Algal Research 16 (2016) 458–464
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Algal Research journal homepage: www.elsevier.com/locate/algal
The impact of temperature on membrane fouling in algae harvesting Huaqiang Chu, Fangchao Zhao, Xiaobo Tan, Libin Yang, Xuefei Zhou, Jianfu Zhao, Yalei Zhang ⁎ State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China
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Article history: Received 14 December 2015 Received in revised form 25 February 2016 Accepted 16 April 2016 Available online 23 April 2016 Keywords: Algae harvesting Temperature Flux Membrane fouling EOM
a b s t r a c t Temperature has a signiﬁcant impact on both the secretion of extracellular organic matter (EOM) and the performance of membrane ﬁltration, which inﬂuences the efﬁciency of algae harvesting by membrane technology. In this study, the performance of membranes for microalgae harvesting was investigated under different temperatures. The results showed that the critical ﬂux (Jc) increased from 27.0 to 42.0 L/(m2·h) when the solution temperature increased from 15 to 35 °C. In continuous ﬁltration tests, membranes at high temperature had a low ﬂux decline. At 35 °C, the decrease rate of ﬂux was 59.7%, whereas at 15 °C, the decrease rate of ﬂux was 85.5%.The higher ﬂux and lower membrane fouling at higher temperature were partly due to the decline of liquid viscosity, which further decreased the permeate drag force (FD). Lower FD retarded the deposition of particles on the membrane surface, which reduced the quantity of algae deposited on the membrane. At low temperatures, the high content of EOM was another possible cause of membrane fouling. At 15 °C, the algae solution contained a high concentration of total extracellular organic matter (TEOM), 263.2 mg/L; moreover, the speciﬁc EOM secretion rate (SEOM) decreased with a temperature increase from 15 to 35 °C. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Biofuel can reduce global warming caused by fossil fuels and has very promising application prospects . Microalgae are a good source for biofuel, but there are still substantial challenges in achieving efﬁcient harvesting for commercial use. Membrane technology is a promising separation and ﬁltration process due to its stable and clean efﬂuent water . Currently, a growing number of researchers have studied how to make better use of membranes for microalgae harvesting [3,4]. However, the primary problem of membrane technology is membrane fouling . During ﬁltration, algae cells and extracellular organic matter (EOM), can accumulate on the membrane surface, and some even enter into the pores of the membrane, resulting in a decrease of membrane permeability and an increase of ﬁltering resistance . Currently, EOM in either the bound or dissolved form is considered a signiﬁcant cause of membrane fouling in the algae harvesting process . Qu et al.  found that ﬂux decline and irreversible membrane fouling were more serious when microalgae were combined with EOM. However, in our previous research, it was found that seasonal changes of temperature had a signiﬁcant inﬂuence on algae growth in Abbreviations: EOM, extracellular organic matter; dEOM, dissolved extracellular organic matter; bEOM, bound extracellular organic matter; TEOM, total extracellular organic matter; SEOM, the speciﬁc EOM secretion rate; STEOM, the speciﬁc TEOM secretion rate; SdEOM, the speciﬁc dEOM secretion rate; SbEOM, the speciﬁc bEOM secretion rate; JC, critical ﬂux; TMP, transmembrane pressure; IFM, improved ﬂux-step method; JL, low ﬂux; JH, high ﬂux; JI, a ﬁxed initial ﬂux; ΔJ, the speciﬁc rate of change of ﬂux; JC35, critical ﬂux at 35 °C; FD, the permeate drag force; Rm, the membrane hydraulic resistance. ⁎ Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.algal.2016.04.012 2211-9264/© 2016 Elsevier B.V. All rights reserved.
outdoor cultivation [9,10]. Temperature also has important effects on permeate ﬂux and membrane fouling l. Particularly, for outdoorcultured microalgae, the effect of temperature on microalgae harvesting is more evident. Eziyi et al.  reported that the membrane ﬂux was highly sensitive to temperature variations. Within a range of 33 to 36 °C, a 1 °C decline of liquid temperature can result in a decrease of approximately 1 L/(h·m2) of permeation ﬂux . However, the impact of temperature on the permeate ﬂux is complicated when the algae solution contains a relatively high concentration of cells and EOM. Within a suitable temperature range, the higher the temperature, the faster the metabolism and the more EOM the microalgae releases [4,13]. It is possible that with an increase in temperature, the permeate ﬂux of the membrane does not increase because of the aggravated membrane fouling at high levels of EOM in the feed. Although several researchers have studied the effect of temperature on membrane fouling, microalgae growth or secretion of EOM [11–14], there is little literature about membrane fouling in association with the performance of algae at different temperatures. Lee et al.  showed that high water temperature can reduce the viscosity of solution and thus increase the permeability of the membrane. However, how and why the viscosity inﬂuences the performance of the membrane has not been reported. Therefore, to determine the appropriate operation temperature to reduce membrane fouling and to understand how temperature inﬂuences the performance of microalgae harvesting by membrane ﬁltration, it is necessary to investigate the effect of temperature on the performance of the membrane for microalgae ﬁlters. Tan et al.  reported that when temperature was above 40 °C or below 10 °C, the algae growth rate was very low. Therefore, in this study, 15, 20, 25, 30 and 35 °C use to study the changes of membrane fouling,
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permeate ﬂux, algae growth and EOM concentration. First, at different temperatures, the critical ﬂuxes were measured; meanwhile, the calculation of the permeate drag force at different temperatures was used to explain why high temperature could reduce membrane fouling. Second, we studied the changes of the concentration of algae and EOM under different temperatures. Finally, to further verify the effects of temperature on the variation in ﬂux and membrane fouling, a continuous ﬁltration experiment was conducted. Through this study, we hope to provide some valuable information to further the understanding of why and how temperature inﬂuences the performance of the membrane and membrane fouling in algae harvesting; in addition, we also hope to provide a reference for algae harvesting by membrane technology. 2. Materials and methods 2.1. Cultivation of microalgae Chlorella pyrenoidosa (C. pyrenoidosa, FACHB-9) was obtained from the Institute of Hydrobiology at the Chinese Academy of Sciences in China. C. pyrenoidosa was cultured in Basal medium prepared in sterile distilled water. The Basal medium consisted of KNO3 (1250 mg/L), KH2PO4 (1250 mg/L), MgSO•47H2O (1000 mg/L), EDTA (500 mg/L), H3BO3 (114.2 mg/L), CaCl•22H2O (111 mg/L), FeSO•47H2O (49.8 mg/L), ZnSO•47H2O (88.2 mg/L), MnCl•34H2O (14.2 mg/L), MoO3 (7.1 mg/L), CuSO•45H2O (15.7 mg/L), Co(NO3)•36H2O (4.9 mg/L) and glucose (1000 mg/L). The pH of the culture solution was adjusted to 6.1 using NaOH before being inoculated with algae. The algae was inoculated in 3 L conical ﬂasks containing 2 L of autoclaved Basal medium and placed in an incubator (GZX-300BS-III, CIMO Medical Instrument, Shanghai, China). All experiments were conducted in three parallel cultures. The initial concentration of algae was 0.08 g/L. The temperatures of the incubators were maintained at 15 ± 0.5, 20 ± 0.5, 25 ± 0.5, 30 ± 0.5 and 35 ± 0.5 °C, respectively. The culture conditions were as follows: light/dark = 14 h/10 h, light intensity = 127 μmol/m2·s. For the continuous ﬁltration experiments, algae were cultivated in transparent plastic buckets, with 20 L placed in the incubator under different temperature conditions. 2.2. Extraction and analysis of extracellular organic matter (EOM) The extracellular organic matter (EOM) was divided into boundEOM (bEOM) and dissolved-EOM (dEOM). Algae solution (5 mL) was ﬁrst centrifuged at 4000 rpm for 15 min in a centrifuge (CT15RT, Shanghai, China), and the supernatant was ﬁltered through a 0.45 μm ﬁlter to obtain the dEOM. For the bEOM extraction, the residual algae (added to 5 mL using distilled water) was centrifuged at 10,000 rpm for 15 min and subsequently ﬁltered through a 0.45 μm ﬁlter to obtain the bEOM [8,16]. C. pyrenoidosa was cultured for 11 days, and the biomass and EOM (including dEOM and bEOM) were collected for daily analysis. The concentration of C. pyrenoidosa was measured using the OD680 method. Polysaccharides and proteins were the main compositions in EOM (accounting for 90% of EOM) and had a signiﬁcant inﬂuence on membrane fouling [17,18]. Polysaccharides and proteins in dEOM were named dpolysaccharides and d-proteins, respectively; polysaccharides and proteins in bEOM were named b-polysaccharides and b-proteins, respectively. The contents of the proteins and polysaccharides were measured using the anthrone-sulfuric acid method and the modiﬁed Lowry method, respectively . To determine the capacity of EOM release by C. pyrenoidosa at different temperatures, a speciﬁc EOM secretion rate (SEOM) was used, as in Eq. (1):
where SEOM is the speciﬁc EOM secretion rate (mg/g), CEOM is the concentration of EOM (mg/L), and Calgae is the concentration of C. pyrenoidosa (g/L).
2.3. Reactor set-up The ﬁltration experiments were all conducted in a lab-scale ﬁltration tank, as illustrated in Fig. 1. The ﬁltration tank had a working volume of 2.5 L. A micro-porous pipe was located on the bottom to reduce membrane fouling. The volume of aeration was 30 m3.h−1.m− 2. In this study, a hydrophilic PVDF membrane (pore size of 0.1 μm) was used. The membrane had an effective ﬁltration area of 0.02 m2. The ﬁltrate was pumped by a peristaltic pump (BT100-LJ, Kejian, China). The ﬂow velocity was automatically recorded by an electronic balance connected to a computer. A vacuum meter was installed on the module measuring the transmembrane pressure (TMP).
2.4. Filtration experiments 2.4.1. Critical ﬂux test Critical ﬂux (JC) is a quantitative parameter for the ﬁlterability of a membrane, which is generally regarded as the ﬂux above which the membrane fouling rate will be aggravated [20,21]. Practically, a higher ﬂux will cause serious membrane fouling and a sharp decline in ﬂux, but a lower ﬂux affects the working efﬁciency of the membrane. Selecting an appropriate ﬂux under the critical ﬂux, namely, an appropriate sub-critical ﬂux, will prolong the service life of a membrane and reduce the cleaning frequency. In addition, the critical ﬂux can also be employed to compare the fouling propensities between membranes. Thus, it was essential to measure the critical ﬂuxes under different temperature conditions. The values of critical ﬂux were determined by an improved ﬂux-step method (IFM) . The algae used in the IFM tests were all cultured at 25 °C and had similar concentrations of 0.3 mg/L. During each test, one other algae sample was placed at the same temperature to verify if there was a difference in the content of the algae and EOM after several hours, and it was found that there was a small difference in every group. Thus, in IFM, the performances of the membranes were considered to be affected only by temperature. The permeate was measured and recorded using an electric balance and a computer and was subsequently recycled into the ﬁltration tank to keep the cell concentration constant. In IFM, a stepwise increasing ﬂux (JH) was applied and an intermediate decrease in ﬂux to a low ﬂux (JL) was employed after each JH [22,23]. In IFM, no fouling occurred at JL, and ﬁltration at JL was considered as a form of relaxation , although the real relaxation ﬁltration should be 0 L/m2h. However, a ﬂux larger than 0 L/m2h must be applied to measure a value for TMP before and after a JH . At the low ﬂux, the convective ﬂow towards the membrane is reduced and the foulants are removed due to air scouring. Several studies have shown that continuous air bubbling was able to control the deposition at a very low level [24,25]. At different temperatures, the JL of each module was identical (10 L/ (h·m2)). JH started from 15 and stepwise increased by 3 L/(h·m2), respectively. All JL and JH processes lasted 15 min . In this research, an arbitrary minimum increase in the TMP of 20 Pa min−1 was used to determine JC.
2.4.2. Continuous ﬁltration experiments During the ﬁltration process, the reactor temperatures were consistent with the cultivation temperature. The initial concentrations of the microalgae were all adjusted to 0.3 g/L (dry weight) by adding water in every group. During the test, the pipe became ﬂat in several hours if the ﬂux stayed at 30 L/(h·m2). To conduct a 24-h ﬁltration, the peristaltic pump was kept at a constant rotation speed .
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Fig. 1. Schematic diagram of the experimental setup in continuous ﬁltration experiment.
During the ﬁltration, a ﬁxed initial ﬂux (JI) was applied, as calculated using Eq. (2): JS ¼ 0:7JC35
where JC35 is the critical ﬂux determined at 35 °C (L/(h·m2)). To reduce the effects of the discrepancies between the experimental conditions as much as possible, the ﬁxed initial ﬂux (JI) was kept constant for the same membranes under different temperatures. One reason that 0.7JC35 was selected as JI is because the membranes had the highest critical ﬂux (JC) at 35 °C. The other reason was that 0.7JC35 approximated the value of JC at 15 °C, and there was sub-critical ﬂux at 20, 25, 30, and 35 °C that would prolong the ﬁltration time and reduce the fouling rate. During ﬁltration, the speciﬁc rate of change of the ﬂux was calculated using Eq. (3):
serious membrane fouling. When the membranes were at the same ﬂux, the values of TMP at low temperatures were higher than those at high temperatures. Huang et al.  also found that membrane resistance increased as the temperature decreased from 25 to 5 °C during wastewater treatment using an aerobic dynamic membrane bioreactor. In the descending phase, the membranes had higher TMP than the corresponding ﬂuxes in the ascending phase, which suggested that the membranes were already fouled in the ascending phase . In particular, at large ﬂuxes, the membranes had more severe fouling and higher TMP, which could not be completely removed by bubbles in the descending phase. When the ﬂux was 39 or 36 L/(h·m2) in the descending phase, the values of TMP were even higher than those at 42 L/(h·m2). The membrane was fouled more seriously due to the long test time and the high ﬂux. 3.2. Effect of temperature on the permeate drag force
dJ JI dt
where ΔJ is the speciﬁc rate of change of the ﬂux (1/s), J is the ﬁltration ﬂux (L/(h·m2)), JI is the ﬁxed initial ﬂux (L/(h·m2)), and t is the ﬁltration time (h). The speciﬁc rate of change of the ﬂux is divided into three stages: the rapidly declining stage (ΔJ N 0.1), the slowly declining stage (0.01 b ΔJ b 0.1) and the stable stage (ΔJ b 0.01) . After continuous ﬁltration experiments, algae deposited on the membrane was rinsed with water, and the concentration of the algae was measured, which was used to evaluate the effect of temperature on algae deposition. 3. Results and discussion
In membrane ﬁltration, algae particle deposition onto membrane is inﬂuenced by the permeate drag force (FD) [30,31]. The increase of FD enhances the velocity of particles towards the membrane, which accelerates the deposition of particles on the membrane surface, accelerating membrane fouling [30,32]. The permeate drag force is expressed as FD ¼ 6πμ w ra vw φH
where FD is the permeate drag force (nN), μw is the solution viscosity (Pa·s), ra is the particle radius (m), vw is the permeate water velocity (m/s), and φH (F/F∞) is the hydrodynamic correction factor [30,32]. The negative sign in Eq. (4) indicates the permeate drag force as attraction towards the membrane surface. φH was a complex function of the surface's Darcy permeability, separation distance, and particle radius [30–32]. This correction factor at the point of contact was described by
3.1. Effect of temperature on critical ﬂux Fig. 2 shows that JC increases along with the increase of temperature. The membrane permeate ﬂux was sensitive to the temperature change of the feed solution, and the viscosity decreased when the temperature increased [11,28]. Goosen et al.  found that the permeate ﬂux increased by 60% when the water temperature increased from 20 to 40 °C. In this study, the JC of the 0.1 μm membrane increased by 56%, when the algae solution temperature increased from 15 to 35 °C. As shown in Table 1, the membranes had the same JC of 30 L/(h·m2) at 20 and 25 °C, but the values of TMP were 1.1 and 0.5 kPa, respectively. That is to say, although some JC had the same value at different temperatures, the membranes with higher TMP at low temperatures had more
0:5 2Rm ra þ 1:0722 3
where Rm is the membrane hydraulic resistance obtained by Darcy's law (m−1). The preliminary results suggested that when the temperature increased, the critical ﬂux increased and the fouling rate decreased. The changes in the physical-chemical properties of the membrane, such as the pore size or the diffusivity of the solution in the membrane, may be the cause of the increase in the critical ﬂux at high temperatures . However, the solution viscosity change at different temperatures should not be neglected because it will affect the permeate drag force.
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Fig. 2. Flux-TMP proﬁles of the improved ﬂux-step method under different temperatures.
According to Eq. (4), the permeate drag force (FD) changes with the viscosity and Rm. Fig. 3 shows that the temperature has an obvious inﬂuence on the viscosity of the ﬂuid. When the temperature increases from 15 to 35 °C, the viscosity decreases from 2.08 to 1.30 mPa·s. The temperature also affects the membrane hydraulic resistance (Rm), but Lee et al.  found that Rm did not change with temperature due to the difference of the membranes. In this study, however, Fig. 3 shows that with increasing temperature, Rm declined. During the critical ﬂux test, the ﬂux was not constant, hence there was not a constant FD in the entire test. However, FD at different Table 1 The values of critical ﬂux and TMP when ﬂux reaching JC at different temperatures. Temperature (°C)
JC (L/(h·m2)) TMP (kPa)
temperatures could be compared at a given ﬂux. In this study, the average particle radius was 2 μm, and 20 L/(h·m2) was arbitrarily selected as the given ﬂux. Under the same ﬂux of 20 L/(h·m2), FD at the surface membranes increased with the decrease of temperature (shown in Fig. 3). The increase of FD enhanced the velocity of particles towards the membrane, which accelerated the deposition of particles on the membrane surface and aggravated membrane fouling [30,32]. Therefore, at the same ﬂux, with a decrease of temperature, more algae particles deposited onto the membrane, and membrane fouling was more severe, which was why the membrane had a high critical ﬂux at high temperature. 3.3. Effect of temperature on algae In the process of algae ﬁltering, accumulation of algal cells and EOM on the membrane surface can cause serious membrane fouling, leading to a remarkable decrease of membrane ﬂux . Temperature has a
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Fig. 3. At different temperatures the changes of viscosity; and the changes of membrane hydraulic resistances (Rm); and the changes of drag force at the surface of membranes with the same ﬂux of 20 L/m2h (the average particle radius is 2 μm).
signiﬁcant effect not only on membrane but also on algae growth and secretion of EOM. Therefore, tracking the trend of the algae and EOM concentration under different temperatures could contribute to explaining the cause of membrane fouling.
3.3.1. Effect of temperature on growth The growth performance of C. pyrenoidosa is different at different temperatures (shown in Fig. 4). The concentration of algae increased with the increase of temperature from 15 to 30 °C, and the highest concentration was 0.66 g/L (dry weight) at 30 °C. However, the concentration of biomass decreased to only 0.41 g/L when the culturing temperature subsequently increased to 35 °C. Han et al.  found that algae had a lower growth rate at 36 °C due to stronger respiration during the night. In the previous study, it was found that due to the lack of glucose, there was a low algae concentration at 35 °C, although
it had the fastest growth rate at the earlier stage with sufﬁcient glucose . 3.3.2. Effect of temperature on EOM During membrane ﬁltration, EOM, including the bound and dissolved forms, is considered to be an important factor in membrane fouling . Temperature is an important factor inﬂuencing the production of EOM. As shown in Fig. 4, the secretion of dEOM (d-polysaccharides and d-proteins), bEOM (b-polysaccharides and b-proteins) and total EOM (TEOM, including dEOM and bEOM) by microalgae varied at different culturing temperatures. At 15 °C, the algae solution had the lowest cell concentration, but the highest TEOM (263.2 mg/L). At low temperature, algae did not grow as fast as at high temperature due to the adjustment of synthesis, and the production of EOM was closely linked to synthesis . There were also reports that at low temperature, the ﬁxed carbon was transferred to
Fig. 4. Concentrations of total EOM and algae; and speciﬁc EOM secretion rate (SEOM) under different temperature conditions on the 11th day.
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EOM and overﬂow metabolism occurred . As the temperature increased, the growth of the algae cells gradually recovered, and the content of TEOM decreased from 263.2 to 198.5 mg/L when the temperature increased from 15 to 20 °C. From 20 to 25 °C, as the concentration of algae increased, the volume of TEOM increased to 258.4 mg/L. At 25 and 30 °C, the concentration of TEOM exhibited small differences. However, at 35 °C, the algae solution contained the lowest amounts of TEOM (139 mg/L) due to the lower algae concentration. As shown in Fig. 4, the variations of dEOM and bEOM were similar to TEOM. The speciﬁc EOM secretion rate (SEOM) was introduced to elucidate the capacity of EOM secretion (Fig. 4). At 15 °C, the algae had the highest speciﬁc TEOM secretion rate (STEOM), speciﬁc dEOM secretion rate (SdEOM) and speciﬁc bEOM secretion rate (SbEOM), and the values were 822.5, 425.1 and 397.4 mg/g, respectively. As the temperature increased from 15 to 20 °C, SEOM (STEOM, SdEOM and SbEOM) declined sharply due to the recovery of growth activity . Although the value of SEOM decreased with the increase of temperature from 15 to 35 °C, the SEOM did not vary signiﬁcantly when the algae grew at temperatures of 20, 25, 30 and 35 °C. Wolfstein et al.  also found that the algae had the highest SEOM at the lowest temperatures (4 and10 °C), and the difference of SEOM at 15, 25 and 35 °C decreased.
3.4. Effect of temperature on continuous ﬁltration During the continuous ﬁltration, the concentration of algae was almost constant (0.3 g/L) in every group, and the SEOM (STEOM, SdEOM and SbEOM) declined with the increase of temperature. Therefore, the content of EOM in the tank decreased with the increase of temperature. Fig. 5 shows the ﬂux and TMP variation with time during ﬁltration under different temperature conditions. At 35 °C, because the ﬁxed initial ﬂux was 0.7 times JC35, the ﬂux and TMP were relatively steady during the ﬁrst 60 min. As time passed, the ﬂux/TMP experienced a sharp decrease/increase stage followed by a relatively slow period of reduction until a stable stage was obtained. At 15, 20, 25 and 30 °C, the ﬂuxes of all of the modules decreased rapidly at the beginning, followed by a slowly declining stage and a subsequent stable period. Fig. 5 shows that there was a higher ﬁnal ﬂux and lower ﬁnal TMP at higher temperatures. As shown in Fig. 5, the ﬁnal ﬂux at 35 °C was approximately 2.5 times than at 15 °C. When the continuous ﬁltration tests were completed, the decrease rates of ﬂux were 85.5%, 79.6%, 70.5%, 64.3% and 59.7% at 15, 20, 25, 30 and 35 °C, respectively. At 15 °C, in addition to being the lowest temperature, the concentration of EOM was much higher than at other temperatures; thus, the membrane exhibited the highest rate of decreasing ﬂux . In continuous ﬁltration, the membrane had less fouling and higher ﬂux at higher temperatures, which indicated that high temperature is beneﬁcial for algae harvesting by the membrane. As found in the research of Lee et al. , there was a high initial ﬂux at high temperature, but there was also a high fouling rate
Fig. 6. The deposition of algae on membrane under different temperatures.
due to the differences of the operating conditions between Lee's experiment and this experiment. 3.5. Effect of temperature on the deposition of algae During the ﬁltration of algae by a membrane, with the increase of algae cells deposited on the membrane, there is more serious membrane fouling and a faster ﬂux decline. The content of algae on the membrane was measured to estimate the effect of temperature on the deposition on the membrane. With the increase of temperature, the value of deposited algae decreased from 4.26 to 3.31 g/m2 (shown in Fig. 6), indicating that temperature had a signiﬁcant inﬂuence on algae deposition. Based on the calculation of the permeate drag force, at higher temperature, there was a lower drag force, which could slow the deposition rate of algae on the membrane, which explained why there was less algae deposited on the membrane at higher temperature. The more algae that is deposited on the membrane, the more serious the membrane fouling is , which could explain the reason why there was a higher ﬂux and less fouling at higher temperature. 4. Conclusions This study showed that although a temperature of 35 °C led to the highest critical ﬂux and best ﬁltration performance, the algae growth rate was very low. From 25 to 30 °C, the highest algae concentration occurred, and the secretion capacity of EOM at 25 and 30 °C was only slightly different from that at 35 °C. Thus, the optimum strategy is to maintain the algae cultivation temperature at 25 to 30 °C and the ﬁltration temperature at 35 °C. However, for large-scale outdoor cultivation, seasonal changes in the temperature are not easily controlled; although the temperature increase can reduce EOM secretion and membrane fouling, heating to increase the cultivation or ﬁltration temperature will require an additional cost. Thus, in algae cultivation and harvesting, a high temperature should be maintained without extra cost for as long as possible. At 30 to 35 °C, the algae concentration decreased, so the cultivation temperature should not exceed 30 °C. Acknowledgements This work was ﬁnancially supported by the National Key Technologies R&D Program of China (No. 2012BAJ25B02). References
Fig. 5. Variations in ﬂux and TMP during continuous ﬁltration under different temperatures ( is the stable stage during the ﬁrst 60 min).
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