Enhanced removal of natural organic matter by hybrid process of electrocoagulation and dead-end microfiltration

Chemical Engineering Journal 232 (2013) 338–345

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Enhanced removal of natural organic matter by hybrid process of electrocoagulation and dead-end microfiltration Moshe Ben-Sasson ⇑, Yehoyada Zidon, Rivka Calvo, Avner Adin The Department of Soil and Water Sciences, The Robert H. Smith Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel

h i g h l i g h t s  There is a critical need to optimize NOM removal by membrane filtration.  EC pretreatment significantly improved NOM removal rates and may dramatically mitigates fouling, in MF.  The differences between improved NOM removal mechanism and fouling mitigation mechanism were discussed.  The effects of solution pH, electrodes type and EC operation times were investigated.  The filtration performance of hybrid EC&MF were supreme to UF, demonstrating its potential as efficient NOM removal method.

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Article history: Received 9 January 2013 Received in revised form 19 July 2013 Accepted 22 July 2013 Available online 3 August 2013 Keywords: Microfiltration Ultrafiltration Coagulation Electrocoagulation Electroflocculation Natural organic matter

a b s t r a c t The significant role played by natural organic matter (NOM) in aquatic environments and water purification processes motivates the critical need to optimize its removal by membrane filtration. This work studied the performances of hybrid process that combines electrocoagulation (EC) and dead-end microfiltration (MF) to remove NOM. Both iron and aluminum were used as the anode materials under experimental conditions of pH 6, 7 and 8. Filtration performance was characterized by NOM removal rates and the time needed to filter 750 mL of solution. The results revealed that both iron- and aluminum-based electrocoagulation pretreatment can mitigate NOM fouling and improve NOM removal rates. Still, improved NOM removal due to EC pretreatment was not necessarily followed by fouling mitigation and, in some conditions, a severe fouling effect coupled with improved removal rates was observed. In general, the positive effect of EC on fouling was observed both at the initial stage and during the late stages of filtration. Both fouling mitigation intensity and improved removal rates were strongly dependent on initial pH value, EC operation time and type of electrode. The most significant improvement in MF performance due to iron- or aluminum-based EC was observed at pH 6. At higher pHs (7 and 8), the iron electrode was favored over the aluminum electrode. At optimal conditions, a 36-fold shorter filtration time and a 20% increase in NOM removal rates were observed with the EC-MF hybrid process as compared to 100 kDa ultrafiltration alone. These observations emphasize the high potential to incorporate an EC-MF hybrid process into NOM removal methods, due to its ability to facilitate high fluxes, low fouling, and high colloidal/NOM removal. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Removal of natural organic matter (NOM) by membrane filtration has gained significant attention in the scientific literature during the past decade [1,2]. This interest arises from the unique physical and chemical properties of NOM and the significant and active role that NOM plays in water treatment processes. NOM tends to complex with heavy metals and consequently, may affect the fate of these contaminants in natural aquatic environments [3]. The presence of NOM may result in odor, taste, and aesthetic issues ⇑ Corresponding author. Tel.: +972-8-9353308. E-mail address: [email protected] (M. Ben-Sasson). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.07.101

[4]. For both drinking and reclaim water, NOM may affect the disinfection process by forming undesirable DBP (disinfection byproducts) [5], or affect advance oxidation processes such as UV or H2O2 [6]. In reverse osmosis (RO) desalination plants, NOM that is not efficiently removed during pre-desalination treatment causes organic fouling by accumulating on the RO membrane surface. This accumulation leads to an increase in the required effective osmotic pressure and to lower salt rejection, a phenomenon known as ‘‘cake enhanced osmotic pressure’’ [7]. Moreover, NOM cake on the membrane surface can stimulate biofilm formation because of the use of NOM as carbon, nutrient, and energy sources by bacteria [8]. For all these reasons, there is great incentive to study NOM removal methods.

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B D C A F E Fig. 1. The membrane system: A: nitrogen pressure balloon, B: pressure regulator, C: supply chamber, D: membrane cell, E: technical balance, F: BAL6 computer program.

Microfiltration (MF) is widely used in a large variety of filtration processes of aquatic solutions containing NOM, such as: membrane bio-reactor (MBR) [9], pretreatment for seawater and wastewater desalination plants [10], filtration of drinking water [11], tertiary treatment of wastewater for agricultural irrigation [12], and treatment of industrial wastes [13]. However, attempts to remove NOM or to filter aquatic-NOM solutions by MF are highly challenging because of the accompanied severe NOM–colloidal fouling [14]. The fouling intensity is governed by very complex relationships between the NOM properties (size, hydrophilicity, and charge), membrane characteristics (hydrophilicity, surface charge, and roughness), and the solution chemistry (e.g., pH, divalent ions such as Ca) [15]. Despite the substantive discussion found in the literature, it is difficult to find a precise and definitive description of NOM fouling and its mechanisms. This difficulty stems from the complex chemistry, high diversity of NOM, including location or seasonal variability, as well as the different experimental conditions (such as types of membranes used) in the published literature. Nevertheless, there is evidence that NOM–colloidal fouling will be more pronounced in MF as compared to ultrafiltration (UF), because of the tendency for pore blockage fouling of the former [16]. Moreover, the actual flux decline due to colloidal fouling is strongly dependent on the intrinsic hydraulic resistance of the pristine membrane. For the same filtrated solution, the lower the intrinsic/pristine membrane resistance the more it will suffer from relative flux reduction due to fouling [17]. Therefore, fundamentally, the NOM fouling will be more pronounced in MF than in UF. In addition, NOM removal will be less efficient in MF because of its larger pore size as compared to UF [18]. Consequently, currently it is well established that UF is more suitable than MF for NOM removal [19]. However, since MF hydraulic resistance is essentially lower than UF, MF has the potential to consume significantly less filtration energy, which provides incentive to optimize and to find improved strategies for MF filtration of aquatic-NOM solutions. The severe NOM–colloidal fouling in MF has motivated intensive scientific efforts to develop and research fouling mitigation strategies. One potential MF fouling mitigation method that was suggested in recent years is pretreatment by electrocoagulation (EC) [20,21]. EC (also called electroflocculation) poses an alternative method to conventional (chemical) flocculation because of several advantages, including: easy operation, lower quantities of produced sludge, avoidance of chemical usage, and importantly, the fact that no anions such as chloride or sulfate need to be added to the solution [22]. Unlike conventional flocculation in which the coagulants are added to the water as salts, in the EC process the coagulants (iron or aluminum) are added to the solution by dissolving the anode in an electrochemical cell. These coagulant ions ultimately lead to aggregation of the original particles in the water, which are later removed by sedimentation or filtration processes [23]. While several research groups have observed significant colloidal fouling mitigation in MF as a result of pretreatment with alu-

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minum-based EC ([24,25]), the effects of iron-based EC pretreatment in solutions that contain NOM are still being debated. Bagga et al. observed only marginal fouling mitigation due to pretreatment of iron-based EC in dead-end MF of river water [26]. This marginal effect on fouling was attributed mostly to the presence of NOM, which is prone to complex with electrochemically dissolved ferric ions, and thus reduces coagulation process efficiency. On the other hand, Adin et al. obtained significant fouling mitigation due to iron-based EC pretreatment in MF of both synthetic silica solution without NOM and secondary effluent that contained organic matter [27,28]. Unlike the effect on fouling, the effects of EC pretreatment on contaminant removal abilities in MF were seldom investigated. Improvement resulting in 4-log or greater virus removal rates was observed in synthetic water without NOM, as a result of pretreatment of iron-based EC with MF [29,30]. However, the same authors observed low virus removal rates in hybrid EC + MF in the presence of NOM [30]. This observation was explained by the tendency of NOM to complex with ferric ions and thus to lower flocculation efficiency. Regarding heavy metals, Mavrov et al. [31] observed very high and improved removal (>98%) of several types of heavy metals in hybrid iron-based EC and MF. Regarding aluminum-based EC, to the best of the authors’ knowledge, the effect on MFcontaminant removal rates was not previously reported in the scientific literature. However, to date, there has been no comprehensive research on the effect of EC on the performances of MF for NOM removal, meaning, both the effects on fouling intensity and on removal rates. Consequently, the overall potential of EC&MF as NOM removal methods is not clear. The impreciseness regarding the performance of hybrid EC&MF is enhanced by the ambiguous conclusions found in the literature about the efficiency of ironbased EC as a fouling mitigation method and its effect on virus removal rates in the presence of NOM. Therefore, the current study was designed to bridge this gap in knowledge and to further clarify the potential of hybrid EC&MF as a NOM removal method. The fouling mechanisms and NOM removal rates were explored for both aluminum and iron anodes under near neutral conditions (pH 6–8). The performance of the hybrid EC-MF process was compared to UF in order to evaluate its desirability for NOM removal. 2. Experimental 2.1. Materials 2.1.1. Membrane system and EC cell The membrane system (Fig. 1) consisted of the following components: (A) N2 pressure balloon, (B) pressure regulator (SYC IR2010), (C) 2-L supply chamber, and (D) 100-mL membrane cell. The flux was measured every 10 s by technical balance (Sartorius, BS3100S) and processed by a BAL6 computer program. The EC device consisted of an inner-cylinder cathode made of stainless steel, measuring 2.5 cm in diameter and 27 cm in length, and an outer-cylinder anode made of aluminum or iron, measuring 10 cm in diameter and 27 cm in length. The electrodes were inserted into the supply chamber (Fig. 1C) for operation of the EC process. 2.1.2. Membrane A polycarbonate track etched MF membrane (Sterlitech) with a 0.1-lm pore size was used in all MF experiments. These membranes were selected because of their smooth surface, very simple pore shape (cylindrical), and uniform pore size, which helped to lower the diversity of the intrinsic hydraulic resistances. UE10: polyethersulfone (PES) on polyester backing with 10 kDa MWCO and UE50: Polyethersulfone (PES) on polyester backing with

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100 kDa were used as UF membranes and purchased from TriSep, USA. Both UF and MF membranes were cut into 2.5-cm diameter disks. 2.1.3. Solution preparation The experimental solutions were prepared using highly pure water (Haifa Chemicals Ltd.) after undergoing RO and ionic-exchange treatment for removal of silica (conductivity: less than 1 lS/cm2). 1 mg/L NaHCO3 (>99% purity) was added as a buffer to 1 L of solution. Further addition of 0.2 mg/L NaNO3 provided an electric conductivity of 1 mS/cm, simulating the average electric conductivity of tap water. 2.1.4. NOM NOM stock solution was prepared by dissolving 1.5 g of standard humic acid (IHSS Leonardite, Humic Acid Standard, 1S104H5) in 1 L of NaOH solution (1 M). 1 mL of that NOM stock solution was mixed into 1 L of the basic solution described above, to achieve a NOM concentration of 15 mg/L or 10 mg/L as organic carbon. The solution pH was adjusted with dilute HNO3 to achieve pH values of 6, 7, and 8.

behavior [32]. Therefore, the fouling behaviors at the early and late stages of filtration were examined by analyzing the curves of all the filtration experiments. Fouling at the initial filtration stage was represented by Radd (50 mL), which was determined by the absolute additional resistance after filtration of 50 mL of the 750 mL solution. The effect of EC on fouling in the last stage of filtration is represented by the rate of the additional hydraulic resistance increment, as described below.When presuming that the cake layer mechanism is the cause of fouling, it is common to describe the additional resistance (or cake resistance) as a function of filtrated volume with the following equation [33]:

Radd ðVÞ ¼ Rcake ðVÞ ¼ aV þ b

ð2Þ

where V is the filtrated volume (m3), a is the rate of additional resistance increment when the cake fouling mechanism dominates (m4), and b refers to additional resistance in the early stages of filtration (m1).The rate of the additional resistance increment (which can also be referred to as the specific cake resistance coefficient), a, was calculated by fitting a linear line to the additional resistance values in the filtration stage between 350 and 750 mL. 3. Results and discussion

2.2. Methods Before each experiment the iron and aluminum anodes were soaked in HCl for 20 min and then rinsed with distilled water, in order to remove the oxide and passivation layer on the anode. The NOM solution was placed in the supply chamber – EC cell (Fig. 1C) – and then an electrochemical reaction was conducted with a constant electric current of 0.3 A (DC power supply: Topward 6030D). The duration of the electrochemical reaction was varied (0, 4, 8, and 12 min) under conditions of fast mixing (350 rpm). The next flocculation stage was carried out for 10 min with slow mixing at 40 rpm. Immediately after that, a filtration process was conducted without a pre-sedimentation step (in-line coagulation). A new membrane was used in each filtration test. The filtration of 750 mL of NOM solution was performed under a constant pressure of 2 bars for MF, both with and without EC pretreatment, and under a constant pressure of 3 bars for UF. In order to simulate dead-end conditions, there was no stirring in the membrane cell. Solution samples (30 mL) were taken from the initial solution (before EC and MF/UF) and from the permeate after filtration. The samples were acidified (with HCl), and TOC removal for the hybrid EC-MF process and for UF/MF alone were evaluated using a Shimadzu TOC analyzer (model: TOC-VCPN). 2.3. Fouling analysis In order to more thoroughly analyze the fouling, the normalized flux and the absolute additional hydraulic resistance were calculated for the filtration curves of 12 min aluminum-based and iron-based EC&MF, MF alone, and UF alone at pH 7. The absolute additional hydraulic resistance (Radd (V)) was calculated according to the following equation:

Radd ðVÞ ¼

DP  R0 JðVÞg

ð1Þ

where DP is the pressure in units of Pa, g, the dynamic viscosity (103 N s1 m2), R0, the hydraulic resistance at the initial filtration test (units of Mega m1), and J(V) is the observed absolute flux as a function of filtrated volume (units of m s1). In addition, it is well-accepted to distinguish between two stages in the filtration process. In the early stage, the internal fouling (pore blocking) mechanism plays an important role, while in the later stage, the cake layer mechanism solely dominates fouling

The significant role played by NOM in aquatic environments and water purification processes highlights the critical need to optimize its removal by membrane filtration. Optimization of the membrane filtration process is mostly dependent on two parameters: (a) filtration energy consumption/hydraulic retention time as represented by the water fluxes/filtration times which are governed by fouling intensity, and (b) the removal abilities of the targeted contaminants. In practice, optimization of the filtration process also depends on chemical consumption, as well as cleaning and waste (sludge) disposal costs. However, since the current study tried to assess whether EC&MF basically has the potential to efficiently remove NOM, and because of the specification of the economical calculations involved, only the filtration performances (i.e. filtration time, NOM removal) were considered. Significant flux reduction due to NOM–colloidal fouling was observed in all the filtration experiments. When the NOM solution was filtered through a UF 10 kDa (UE-10) membrane it took more than 1 week to filter half of the targeted solution volume (325 mL), while the MF and UF 100 kDa (UE-50) membranes filtered 750 mL of solution in less than 1 day. The very low fluxes observed with the UF 10 kDa membrane prevented fouling analysis. Because of this technical problem, the authors decided to abandon discussion about this membrane. Still, the very long filtration times of the UF 10 kDa membrane emphasize the high-energy/long-hydraulic retention time, and consequently, the lower performance of this membrane during filtration of the examined NOM solution. 3.1. NOM removal rates and filtration times The NOM removal rates of MF, UF 100 kDa, and hybrid EC&MF for filtration of 750 mL solution are shown for solutions at pH 6 (Fig. 2B), pH 7 (Fig. 3B), and pH 8 (Fig. 4B). MF conducted without EC resulted in poor NOM removal rates for all of the pH values examined (removal rates were of less than 33%). Both iron- and aluminum-; based EC significantly improved NOM removal rates for all pH values and EC operation times. At pH 6, short EC operation times led to an increase in the NOM removal rate to 65–70%. At longer EC operation times (12 min), NOM removal rates further increased to about 80%. EC significantly improved NOM removal also at pH 7, with higher removal rates for iron- than aluminumbased EC (71–82% and 52–59% for iron and aluminum, respectively). At pH 8, relatively lower NOM removal rates were observed

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Fig. 2. Filtration performances at pH 6. (A) Filtration times (s) and (B) NOM removal (%), after filtration of 750 mL through: MF alone (red), UF 100 kDa alone (brown), and MF pretreated by iron- (green) or aluminum- (blue) based EC. The numbers on the X axis represent the EC operation time in minutes (e.g., MF + 4Fe and MF + 12Al represent MF with 4 min of pretreatment with iron-based EC and 12 min of pretreatment with aluminum-based EC, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3. Filtration performances at pH 7. (A) Filtration times (s) and (B) NOM removal (%), after filtration of 750 mL NOM solution through: MF alone (red), UF 100 kDa alone (brown), and MF pretreated with iron- (green) or aluminum- (blue) based EC. The numbers in the X axis represent the EC operation time in minutes (e.g., MF + 4Fe and MF + 12Al represent MF with 4 min of pretreatment with iron-based EC and 12 min of pretreatment with aluminum-based EC, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4. Filtration performances at pH 8. (A) Filtration times (s) and (B) NOM removal (%), after filtration of 750 mL through: MF alone (red), UF 100 kDa alone (brown), and MF pretreated with iron- (green) or aluminum- (blue) based EC. The numbers on the X axis represent the EC operation time in minutes (e.g., MF + 4Fe and MF + 12Al represent MF with 4 min of pretreatment with iron-based EC and 12 min of pretreatment with aluminum-based EC, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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by hybrid EC&MF (51–67%) as compared to lower pHs, with ironbased slightly outperforming aluminum- based EC. It must be emphasized that the observed removal rates for EC-treated solutions were mostly higher than or at least similar to the removal rates observed for solutions treated with UF 100 kDa (49–59% for UF 100 kDa). The effect of EC pretreatment on filtration time was not as apparent as its effect on NOM removal rates, as can be seen for pH 6 (Fig. 2A), pH 7 (Fig. 3A), and pH 8 (4A). It took between 4 and 5.5 h to filter the solution through MF without EC treatment at all of the examined pH values. At pH 6, pretreatment with both aluminum- and iron-based EC resulted in significant fouling mitigation, and consequently, a reduction in filtration time. During long EC operation times (8 min for aluminum-based EC; 8 and 12 min for iron-based EC), filtration time was reduced to less than 30 min. At pH 7, short EC operation times of 4 min resulted in increased fouling intensity and longer filtration times (9–11 h), while longer EC operation times led to improvement in filtration performance with filtration times of less than 1 h in 12 min iron-based EC. The trends at pH 8 were similar to those at pH 7, in which short EC operation times led to longer filtration times that exceeded those for MF alone (7–8.5 h for 4-min treatments of iron- and aluminum-based EC). Longer EC operation times led to improvement in fluxes and consequently to reduction in filtration time. Both at pH 7 and pH 8, the iron electrode outperformed the aluminum electrode in terms of filtration time. Although the operational pressure was 1.5 times higher as compared to the MF experiments (3 bar vs. 2 bar, respectively), the time needed to filter 750 mL through UF 100 kDa was much longer at 13, 20.5, and 23 h for solutions with pH values of 6, 7, and 8. 3.2. Factors that affect the performance of hybrid EC&MF (EC operation time and pH) In the above paragraphs it can be noticed that improvement in NOM removal due to EC treatment is not necessarily followed by fouling mitigation. At pH 7 and 8, short EC operation times (4 min of both iron and aluminum electrodes) led to improved NOM removal, while it resulted in longer filtration times, meaning severe fouling as compared to MF without EC (Figs. 3 and 4). At longer EC operation times of 8 and 12 min, both NOM removal and fouling mitigation were observed. Consequently, EC operation time has a crucial effect, which dictates whether deterioration or improvement in MF filtration performance will be observed. Judd and Hillis [34] also observed the intensification of fouling at low coagulant doses and explained it by the incomplete coagulation followed by intensive internal pore blockage. Here we suggest another explanation, based on the need to distinguish between the improved NOM removal mechanisms and fouling mitigation mechanisms due to EC treatment. In the past, two main mechanisms were offered to explain the flocculation of NOM by metal coagulants. The first mechanism involves particle destabilization, either by charge neutralization or double layer compaction [35]. This mechanism dominates when the concentration of the coagulant is below the solubility values, and leads to aggregation of NOM particles. The second flocculation mechanism is sweep coagulation, which occurs when large flocs made of metal hydroxides are formed when iron or aluminum concentrations exceed the saturation values. These hydroxide-metal flocs then adsorb the NOM [35]. Both mechanisms lead to an increase in NOM particle size and consequently, to better removal by the membrane. Still, the mechanisms inherently differ in regards to the properties of the flocs that form, which, for the former, consist of pure NOM aggregates, and for the latter, are metal-hydroxide flocs with adsorbed NOM. When NOM particles aggregate because of the destabilization flocculation mechanism, the higher removal rate due to the in-

crease in particle size is followed by an increase of the accumulated mass load on the membrane. This higher accumulation rate occurs without significant change to the properties of the components that form the cake. Therefore, while high removal rates will be observed, fouling intensity will increase. On the other hand, during sweep coagulation, the large and hydrophilic metalhydroxide flocs completely change the cake structure and tend to form relatively high permeable cake [36]. Consequently, both improved removal of contaminants and fouling mitigation may be observed. The filtration performance of the hybrid EC-MF process was also strongly affected by solution pH. In the examined pH range (6–8), the following trends were observed: (1) the lower the pH, the higher the NOM removal rates for both iron and aluminum electrodes, (2) the lower the pH, the better the observed potential for fouling mitigation (i.e. shorter filtration times), and (3) at 4 min EC operation time for pH 7 and 8, the fouling deteriorated, while at pH 6 with the same EC operation time, the fouling was mitigated, as compared to MF without EC. Several mechanisms are suggested in the literature to explain the dependency of filtration performance in hybrid EC/chemical coagulation & MF on pH: (1) Solubility of aluminum: the solubility of aluminum and hence, aluminum-hydroxide floc formation, is highly dependent on pH, with minimum solubility occurring around pH 6 [35]. Consequently, at pH 6, lower doses of coagulant or shorter EC operation times are needed in order to reach concentrations at which the aluminum-hydroxide sweep coagulation mechanism will dominate. (2) Ferrous to ferric oxidation rates: the solubility of iron does not change significantly within the examined pH range [35]. Still the oxidation rates of the electrochemically dissolved ferrous ions to ferric ions are highly dependent on pH, with very slow rates occurring at the lowest pH values [37]. Bagga at el. [26] claim that ferrous has poorer coagulation performance than ferric and consequently, the oxidation rates of ferrous to ferric will determine the coagulation efficiency. (3) Metal-hydroxide size: the size, structure, and the charge of the metal-hydroxide flocs that form during coagulation depend on pH [35]. These properties of the particles dictate the cake resistance according to the Carman–Kozeny theory [38]. (4) NOM charge and size – the charge of the NOM particles changes as a result of protonation of its main functional group: carboxylic, amines and phenolic. As the charge of the NOM particles reduces because of the decreasing pH, the NOM configuration reduces as a result of weaker repulsion between the functional groups [39]. This may affect the cake properties and the coagulation efficiency. It is hard to distinguish between the effects of each one of the above mechanisms for a given pH value. Still, the optimal performances at pH 6 are in accordance with the accepted optimal pH for NOM coagulation for both ferric and aluminum salts [40]. A pH value of 6 was found to be optimal for hybrid low pressure membrane and coagulation of iron [36,41] and aluminum [42] salts. However, authors of other studies claimed a different optimal pH value for hybrid conventional coagulation combined with MF, such as Judd and Hillis [34], who concluded that a pH of 7.5–8 is best for hybrid aluminum-based coagulation and MF. Bagga et al. [26] claimed that fouling was insensitive to pH changes in a pH range of 6.4–8.3 for hybrid iron-based coagulation and MF. The gap between the findings from these studies may be explained by differences in the type of NOM and membranes used, as well as the experimental conditions. All the same, we can conclude that

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hybrid EC-MF process performance is highly dependent on pH value, and consequently, for each pH a different optimal EC operation time is expected.

ized flux reduction as compared to MF implies that fouling mitigation by EC can be more beneficial in MF. MF relative flux is more sensitive to the value of the additional absolute hydraulic resistance. Consequently, for the same improvement in the absolute additional hydraulic resistance by EC treatment, MF will show significantly more improvement in relative flux reduction as compared to UF. The experimental results showed that EC pretreatment significantly decreased both the relative flux reduction, and more significantly, the absolute additional hydraulic resistance. This fouling mitigation may be explained by the ability of EC both to prevent internal fouling, since fewer particles penetrate the membrane pores, and also to change the cake structure, because of the metal-hydroxide flocs that form a highly permeable cake [24,28]. It is well-accepted to distinguish between two stages in the filtration process. In the early stage, the internal fouling (pore blocking) mechanism plays an important role, while in the later stage, the cake layer mechanism solely dominates fouling behavior [32]. Consequently, it is important to understand if the observed mitigated fouling is only a result of reduction in internal fouling or related only to changes in the hydraulic cake resistance, or to both of them. If EC only mitigated one of the fouling mechanisms, and because each mechanism dominates in different filtration stages, an opportunity for chemical saving would emerge, since only the relevant filtered volume would require EC pretreatment. The trends of Radd (50 mL) (represents the additional hydraulic resistance at first stage after filtration of 50 mL; Fig. 6A) and a (represents the rate of additional hydraulic resistance increments at the late filtration stage, Fig. 6B), were almost similar to the trends observed for filtration time (Figs. 3–5). At pH 6, EC pretreatment reduced both the Radd (50 mL) and a as compared to the values of MF without EC, with the strongest reduction observed after 8 and 12 min treatments with both iron- and aluminum-based EC (see also SI Tables S1 and S2). At pH 7 and 8, a short EC operation time of 4 min led to an increase in both Radd (50 mL) and a as compared to MF alone. At these pH values, longer EC operation times (8 or 12 min) decreased the Radd (50 mL) and the a as compared to MF alone. In addition, at pH values of 7 and 8, lower values of Radd (50 mL) and a were observed in iron- as compared to aluminumbased EC. Because of the similar trends between the 750 mL filtration times, Radd (50 mL) and a, we can conclude that EC mitigates the fouling both at the early and the late stage of filtration. Therefore, it is important to coagulate the entire solution in order to mitigate fouling during all filtration stages.

3.3. Analysis of MF, UF, and EC&MF fouling In order to further explore the NOM fouling in the EC-MF hybrid process, the normalized flux curves were calculated as a function of the filtrated volume (Fig. 5A). Pretreatment times of 12 min for both aluminum and iron electrodes at pH 7 were chosen as the case studies and compared to MF and UF 100 kDa alone at the same pH. The severe fouling effect caused by NOM can clearly be seen, as the flux after MF of 750 mL without EC decreased to 2% of its initial value. The fouling effect was significant, but less severe as compared to MF alone, with normalized fluxes of 6% and 14% by the end of filtration test, in 12 min aluminum- or 12 min iron-based EC respectively. UF 100 kDa showed the least fouling effect, with a final normalized flux of 25% measured at the end of 750 mL filtration. The results of the absolute additional hydraulic resistance (Fig. 5B) give another dimension of information to the fouling behavior, in addition to the normalized flux (Fig. 5A). UF 100 kDa, which showed the lowest relative flux reduction, had the highest additional resistance by the end of filtration (14,000 Mm1); while MF, which had the severest flux reduction according to the normalized flux, had moderate additional resistance of only 4000 Mm1. Pretreatment with EC reduced the additional resistance by the end of filtration to 500 and 750 Mm1 for 12 min of iron- and 12 min of aluminum-based EC, respectively. Several articles claim that the severe fouling observed in MF as compared to UF is related to the larger ratio of pore-particle size and consequently, to the tendency of MF membranes to be fouled because of pore blockage (internal fouling). This argument claims that since internal pore blocking strongly decreases membrane permeability, the actual absolute fouling resistance is higher for MF than UF. This explanation is not valid for this study since UF was shown to have the highest absolute additional hydraulic resistance and yet had the lower relative flux reduction (Fig. 5). Therefore, it can be concluded that it is not the additional fouling hydraulic resistance alone, but rather the ratio between the additional hydraulic resistance and the pristine membrane resistance, that dictates the actual observed fouling intensity as demonstrated by the relative flux reduction. The fact that despite the very high additional hydraulic, UF showed only relatively moderate normal-

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3.4. The potential of EC&MF as a NOM removal method When summarizing the above, MF alone showed very poor NOM removal rates (lower than 35%), a strong fouling effect indicated by the low relative flux (less than 2% at the end of filtration), and moderate filtration time for a 750 mL solution (4–5.5 h). UF 100 kDa, on the other hand, succeeded in removing a larger portion of NOM (up to 60%). The process underwent less reduction in relative flux, but had filtration times that were 2.5–4 times longer than those for MF, although UF was filtered at a 1.5-fold higher pressure. Filtration conducted with the hybrid EC-MF process showed much better performance. At optimized conditions (pH 6, 12-min treatment of iron- or aluminum-based EC), the NOM removal rate increased to 80%, while the filtration time was reduced to less than 30 min. This filtration time, which is 36 times less than that required for UF 100 kDa filtration, resulted in 20% higher NOM removal. Consequently, it can be stated that the hybrid EC-MF process has the potential to efficiently remove NOM from aquatic environments. 4. Conclusions Pretreatment with both iron- and aluminum-based EC can improve MF filtration of solutions containing NOM in two ways: (1) it may significantly mitigate NOM fouling and consequently, reduce the filtration energy consumption, and (2) it dramatically improves the ability of the process to remove NOM. The effect of EC on filtration performance is highly dependent on solution pH, anode material, EC treatment time (coagulant dose), and NOM type and concentration. EC treatment times that are too short may lead to deterioration of fouling as compared to MF without EC. A pH value of 6 seems to be the best for achieving lower fouling and high NOM removal for both iron and aluminum electrodes. At pH values of 7 and above, iron-based EC led to stronger fouling mitigation and better NOM removal than aluminum-based EC. The filtration performance and NOM removal ability of the EC-MF hybrid process were superior to those of UF. This emphasizes the potential of using a hybrid EC-MF process as an alternative treatment to UF for removal of NOM. Acknowledgments The research was partly sponsored by the EU (SWITCH project). We wish to thank Arkadi Kushnir and Prof. Chaim Sheindorf from EPT Ltd. for their contribution in obtaining the highly pure water.

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