tetradecamethyl-

C14H44O6Si7 14.7

4 Hexasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13-tetradecamethyl-

C12H38O5Si6 8.03

5 Spirost-8-en-11-one, 3-hydroxy-,(3β, 5α, 14β, 20β, 22β, 25R)-

C27H40O4 8.03

6 Cholestan-3-one, cyclic 1,2-ethanediyl aetal, (5β)- C29H50O2 2.4

7 Oleic acid, 3-(octadecyloxy)propyl ester C39H70O3 1.65

8 Cholestan-3-one, cyclic 1,2-ethanediyl acetal, (5α)- C29H50O2 1.39

9 Octadecane, 1,1'-[1,3-propanediylbis(oxy)]bis- C39H80O2 1.3910 Glycine, N-[(3α, 5β, 7α, 12α)-24-oxo-3,7,12-tris

[trimethylsilyloxy]cholan-24-yl]C36H69NO6Si3 1.04

After photocatalysis 1 Octasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13,15,15-hexadecamethyl36-

C16H50O7Si8 51

2 Heptasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13-tetradecamethyl-

C14H44O6Si7 36

3 Hexasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13-tetradecamethyl-

C12H38O5Si6 10.2

52 C.S. Ong et al. / Desalination 353 (2014) 48–56

period, which can be attributed to the smaller pore size of membranesthan the mean diameter of oil particles, as reported in our previousstudies [32,33].

Fig. 5(b) on the other hand shows the TOCdegradation of TiO2 incor-porated PVDF compositemembrane under UV irradiation. The TOC deg-radation increasedwhen TiO2 loading increased from1 to 2 wt.%, owing

to photoinduced-hydrophilicity on the membrane surface by TiO2 cata-lyst upon UV illumination. Photo-induced electrons are generated dur-ing UV illumination and large amount of OH ∙ radicals is formed due todissociation of water molecules through oxygen vacancies. However,further increase in TiO2 loading was detrimental to the oil degradationas TiO2 agglomerationmight lead to inefficient adsorption of UV energy

Fig. 5. Effect of TiO2 catalyst loading on (a) permeate flux and oil rejection and (b) TOC degradation of PVDF-TiO2 composite membrane in the sMPR system (operating conditions:temperature = 25 °C, vacuum pump flow rate = 15 ml/min and pH= 7).

Table 3Apparent rate constant (kapp) and correlation coefficient (R2) for TOC degradation withPVDF-TiO2 composite membranes at different TiO2 catalyst loading.

TiO2 (wt.%) kapp (×10−3 min−1) R2

0 0.20 0.92981 0.80 0.85222 2.80 0.93553 2.10 0.86334 1.30 0.8961

53C.S. Ong et al. / Desalination 353 (2014) 48–56

and consequently caused the deterioration of photocatalytic activityand less OH ∙ radicals generated on membrane surface. This phenome-non was supported by the decreased value of kapp in the PVDF mem-brane embedded with 3 and 4 wt.% TiO2 in comparison to themembrane with 2 wt.% TiO2 (see Table 3).

In order to better understand the effect of TiO2 loading on mem-brane performance, surface charge of each membrane with respect tozeta potential was determined. The zeta potential measured for the or-igin PVDF membrane and PVDF membranes modified with 1, 2, 3 and4 wt.% TiO2 were −3.5, −8, −8.9, −8 and −3.5 mV, respectively, atpH 7. It can be clearly seen that the zeta potential of the modifiedPVDF membranes with 1–2 wt.% TiO2 loading was significantly higherthan that of the origin PVDFmembrane, confirming the role of the neg-ative charged TiO2 nanoparticles in inducingmembrane charge proper-ties [34]. However, the use of excessive loadings of TiO2 (N2 wt.%) forPVDF membrane tended to reduce negative surface charges, owingto the enhancement of electrostatic attractive force among TiO2

Fig. 6. Effect of module packing density on (a) permeate flux and oil rejection and (b) TOC detemperature = 25 °C, membrane type: PVDF with 2 wt.% TiO2, vacuum pump flow rate = 15

nanoparticles as a result of TiO2 agglomeration [35,36]. This trend is ex-actly the same as those observed in permeate flux and TOC degradation,revealing that membrane surface charge does to some extent governmembrane performance in this study. Based on the results obtainedhere, it can be said that the membrane separation performance andthe photocatalytic activity are not favored at excessive TiO2 loadingand 2 wt.% TiO2 was therefore selected as optimal catalyst loading inthe following studies.

3.3. Effect of module packing density

To further investigate the performance of PVDF-TiO2 membraneduring sMPR process, the membrane module packing density alsoneeds to be considered. As reported in literature [37–41], mass transferwas able to be enhancedwith highermembranemodule packing densi-ty, which contributed to greater membrane flux. Yet the effect of mod-ule packing density on the corresponding photocatalytic degradationhas not been reported so far. Thus, an attempt has been made in thiswork to investigate the permeate flux, oil rejection andTOC degradationas a function of module packing density. It can be clearly seen fromFig. 6(a) that permeate flux increased with increasing module packingdensity from 17.6% to 35.3%, owing to the enhanced mass transfer be-tween the water molecules and membrane surface [38]. However,when further increased the module packing density to 52.9%, the fiberswere likely to attach to each other and resulted in the limited spacesavailable between adjacentfibers, causing both the effectivemass trans-fer area and membrane flux to decrease [37]. The experimental workconducted by Kiat et al. [42] showed that severe inter-fiber fouling

gradation of PVDF-TiO2 composite membrane in the sMPR system (operating conditions:ml/min and pH= 7).

Table 4Apparent rate constant (kapp) and correlation coefficient (R2) for TOC degradation withPVDF-TiO2 composite membranes at different module packing density.

Module packing density (%) kapp (×10−3 min−1) R2

17.6 0.09 0.967835.3 2.30 0.958952.9 1.00 0.9576

F1 F2 F3 F4

P1 P2 P3 P4

Fig. 8. Feed and permeate solution at varied concentration (a) feed oil concentration of250 ppm (F1), 1000 ppm (F2), 5000 ppm (F3) and 10,000 ppm (F4) and (b) their respec-tive permeate sample (P1-4).

54 C.S. Ong et al. / Desalination 353 (2014) 48–56

tended to occur when the packing density of the module exceeded acritical value, i.e. 30.8% reported in their work. However, the criticalpacking density might vary, depending on feed solution properties,module dimension and membrane material. Yeo et al. [39] on theother hand also reported that high membrane packing density couldlead to enhanced permeability, provided the packing density was notexcessive. It is because excessive packing density would cause foulantaccumulation to occur inside the fiber bundle.

As illustrated in Fig. 6(b), TOC degradation increasedwhen themod-ule packing density was increased to 35.3%, but decreased with furtherincreased themodule packing density to 52.9%. This can be explained bythe fact that when large amount of fibers is packed in amodule, they aretended to attach to each other, reducing the active surface sites for UVirradiation and photocatalysis. Because of this, lower value of kapp wasobtained (see Table 4). Despite the deterioration of flux and photocata-lytic activity at high packing density, high rejection rate could be consis-tently achieved throughout the experiment. Based on these findings, itcan be concluded that high packing density might adversely affectboth filtration performance and TOC degradation, hence, module pack-ing density of 35.3%was selected as optimalmodule scale to apply in thesubsequent studies.

3.4. Effect of feed concentration

To deal with industrial oily wastewater of various oil concentrations,the membrane performance and oil photodegradation at different initialfeed concentration in the range of 250–10,000 ppmwere also investigat-ed based on the optimumTiO2 loading (embedded inmembranematrix)andmodule scale as reported in previous sections. As shown in Fig. 7(a),membrane flux was found to decrease significantly with the increase offeed concentration. The average flux of membrane was decreased fromaround 73 L/m2 h at 250 ppm to b62 L/m2 h at 1000 ppm and furtherto approximately 22 L/m2 h and 15 L/m2 h for concentration of5000 ppm and 10,000 ppm, respectively. These results serve as the com-pelling evidence that thicker oil layer is formed on the membrane sur-face at high feed concentration, leading to increased water transportresistance and decreased membrane permeability [43]. Additionally, atlow oil concentration, the flux decline rate was not as severe as that ofobserved at high oil concentration. At 250 ppm, the membrane flux

Fig. 7. Effect of feed concentration on (a) permeate flux and oil rejection and (b) TOC degradatiature = 25 °C, membrane type: PVDF with 2 wt.% TiO2, module packing density = 35.3%, vacu

declined by around 8% in 480min compared to 43%, 51% and 56% record-ed for 1000 ppm, 5000 ppm and 10,000 ppm of oily solution, respective-ly. This increasing pattern was similar to the findings reported in ourprevious work [32,33], where the flux of PVDF-TiO2 membrane testedunder non-UV irradiation declined with increasing oil concentration.However, this experimental results show that membrane flux with UVirradiation was higher compared to those operated without UV irradia-tion. It is because of the photoinduced-hydrophilicity effect on thePVDF-TiO2 surface under UV illumination as discussed in the previoussection.

Althoughmembrane flux tended to decrease at higher oil concentra-tion, promising oil removal ratewas still able to obtain irrespective of oilconcentration (see sample illustrations in Fig. 8). Furthermore, as ob-served in Fig. 7(b), the degradation of oil under UV irradiationwas high-ly efficient at low concentrations. The formationof thicker oil layer is themain cause for this phenomenon, because it adversely affects the UVlight penetration on the membrane surface which in turn reduces theactive surface sites for UV irradiation and results in drastic reductionin photocatalytic degradation of oil [28,44]. Fig. 9 shows that kapp de-clined with increasing initial feed concentration and the reaction rateat 250 ppmwas remarkably higher compared to 10,000 ppm. This indi-cates that photocatalytic degradationwas seriously deteriorated at highoil concentration.

on of PVDF-TiO2 composite membrane in the sMPR system (operating conditions: temper-um pump flow rate = 15 ml/min and pH= 7).

2.9

2

1

0.2

0

0.5

1

1.5

2

2.5

3

3.5

250 1000 5000 10000

k app

(×10

-3m

in-1

)

Oil concentration (ppm)

Fig. 9. Apparent rate constant (kapp) for TOC degradation at different feed oilconcentrations.

1.3

1.9

2.8

3.5

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 3 5

k app

(×10

-3m

in-1

)

ABFR (L/min)

Fig. 11. Apparent rate constant (kapp) for TOC degradation at different ABFR.

55C.S. Ong et al. / Desalination 353 (2014) 48–56

3.5. Effect of air bubble flow rate (ABFR)

Fouling is always more significant when membrane is used to treatoily wastewater of relatively high concentration. Therefore, in order toeffectively alleviate this problem, the effect of ABFR on the membraneperformance and oil degradation during sMPR process was investigatedand the results are presented in Fig. 10. As can be seen from Fig. 10(a),the average membrane flux was found to increase with increasingABFR fromzero to 5 L/min. Theflux improvement at higher ABFR is like-ly due to the generation of circulation flow in the sMPRwhich limits theoil adsorbed onto themembrane surface and further reduces themem-brane fouling tendency [45].

Fig. 10(b) shows the enhancement of TOC degradation when higherABFR was applied in sMPR system. The detailed mechanism can be ex-plained by the following pathways (Eqs. (7)–(10)). The formation ofmoreOH∙radicals is promptedwith larger number of oxygen bubbles re-sulted from higher ABFR (Eqs. (7)–(8)). Meanwhile, large amount ofOH∙ radicals is formed due to dissociation of water molecules upon UVillumination (Eq. (9)). These OH∙radicals will mineralize those hydro-carbon groups in the oilywastewater to become CO2 andH2O (Eq. (10)).

With air flow (O2 ~ 78%) under 365 nm irradiation [46]:

Oxygen bubbles →hv

Dissolved oxygen→hv

Ozone O3ð Þ ð7Þ

OzoneðO3Þ→Oð1dayÞ þ H2O→2OH∙ðformation of hydroxyl radicalsÞð8Þ

H2O→hv

H � þOH� ð9Þ

Oil þ 3OH∙→xCO2 þ yH2OðmineralizationÞ: ð10Þ

Fig. 10. Effect of ABFR on (a) permeate flux and oil rejection and (b) TOCdegradation of PVDF-Timembrane type: PVDF with 2 wt.% TiO2, module packing density = 35.3%, initial feed concent

The enhanced TOC degradation at higher ABFR was further con-firmed by kapp shown in Fig. 11. Based on these findings, it can be con-cluded that air bubbling under UV irradiation was crucial for theenhancement of TOC degradation. It is worth mentioning that high oilrejection was consistently achieved regardless of ABFR, indicating thatsmaller membrane pore size is the dominant factor in ensuring excel-lent oil separation process.

4. Conclusion

A laboratory-scale sMPR exhibited remarkably improved perfor-mances not only in degrading synthetic cutting oil wastewater butalso in producing permeate of high quality at relatively low operatingcost. The results showed that TOC degradation using PVDF-TiO2 mem-brane was remarkably higher compared to neat PVDF membrane. Fur-thermore, GC-MS analyses showed that synthetic oily wastewatercould be efficiently degraded using PVDF-TiO2 composite membraneunder UV irradiation. The influence of several key operating parameters,such as TiO2 loading, membrane module packing density, initial feedconcentration and ABFR onmembrane performances were investigatedbased on permeate flux and its quality. It was found that both TOC deg-radation and membrane flux were seriously deteriorated when exces-sive TiO2 loading and module packing density were used. Althoughthicker oil layer tended to form at high feed concentration which ad-versely affected photocatalytic degradation and water flux, the intro-duction of higher ABFR could reduce the impacts to certain extent.With respect to TOC degradation and oil rejection, the in-house mademembrane could achieve 80% TOC degradation and N90% rejectionwhen it was operated under optimized conditions. Overall, this studynot only provides some useful information for the simultaneous

O2 compositemembrane in the sMPR system (operating conditions: temperature=25 °C,ration = 1000 ppm, vacuum pump flow rate = 15 ml/min and pH= 7).

56 C.S. Ong et al. / Desalination 353 (2014) 48–56

separation and degradation of oily wastewater, but also facilitates thedevelopment of hybrid sMPR in real oily wastewater industry.

Acknowledgments

The authors gratefully acknowledge the financial support by theLIMPID FEP-7 Collaborative European Project Nanocomposite Materialsfor Photocatalytic Degradation of Pollutants (Project number: NMP3-SL-2012-310177) and Fundamental Research Grant Scheme (Projectnumber: 4F306).

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