Electroconductive Feed Spacer as a Tool for Biofouling Control in a

Letter pubs.acs.org/journal/estlcu

Electroconductive Feed Spacer as a Tool for Biofouling Control in a Membrane System for Water Treatment Youngbin Baek, Hongsik Yoon, Soojin Shim, Jusol Choi, and Jeyong Yoon* World Class University (WCU) Program of Chemical Convergence for Energy & Environment (C2E2), School of Chemical and Biological Engineering, College of Engineering, Institute of Chemical Process, Seoul National University (SNU), Daehak-dong, Gwanak-gu, Seoul 151-744, Republic of Korea S Supporting Information *

ABSTRACT: This study investigated the application of electrical potentials to an electroconductive feed spacer (ECFS) as a tool for controlling biofouling in a lab-scale cross-flow membrane system. When the ECFS was electrically polarized for 30 min after a 24 h biofouling occurrence, 33−44% of the permeate flux was recovered without any damage to the membrane. This recovery can be explained by the effective detachment of the attached bacteria or biofilms on the membrane surface as well as the ECFC. Overall, the results of this study suggest that an ECFS with a proper electrical potential is an effective method for biofouling control in membrane systems for water treatment.

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caused by electro-hydrodynamic flow and electro-osmotic flow.21 Electro-hydrodynamic flow generates a convective flow through the migration of ions,27 while an electro-osmotic force generates the motion of a liquid induced by an applied electrical potential.28 On the other hand, the detachment of microorganisms by a negative potential can be explained by an electrical repulsion force.21,23,24,26 This indicates that a negative potential induces an electrostatic repulsion between the negatively charged microorganisms and the negatively charged electrode surface. An alternating potential has a combined effect from the positive and negative potentials.29,30 These electrical methods have some advantages because they are environmentally friendly and can be potentially automated.29,31 Recently, an electric current was used to prevent aquatic biofouling on an electrically conductive polymer nanocomposite membrane (ECPNC). Significant flux recovery was observed, which is explained by the electrostatic repulsion between the bacteria and the ECPNC or by microbial inactivation by oxidant generation despite limited experimental evidence.30 However, to the best of our knowledge, no electrical method for the feed spacer in the spiral wound membrane process has been introduced for controlling a biofouling occurrence without oxidant generation. This study investigated an electroconductive feed spacer (ECFS) as a tool for controlling biofouling in a lab-scale crossflow membrane system, in which low electrical potentials were

INTRODUCTION A biofouling occurrence on a membrane surface reduces both the efficiency of the membrane process and the water quality of the product, both of which increase the operating and maintenance costs.1,2 Generally, biofouling occurs because surviving microorganisms attach to the membrane surface forming a thick sticky layer known as a biofilm.3,4 Deterioration in membrane performance from a biofouling occurrence consists of a decreased permeate flux and salt rejection because of the blocked membrane pores, that is, a biofilm-enhanced osmotic pressure (BEOP) is generated.5,6 It has also been reported that the feed spacer may be responsible for the biofouling process. For example, in a spiral wound membrane system (i.e., NF/RO membrane system), a feed channel pressure drop develops from the biofouling occurrence of the feed spacer7,8 or facilitates biofouling on the membrane surface through the initial biofouling of the feed spacer.9 Biofouling is difficult to remove because of its sticky matrix and the regrowth of microorganisms even after various cleaning treatments (i.e., acid/alkali, biocides, etc.).10−12 Methods for controlling biofouling focusing on the feed spacer have been studied including introducing periodic air/water cleaning,13 changing the thickness or orientation of the feed spacer,14−16 and coating antibiotics on the feed spacer.15,17−20 Application of an electrical field has been suggested for controlling the attachment of microorganisms or the formation of biofilms on surfaces through electrical interactions between the microorganisms and conductive surfaces through electron transfer.21−23 According to previous studies, a positive potential is known to detach microorganisms by generating translational motion.21,23−26 The translational motion of microorganisms is © 2014 American Chemical Society

Received: Revised: Accepted: Published: 179

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Figure 1. Schematic of the lab-scale cross-flow membrane system loaded with the electroconductive feed spacer (ECFS). A Ti mesh and SUS mesh were used as the ECFS and permeate spacer, respectively. The ECFS was polarized by applying positive (+1.0 V), negative (−1.0 V), and alternating (+1.0 V and −1.0 V) potentials. The effective membrane area was 3.3 cm × 6.8 cm with a channel height of 0.1 cm.

min repeatedly). For example, the ECFS was positively polarized when a positive potential was applied to the ECFS and vice versa. Details on measuring the electrode polarization are described in the Supporting Information. The electrical potential was applied with a potentiostat (PARSTAT 2273A, Princeton Applied Research, Oak Ridge, TN, U.S.A.) under standard conditions (25 °C, pH 7.4, and 1 atm). When the electrical potential was applied to the ECFS, only the cross-flow was maintained without applying pressure. This process is generally used for RO membrane cleaning in practical applications.32 A low electrical potential (±1.0 V) was selected to minimize the electrochemical reactions, especially for chlorine evolution, which can deteriorate the polyamide RO membrane.33−35 Note that chlorine gas (Cl2) is generated as follows: 2Cl− = Cl2 + 2e− (E0 = 1.36 V/SHE). The chlorine concentration was measured with the N,N-diethyl-p-phenylenediamine (DPD) method using a portable spectro-photometer (DR/2010, Hach Co., Loveland, CO, U.S.A.). Permeate Flux Recovery after Biofouling Occurrence. A biofouling occurrence was carried out in a lab-scale cross-flow membrane system equipped with the ECFS. More details on the lab-scale cross-flow membrane system are available in our previous study.36 This lab-scale cross-flow membrane system was used with the intention to simulate a spiral wound membrane system used in practical applications.8,15 Biofouling was induced as follows: (i) membrane compaction by deionized water for 18 h, (ii) membrane conditioning by the feed solution for 6 h, and (iii) membrane biofouling with P. aeruginosa PA01 GFP (initial concentration: ∼1 × 107 CFU/mL) for 24 h. The initial flux, cross-flow velocity, and temperature were fixed at 40 L m−2 h−1, 7.6 cm s−1, and 25 °C, respectively. After a 24 h biofouling occurrence, the ECFS was polarized by applying three types of electrical potentials for 30 min (positive potential (+1.0 V), negative potential (−1.0 V), and alternating potential (VAC; +1.0 V for 1 min and −1.0 V for 1 min, repeated 15 times)). After the ECFS was polarized, the permeate flux recovery was measured with the feed solution for another 3 h. Additionally, the bacterial concentration was measured with the plate count method, and the morphology of the residual bacteria or biofilms on both the ECFS and membrane surface was observed by confocal laser scanning

used to minimize chlorine gas generation. Biofouling control was evaluated with permeate flux recovery and biofilm morphology. In addition, a membrane integrity test was performed to examine whether this electrical approach was detrimental to the polyamide RO membrane material.

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EXPERIMENTAL SECTION

Materials. A commercial RO membrane (LFC1; Hydranautics a Nitto Denko, CA, U.S.A.) was used. The effective area of the RO membrane was 3.3 cm × 6.8 cm, and the channel height was fixed at 0.1 cm, which was adjusted to the thickness of the feed spacer. A titanium mesh (Ti mesh; purity >99.6%, nominal aperture 0.19 mm, wire diameter 0.23 mm, wires/inch 60 × 60, open area 20%, twill weave, Sigma-Aldrich Co., MO, U.S.A.) and a compressed five-folded stainless steel mesh were used as the model electroconductive feed spacer (ECFS) and model permeate spacer, respectively, because they are electroconductive and similar to conventional spacers. The feed solution consisted of 10 mM NaCl (Aldrich, St. Louis, MO, U.S.A.), 10 mM sodium citrate (Aldrich, St. Louis, MO, U.S.A.), and 0.1% tryptic soy broth solution (Bacto, Franklin Lakes, NJ, U.S.A.) in 6 L of deionized water (Milli-Q, Millipore, Billerica, MA, U.S.A.). A carbenicillin-resistant derivative of Pseudomonas aeruginosa PAO1 tagged with the green fluorescent protein (GFP) (Center for Biofilm Engineering, Montana State University, Bozeman, MT, U.S.A.) was used as a model bacterial strain for biofouling occurrence. Electrode-Modified Membrane Systems Loaded with ECFS. Figure 1 illustrates a schematic of the lab-scale cross-flow membrane system loaded with the ECFS. A two-electrode system consisting of the ECFS (Ti mesh) and the permeate spacer (SUS mesh) was included in the system. The RO membrane acted as a spacer between the two electrodes. A twoelectrode system was selected because there was insufficient space for the reference electrode in not only the lab-scale crossflow membrane system in this study but also in a practical spiral wound membrane system (i.e., NF/RO membrane system). The focus of this study was the ECFS where biofouling occurs. Electrical polarization on the ECFS was induced by applying a positive potential (+1.0 V), negative potential (−1.0 V), and alternating potential (VAC; +1.0 V for 1 min and −1.0 V for 1 180

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Figure 2. Permeate flux recovery by the electrically polarized ECFS (positive, negative, and alternating potentials) for 30 min after a 24 h biofouling occurrence in a lab-scale cross-flow membrane system (biological foulant: ∼1 × 107 CFU/mL of P.aeruginosa PA01 GFP. Feed solution: 6 L of 10 mM NaCl, 10 mM sodium citrate, and 0.1% tryptic soy broth. Initial permeate flux: 40 L m−2 h−1. Cross-flow velocity: 7.6 cm s−1, 25 °C).

Figure 3. CLSM images of biofilm cells on both the electroconductive feed spacer (ECFS) and the membrane surface after a 24 h biofouling occurrence in a lab-scale cross-flow membrane system, and then, the ECFS was polarized with positive, negative, and alternating potentials (membrane: x axis, 202 μm; y axis, 202 μm; and z axis, 9 − 18 μm. ECFS: x axis, 1212 μm; y axis, 1212 μm; and z axis, 96−222 μm).

significantly recovered to 80%, 89%, and 91% when the ECFS was polarized with positive, negative, and alternating electrical potentials for 30 min, respectively. Apparently, the electrically polarized ECFS showed a better flux recovery (33−44%, based on the initial permeate flux) compared to our previous study reporting a permeate flux recovery of only 10−24% with multiple supercritical CO2 treatments under similar conditions and ∼6% with chlorine treatment (1 mg L−1 for 5 min).37 The improved effective flux recovery of the electrically polarized ECFS is attributed to bacterial removal from the membrane surface. This bacterial removal mainly originated from the detachment of bacteria not from bacterial inactivation by oxidants generation because only low electrical potentials were used in this study. No chlorine was detected after applying the electrical potentials, although this does not preclude its generation at very low levels (1.5 V) could inactivate bacteria.30,38 This bacterial detachment by the electrically polarized ECFS can be explained by electrostatic forces between the ECFS and the attached bacteria.21−26 The electrically polarized ECFS

microscopy (CLSM; Nikon 90i, Japan) after staining with the BacLight Live/Dead kit (Molecular Probes, Eugene, OR, U.S.A.).36 Membrane Damage Test. Membrane performance was evaluated by measuring the permeate flux and salt rejection after the electrical potential was applied. The feed solution was 2000 mg L−1 of NaCl solution. Pressure, cross-flow velocity, and temperature were fixed at 15.5 bar (225 psi), 20.2 cm s−1, and 25 °C, respectively. The initial permeate flux and salt rejection were measured, and then, positive (+1.0 V) or negative (−1.0 V) electrical potentials were applied for 30 min. The permeate flux and salt rejection were monitored for 24 h and measured at 1, 18, and 24 h.

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RESULTS AND DISCUSSION Permeate Flux Recovery by Electrically Polarized ECFS. Figure 2 shows the permeate flux recovery when the ECFS was polarized with positive, negative, and alternating electrical potentials for 30 min after a 24 h biofouling occurrence in the lab-scale cross-flow membrane system. Note that the permeate flux was decreased to about 47% after the 24 h biofouling occurrence (Figure S1, Supporting Information). As shown in Figure 2, the permeate fluxes were 181

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Table 1. Integrity Test of the Polyamide RO Membrane permeate flux (L m−2 h−1)a

salt rejection (%)a

a

control +1.0 V for 10 min −1.0 V for 10 min control +1.0 V for 10 min −1.0 V for 10 min

0h

24 h

56 56 51 98.4 98.5 97.0

54 55 50 97.8 97.9 97.7

Feed solution: 2000 mg/L of NaCl solution. Pressure: 15.5 bar. Cross-flow velocity: 20.2 cm s−1. Temperature: 25 °C.

electrolytes, and stirring time, were evaluated to examine their cleaning effect. More biofilm cells were detached under conditions with a larger potential and longer applied time. A higher concentration of electrolyte and faster stirring speed also showed effective biofilm detachment because of a decrease in the resistance between the electrodes and an increase in the shear force, respectively (Figure S5, Supporting Information). The ECFS polarized with an alternating potential, having the most effective recovery of the permeate flux in Figure 2, also exhibited the greatest detachment of biofilm cells (Table S1, Supporting Information). Moreover, the application of the polarized ECFS with negative and alternating potentials reduced by about 20% the residual EPS on the ECFS (Figure S6, Supporting Information). Integrity Test of the RO Membrane with the Electrically Polarized ECFS. Table 1 shows the results of the integrity test of the RO membrane with applied electrical potentials expressed by the changes in the membrane performance (i.e., (a) water flux and (b) salt rejection) when the ECFS was polarized with positive and negative potentials. No significant changes in both the water flux and salt rejection were observed, and chlorine was not detected. These results show that the application of the electrical potentials used in this study did not damage the polyamide RO membrane. Here, we report that a polarized ECFS, by applying electrical potentials, can be used to control biofouling in a spiral wound membrane system for water treatment without chlorine generation. However, this study is somewhat limited in that a Ti mesh was used instead of a conventional feed spacer. Thus, further study is required for a new feed spacer fabricated with electroconductive materials or a conventional feed spacer coated with electroconductive materials.

detached bacteria were directly attached on the ECFS, while bacteria attached to the membrane surface were indirectly detached by the electric field. An electric field was formed around the ECFS with the electrical current from the cross-flow of the electrolyte, and then, this electric field with shear force detached the bacteria on the membrane surface.39 Although the electric field might be small with a weak effect on bacterial detachment, the shear force generated by the cross-flow can enhance bacterial detachment. A schematic of the process for bacterial detachment on both the membrane surface and the ECFS with the electrically polarized ECFS is shown in Figure S2 of the Supporting Information. When the ECFS was positively polarized, bacteria attached to both the membrane surface and the ECFS were randomly moved (translational motion) and eventually detached. On the other hand, under a negatively polarized ECFS, bacteria attached to both the membrane surface and the ECFS were detached by an electrical repulsive force between the negatively polarized ECFS and negatively charged bacteria. Note that P. aeruginosa PA01 GFP is negatively charged (about −20 mV at neutral pH), along with most other microorganisms.24 Figure 3 shows the morphology of the residual bacteria and/ or biofilms on both the membrane surface and the ECFS observed by CLSM when the ECFS was polarized by positive, negative, and alternating potentials for 30 min after a 24 h biofouling occurrence. As shown in Figure 3 (green color, live cells; red color, dead cells), there were less live cells remaining on both the membrane surface and the ECFS, and very few dead cells were observed. The bacterial concentration on the membrane surface (7.4 × 108 CFU/cm2) was reduced to 5.1 × 106 CFU/cm2, 4.6 × 106 CFU/cm2, and 3.5 × 106 CFU/cm2 when the ECFS was polarized by positive, negative, and alternating potentials, respectively. This decrease in bacterial concentration is supported by the flux recovery with the electrically polarized ECFS. The bacterial concentrations on the ECFS when polarized by positive, negative, and alternating potentials were 1.5 × 108 CFU/cm2, 9.2 × 107 CFU/cm2, and 9.2 × 107 CFU/cm2, respectively, which were less than that of the control (1.3 × 109 CFU/cm2). Supporting this result, the electrical currents increased with time, indicating that the resistance of the ECFS decreased because of bacterial detachment (Figure S3, Supporting Information). This effective detachment of bacteria from the ECFS suggests that applying an electrical potential to the ECFS can contribute to preventing a feed channel pressure drop that is caused by a biofouling occurrence in a spiral wound membrane. Separate experiments were performed to determine the various factors that affect the electrically polarized ECFS. Biofilms were formed on the ECFS in a CDC reactor, and electrical potentials were applied to the biofilms in a twoelectrode batch system (Figure S4, Supporting Information). Four factors, which were the amount of potential, applied time,

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ASSOCIATED CONTENT

S Supporting Information *

Measurement of electrode polarization and biofilm formation on the electroconductive feed spacer and treatment with electrical potentials, and permeate flux decline after 24 h biofouling occurrence in the lab-scale cross-flow membrane system (Figure S1). Schematic of process for bacterial detachment on both the membrane surface and the ECFS by the electrically polarized ECFS (Figure S2). Electrical current for applying electrical potential (Figure S3). Two-electrode batch system for biofilm cell detachment (Figure S4). Biofilm detachment with the electrically polarized ECFS under various conditions (Figure S5). Effect of the polarized ECFS with an alternating potential for biofilm detachment (Table S1). Residual polysaccharide concentration representing the amount of EPS on the ECFS (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org. 182

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-880-8927. Fax: +82-2-876-8911. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2010-C1AAA01-0029061). We thank Prof. Yongsug Tak for many valuable and helpful suggestions for revising this manuscript.

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