Algal Research 2 (2013) 437–444

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Algal Research

j ourna l homepage: www.e lsev ie r .com/ locate /a lga l

A comparative study of microfiltration and ultrafiltration foralgae harvesting

Xuefei Sun a, Cunwen Wang a,⁎, Yanjie Tong a, Weiguo Wang a, Jiang Wei b,⁎a Wuhan Institute of Technology, Xiongchu Avenue 693, Wuhan, PR Chinab Alfa Laval Nakskov A/S, Stavangervej 10, DK-4900 Nakskov, Denmark

⁎ Corresponding authors.E-mail addresses: wangcw118@hotmail.com (C. Wang

(J. Wei).

2211-9264/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.algal.2013.08.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 March 2012Received in revised form 26 June 2013Accepted 20 August 2013Available online 13 September 2013

Keywords:AlgaeMicrofiltrationUltrafiltrationHarvestingFouling

The present work deals with the filtration and concentration of algae (Chlorella) from a diluted culture mediumusing six commercial microfiltration membranes (MFP2, MFP5 and MFP8 with different pore sizes) and ultrafil-tration membranes (FS40PP, FS61PP and ETNA10PP with different Molecular Weight Cut-Off (MWCO)). Theeffects of the operating conditions, e.g. feed solution temperature, TMP (transmembrane pressure), VCF (volumeconcentration factor) and cross-flow velocity on the filtration performance were investigated. The resultsshowed that permeate fluxes increased with the increase in feed solution temperature, and the fluxes wereprobably limited by released extracellular polymeric substances (EPS) at higher temperatures. The permeatefluxes increased slowly with increasing TMP up to a certain limit, and after that the fluxes were stable or evendecreased. The higher cross-flow velocity can significantly decrease particles accumulating on the surface ofmembrane, and thus leading to higher permeate flux. Although ETNA10PP exhibited much less fouling thanother membranes, the permeate flux of this membrane was not higher than other membranes most likely dueto the fact that this membrane is the ‘tightest’ membrane with MWCO 10,000. The performance of UF and MFmembranes was compared for this application. The interesting finding of our work is that microfiltration andultrafiltration showed very similar performance in terms of permeate flux under the same operation conditionsat low TMP.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, there has been increasing interest for the productionof biofuels recognizing algae biomass as the raw material [1,2]. Theproduction of biofuels through microalgae has not only attended tothequest for renewable energy source, it also has enormous commercialpotential due to the growth rates of microalgae [3]. Microalgae can becultivated in seawater [4], saline–alkali water [5], agricultural sewage[6] and industrial wastewater [7–9]. More recently, sources of woodymaterial (Lignocellulose hydrolysates) have been considered to be anattractive feedstock for microalgae cultivation, which are the mostwidespread sources of carbon in nature. However, the harvest ofmicroalgae biomass is still a major problem because of the small sizeof algae cells and low biomass concentration.

Although conventional methods, such as flocculation, flotation andcentrifugation have been used as processes for effective removal ofmicroalgae biomass fromculturemedium, there are still someproblemsremaining during practical operations. For example, chemical floccu-lents like alum and ferric chloride were used to harvest microalgae.However, chemical flocculation has not been used for large operations

), jiang.wei@alfalaval.com

ghts reserved.

[10]. Usually, flotation was used in combination with flocculation foralgae harvesting, but the cost of front flotation was estimated to be toohigh for commercial use [11]. Centrifugation and drying are currentlyconsidered too expensive due to low content biomass of the culturemedia.

Membrane technologies have been used for the removal of bacteria,viruses and other microorganisms [12]. As manufacturing techniquesimprove and the range of applications expands, the cost of membranesand membrane systems have steadily decreased, which may make itpossible to use membrane technology for microalgae harvesting. Mostimportantly, membrane filtration can achieve complete removal ofalgae from the culture media [12]. Different membrane filtration tech-nologies have been used for the removal or concentration ofmicroalgae.Zhang [13] evaluated the feasibility of using a cross-flow membraneultrafiltration process to harvest and dewater algae suspension, andthe microalgae was concentrated 150 times and final algae concentra-tion reached 154.85 g/L. Hung [14] studied how operating parametersaffect microfiltration and examined the effect of preozonation on fluxbehavior when using hydrophobic and hydrophilic membranes. Zou[15] investigated the effect of physical and chemical parameters onforward osmosis (FO) fouling during algae separation. In addition, theeffect of solute reverse diffusion on FO fouling was systematically stud-ied. Pressure-driven microfiltration (MF) and ultrafiltration (UF) mem-brane processes are prone to fouling and are relatively energy intensive,

Table 1Membrane type and characteristics.

Membrane process Type Pore size pH Pressure, (bar) Temperature (°C) Material

MF MFP2 0.2 1–12 1–10 0–75 Fluoro polymerMFP5 0.45 1–12 1–10 0–75 Fluoro polymerMFP8 0.8 1–12 1–10 0–75 Fluoro polymer

UF FS40PP MWCO = 100,000 1–11 1–10 0–75 Fluoro polymerFS61PP MWCO = 20,000 1–11 1–10 0–75 Fluoro polymerETNA10PP MWCO = 10,000 1–11 1–10 0–75 Composite Fluoro polymer

Fig. 1. Schematic diagram of experimental system, showing feed (A), cooling/healing (B),pump (C), pressure (D), permeate (E), pressure (F), retentate (G), control value (H).

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while the FOmembrane process showed a very low permeate flux [16].There were a few reports concerning comparison of MF and UF formicroalgae filtration. Chow et al. [16]. compared microfiltration andultrafiltrationmethods and found both techniques attractive for removalof cyanobacterial cells. Rossignol [17] comparedMF andUF technologiesfor continuous filtration of microalgae. The results showed that,although the pure water fluxes of microfiltration membrane werehigher, during separation of microorganisms, fluxes of the ultrafiltrationmembrane became higher than microfiltration membrane.

The effectiveness of membrane separation is greatly affected byfouling. It can be further explained that the accumulation of microor-ganisms on membrane surface or in membrane pores causes declinein permeate flux [18]. Many efforts have been made to understandand reduce fouling, including membrane surface modification andnewmembrane material development [19,20]. Conventional polymericmaterials membranes have been widely used in filtration and concen-tration of microalgae [13,21–23]. Rossignol [24] evaluated the perfor-mances of inorganic filtration membranes. Liu [25] utilized a thin,porous metal sheet membrane to harvest microalgae, which exhibitedhigh properties of membrane area packing density, chemical stability,thermal stability, mechanical strength, high permeability and low cost.

The purpose of our work is to compare the performance ofmicrofiltration and ultrafiltration for algae harvesting by usingmicrofiltration (MF)membraneswith different pore size and ultrafiltra-tion (UF) membranes with different MWCO. All 6 types of the mem-branes used are Polyvinylidene Fluoride (PVDF) based, and ETNA10PPis a surfacemodified PVDFmembrane. ETNA10PP is the onlymembranewith hydrophilic surface [26], which is supposed to show lower foulingtendency. Our intention is to investigate the influence of membranematerials (hydrophobic versus hydrophilic), membrane pore size, andporosity on performance. We have studied how operating parametersaffect MF and UF filtration. MF and UF experiments were carried outseparately including 3 kinds of membranes in each test. Then, theperformance of themicrofiltrationmembrane (MFP8) and ultrafiltrationmembrane (FS40PP)were compared in the same test for thefiltration ofChlorella. The effect of VCF (Volume Concentration Factor = Totalstarting feed volume / retentate volume) on permeate flux was alsostudied during the concentration process of Chlorella.

2. Materials and methods

2.1. Microalgal suspensions

Chlorella pyrenoidosa FACHB-9 cells were cultivated in an open culti-vation system, provided by Algae Innovation Center of Denmark. Thefresh cultures were taken in the middle of the exponential growthphase. Then algae cells were placed in a refrigerator and stored underdarkness at 4 °C. The pH of the culture was 9.0 ± 0.5. In order to com-pare the performance of the tested membranes, all comparative exper-iments have been carried out with the same cell concentration level,0.68 g/L.

2.2. Membrane characteristics

Different commercialMF andUFmembranes fromAlfa Laval NakskovA/S were used in the experiments, using Alfa Laval's cross-flow

membrane module M10 (a small lab-scale membrane module).Performance of different membranes can be compared according to thepermeate flux and cell retention. The membrane characteristics areshown in Table 1.

2.3. Experimental set-up

The schematic diagram of the membrane module is shown in Fig. 1.The membrane module consists of four plates kept together with fourbolts. Themodule contains fourflat-sheetmembrane samples operatingin series, with each having an effective filtration area of 0.0084 m2. Inlet(Pin) and outlet pressures (Pout) aremeasuredwith pressure transducers(D) and (F) mounted on the inlet and outlet of the membranemodule. The transmembrane pressure (TMP) was calculated as TMP =(Pin + Pout) / 2-Ppermeate. A diluted Chlorella culture medium was keptin the feed tank (G).

The membrane filtration was performed in a batch mode operationwith recycling of permeate and retentate back to the feed tank to simu-late a continuous operation. The permeate flow rate was measured bymeasuring the collected permeate in a 500 ml cylinder over a time of60 s. The flux data were measured 2 times to get an average value foreach measurement. The total test time for each membrane test was4.5 h. After each experiment, the M10 module was cleaned withcleaning agents Ultrasil 10 (from Ecolab) for approximately half anhour at 55 °C.

3. Results and discussion

3.1. Effect of temperature

In most microfiltration and ultrafiltration processes, permeate fluxincreases with increasing feed solution temperature [27]. The effect oftemperature on permeate flux may be attributed not only to the effectof temperature on the physical properties (viscosity, solubility, etc.) of

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feed suspension [28], but also to the complex physical changes thatmaybe occurring in the membrane as the temperature is changed [29]. Forthe solution of Chlorella with relatively high cell density, the impact oftemperature on the permeate flux becomes particularly complex. Tem-perature plays an important role in the release of EPS (extracellularpolymeric substances), which accumulates on the membrane surfaceand causes the flux to decline [30].

Fig. 2 shows the effect of temperature on the permeate flux inmicrofiltration and ultrafiltration of Chlorella solution. The temperatureof the feed suspension was varied while transmembrane pressure andcross-flow were kept constant at 1.3 bar, 3.86 m/s (microfiltration),and 2.3 bar, 7.72 m/s (ultrafiltration), respectively. In this process, thetemperature of the feed suspension ranges from 20 °C to 28 °C, whichis within the normal temperature range of the growth of Chlorella. AsFig. 2 demonstrates, membrane permeate flux is sensitive to changesin feed solution temperature. When the solution temperature is 20 °C,the viscosity is higher and the diffusion coefficient is lower, resultingin a relatively low permeate flux. With increasing temperature, theflux of the MF and UF membranes also increases. However, as the tem-perature increases from 24 °C to 28 °C, the permeate fluxes of all MF

Fig. 2. Effect of temperature on permeate flux in cross-flow microfiltration (MFP2, MFP5 and1.3 bar, cross-flow = 3.86 m/s (microfiltration); and TMP = 2.3 bar, cross-flow = 7.72 m/s (

membraneswere similar to each other. It is possible that higher temper-atures favor the metabolism of the Chlorella, and thus concentrations ofextracellular polymeric substances (EPS), e.g., proteins andnucleic acidsincrease in the feed solution [31]. These substances could adsorb themembrane surface, leading to the permeate flux decreasing. The opti-mum temperature for filtration was found to be 24 °C, at which pointthe best growth state of Chlorellawas observed. The change in permeateflux for all membranes shows similar patterns. Typically flux versustime curves show a relatively rapid flux decline in the first 2 h of theprocess, followed by a more gradual decrease, until a steady-state fluxhas been reached.

3.2. Effect of transmembrane pressure (TMP)

Fig. 3 shows the variation of flux with time under different trans-membrane pressures. In most cases, an increase in pressure leads toan increase of the permeate flux. However, with the microfiltrationmembranes only a slight increase was observed as the transmembranepressure increased from 1.3 bar to 1.8 bar. Similar results can be seenfor ultrafiltration. The fluxes increased significantly from TMP of

MFP8) and ultrafiltration (FS40PP, FS61PP and ETNA10PP). Filtration conditions: TMP =ultrafiltration).

Fig. 3. Effect of transmembrane pressure on permeate flux in cross-flowmicrofiltration (MFP2,MFP5 andMFP8) and ultrafiltration (FS40PP, FS61PP and ETNA10PP). Filtration conditions:T = 20 °C, cross-flow = 5.79 m/s (microfiltration); and T = 24 °C, cross-flow = 7.72 m/s (ultrafiltration).

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1.3 bar to 1.8 bar for UF membranes, but not from 1.8 bar to 2.3 bar.Therefore we can assume that there is an optimal pressure, afterwhich further increase in transmembrane pressure will not improveflux. Higher permeate fluxes were observed at the beginning of bothmicrofiltration and ultrafiltration processes, but then the permeatefluxes declined rapidly. The permeate fluxes declined more rapidlywith increasing transmembrane pressure. As shown in Fig. 3, althoughthe initial permeate fluxes of the ultrafiltration membrane at 2.3 barwas higher than the flux at 1.8 bar, the decline rate of the permeateflux was faster at 2.3 bar. The permeate flux at 2.3 bar decreased evenfurther below the flux at 1.8 bar. Such a phenomenon has alreadybeen observed with other biological suspensions (bacteria, apple juice,etc.) due to the presence of polysaccharides in the feed solution [32–34].

Chlorella cells can attach to the membrane surface, which can beseen by visually checking the fouled membrane surfaces. The attachedcells could release a secretion and EPS [30], which might be enhancedat higher transmembrane pressure. The higher pressure can add addi-tional resistance to permeation by compressing the Chlorella cells andEPS into a thicker and denser fouling layer. Further, according toMakardij [35], at high transmembrane pressure, the membrane pore

size and EPS layer porosity decrease, resulting in increase of the cakelayer and hence, more rapid flux decline.

3.3. Effect of cross-flow velocity

The cross-flow velocity is another important parameter which hasan influence on microfiltration and ultrafiltration performance. Fig. 4indicates the effects of cross-flow on performance of the membranes.As the cross-flow velocity increased, the permeate fluxes increased,suggesting that Chlorella and other particles were prevented fromaccumulating on the surface of membrane. During the initial stage ofChlorella filtration, the permeate fluxes are seen to be independent ofcross-flow velocity. As shown in Fig. 4, the initial permeate fluxes ofmicrofiltration and ultrafiltration (FS40PP and FS61PP) are nearlyidentical under different cross-flow velocities. However, as the cakeresistance increased after the first two-hour process, cross-flow velocityshowed a more pronounced effect on permeate fluxes, whereby thesteady-state fluxes increased with the cross-flow velocity. This can beexplained by less particles depositing onto the membrane surface athigh cross-flow velocity. ETNA10PP is a surface-modified PVDF

Fig. 4. Effect of cross-flow velocity on permeate flux in cross-flowmicrofiltration (MFP2, MFP5 and MFP8) and ultrafiltration (FS40PP, FS61PP and ETNA10PP). Filtration conditions: T =20 °C, TMP = 1.3 bar (microfiltration); and T = 24 °C, TMP = 2.3 bar (ultrafiltration).

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membrane, which can reduce fouling by rendering the membrane sur-face hydrophilic whereby it can be cleaned without using cleaningagents [26]. The results in Fig. 4 demonstrate that, after a 4-hour filtra-tion run, an average drop of 10.5% in permeate flux for ETNA10PPmem-branewas observed,while fluxdrops for FS40PP and FS61PPwere 40.7%and 33.3%, respectively. The membrane fouling degree of ultrafiltrationis shown in Fig. 5. It is very obvious that the accumulated material(green in the photos) on the surface of ETNA10PP was much less thanfor FS40PP and FS61PP.

3.4. Direct comparison between microfiltration and ultrafiltration

To compare the performance between microfiltration and ultrafil-tration techniques, a longer period experiment (in recirculation mode)with total 48-hour run was carried out. The highest flux microfiltration

membrane (MFP8) and the highest flux ultrafiltration membrane(FS40PP) andETNA10PPwere testedunder optimal operating conditions(5.79 m/s, 1.8 bar and 24 °C). These operating conditionswere chosen toreduce Chlorella cell damage due to filtration and provide suitable sur-viving conditions for Chlorella. The surface-modified PVDF membraneETNA10PPwas used here to determine whether it has higher permeateflux compared with MFP8 and FS40PP in longer period tests due to itslow fouling properties. Fig. 6 shows performance of this experimentduring a 48-hour run. Significant differences were observed betweenthe microfiltration and ultrafiltration membranes. The MF membraneexhibited a higher initial permeate flux than the UF membranes. How-ever, a sharp decline in flux was observed for the MF membrane in thefirst three hours of the process, resulting in the permeate flux of MFP8being similar or only slightly below FS40PP. Such a rapid drop of initialpermeate flux for the MF membrane may be due to the higher initial

Fig. 5. Photos of used membranes after 4.5 h run: a. FS40PP; b. FS61PP; c. ETNA10PP.Green seen on the photos are foulant material. Red seen on ETNA10PP (c) is the color ofmembrane itself.

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flux of the MF membranes which leads to higher fouling tendency.However, our results shown in Figs. 2, 3 and 4 do not indicate betterperformance of UF membranes over MF membranes as reported byRossignol [17]. The discrepancy observed in the two studies could bedue to different algal types, different membrane types and differentõoperating conditions used by two groups. Membrane flux could beinfluenced bymany parameters. All experimentswe conducted showedvery similar flux level for MF and UF membranes.

Although ETNA10PP exhibited much lower membrane foulingbecause of its anti-fouling properties, the steady state flux was in factstill lower than MFP8 and FS40PP. It was hypothesized that the flux ofthis membrane in long period tests may be higher than other mem-branes due to its low fouling properties. However, the hypothesis wasnot proven in this experiment.

3.5. Concentration of Chlorella suspensions

The above filtration experiments in recirculation modewere carriedout to find the MF or UFmembranewith the best performance (highestpermeate flux and best rejection of Chlorella). The final goal of thiswork is to use MF or UF membranes for the concentration of Chlorellasuspensions.

Fig. 6. Direct comparison betweenmicrofiltration (MFP8 and ultrafiltration membranes (FS40PP

In this study, the volume concentration factor (VCF) is defined as:

VCF ¼ V0= V0−Vtð Þ

where V0 and Vt are the initial feed volume (32 L) and feed volume attime t, respectively. The relationship between VCF and permeationflux was studied. Experiments were performed with the microfiltrationmembrane (MFP8) and ultrafiltration membranes (FS40PP andETNA10PP). The results in Fig. 7 demonstrated that permeate fluxesdeclined much faster at low Chlorella cell concentrations during theinitial stages of the experiment. As the process continued, a stable fluxwas reached at higher Chlorella cell concentrations. It is believed thatthe cake layer (or fouling layer) probably became thicker and denserwith increasing Chlorella concentration at high cell concentrationrange, and thus reduced permeate flux. A stable flux was observedwhen the cake layer did not change at higher cell concentrations.There is a larger flux drop for MFP8 than the other membranes at theearly stages of the concentration, which can be explained by the largerpore size of the microfiltration membrane However, the steady-stateflux seems to be similar for MFP8 and FS40PP, and the flux is slightlyhigher than for ETNA10PP. After 130 min of concentration, the feedvolume in the batch tank was decreased from the initial 32 L to 2.8 L,resulting in a VCF of 11.4.

Since MF membranes have much more open pore structure andmuch higher porosity than UF membranes, one would expect that MFmembranes show much higher permeate flux for algae harvesting.The results in Figs. 6 and 7 show very similar performance of MFP8(pore size of 0.8 μm) and FS40PP (MWCO 100,000), indicating thatthe pore size of membranes is not as an important parameter for thiskind of application as originally hypothesized. A possible explanationfor these results is that the fouling layer caused by deposition of algaecells and EPS was acting as a membrane selective layer [36]. The foulinglayer can be clearly seen in Fig. 5 for FS40PP. The results in this workfurther suggest that the fouling layer, as a membrane selective layer, issimilar for MF and UF membranes, implying that pore size and porosityhave little influence on the formation of the fouling layer. AlthoughETNA10PP showed lower permeate flux than MFP8 and FS40PP inFigs. 6 and 7, the membrane exhibited very low fouling tendency asthe permeate flux was rather stable during the whole filtration period.The low fouling tendency was further shown by very little depositionof foulants on the membrane surface, as seen in Fig. 5. In fact, the lowfouling tendency of thismembrane can be expected, owing to its hydro-philic membrane surface. The lower permeate flux of ETNA10PP can beconsidered to be due to the higher flow resistance of the membrane asthis is a ‘tight’ (much smaller pore size) UF membrane. It may be

, ETNA10PP). Filtration conditions: T = 24 °C, TMP = 1.8 bar and cross-flow = 5.79 m/s.

Fig. 7. Permeate flux versus VCF for microfiltration (MFP8) and ultrafiltration (FS40PP, ETNA10PP) membranes. Filtration conditions: T = 24 °C, TMP = 1.8 bar and cross-flow =5.79 m/s.

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concluded from our work that membrane materials play a very impor-tant role for this application. Membranes with hydrophilic surfaceswill most likely show better performance from the viewpoint of reduc-ing fouling, whereas hydrophobic membranes will experience severefouling for algae filtration.

4. Conclusion

The filtration and concentration of Chlorella from dilute culturemedia and the performance of several commercial MF and UF mem-branes were evaluated and compared. Flux increased with increasingtemperature of the feed solution. However, above a certain tempera-ture, further increase in temperature didn't improve permeate flux,most likely because of the release of EPS by Chlorella cells and/or depo-sition of Chlorella cells. A general trend of increased permeate flux withincreasing TMP was observed. At higher TMP, however, permeate fluxgradually leveled off or even dropped due to the effect of a fouling(cake) layer development. It was also seen that higher cross-flow veloc-ity can significantly decrease particles accumulating on the membranesurface. Although ETNA10PP exhibitedmuch better anti-fouling proper-ties, the steady-state permeate flux of this membrane was not higherthan for MFP8 and FS40PP in a direct comparison test. However, thisis believed to be due to the much lower pore size i.e. MWCO (10,000)of ETNA10PP rather than limitation due to fouling layer buildup (seebelow). Furthermore, the concentration experiments indicate that theMF membrane did not show higher permeate flux than the UF mem-branes under the same operation conditions. MF membranes and UFmembranes show similar flux in this work, indicating that pore sizeand porosity are not important for this application. This suggests thatthe permeate flux of different membranes is controlled by the foulinglayer that acts as the membrane selective layer. Our work also demon-strated that a membrane with hydrophilic surface shows very littlefouling for algae harvesting. The results of ETNA10PP suggest thatmem-branematerials are themost important parameters for reducing foulingtendency. In future work, we will make analysis on the deposition layer(or EPS) attached to membrane surfaces to understand the membranefouling better.

Acknowledgments

Xuefei Sunwishes to acknowledge the Alfa Laval Nakskov A/S for thesupport of the work. Authors would like to thank Jørgen EnggaardBoelsmand at the Algae Innovation Center of Denmark for providingalgae suspensions and helpful discussions. We would also like to

thank Gary Lloyd for proof reading and corrections of the manuscript.This work was supported by the National Natural Science Foundationof China (Grant No. 20976140).

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