Reverse Osmosis Biofilm Dispersal by Osmotic Back-Flushing

Letter pubs.acs.org/journal/estlcu

Reverse Osmosis Biofilm Dispersal by Osmotic Back-Flushing: Cleaning via Substratum Perforation Edo Bar-Zeev and Menachem Elimelech* Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286 S Supporting Information *

ABSTRACT: We have demonstrated the application of osmotic back-flushing (OBF) for the removal of biofilms from reverse osmosis (RO) membranes and proposed a new biofilm dispersal mechanism. OBF was conducted in a laboratory-scale RO test cell by introducing a sequence of hypersaline solution (1.5 M NaCl) flushes into the feedwater, while still maintaining the applied hydraulic pressure (13.8 bar). OBF resulted in significant biofilm detachment, leaving a thin, perforated bacterial film (24 μm thickness) with vertical cavities ranging from 15 to 50 μm in diameter. Application of OBF led to significant reductionin the biovolume (70−79%) and substantial removal of total organic carbon and proteins (78 and 66%, respectively), resulting in 63% permeate water flux recovery. Our findings demonstrate the potential of this chemical-free RO membrane cleaning method while highlighting the possible challenges of the technique.

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osmosis.10−12 Cleaning via osmotic backwashing can be achieved by injecting a pulse of a hypersaline solution into the feedwater, without changing the applied hydraulic pressure.13 It has been suggested that this osmotic cleaning approach can result in physical removal of foulants from the membrane surface, thus improving membrane performance. To date, however, no studies of the application of osmotic backwashing to control biofouling of RO membranes have been published. In this study, we demonstrate, for the first time, osmotic back-flushing as a chemical-free method for biofilm dispersal from reverse osmosis membranes. This technique was evaluated in a lab-scale RO test unit, and the impact of osmotic backflushing on the recovery of permeate water flux and biofilm structure was examined. Analysis of osmotic back-flushing efficiency and potential biofilm removal mechanism is used to estimate the overall feasibility of this method for mitigating biofouling of RO membranes.

INTRODUCTION Biofouling is widely accepted as the “Achilles heel” of membrane-based technologies, including desalination and wastewater reuse by reverse osmosis (RO).1,2 Membrane performance continuously deteriorates as a biofilm develops and proliferates, resulting in a shorter membrane lifetime, a lower water recovery, and an overall increase in the energy cost of product water.3,4 Hence, it is of paramount importance to develop methods for mitigating biofouling of RO membranes. A biofilm is defined as a complex sessile microbial community permanently attached to a surface by gel-like, selfproduced extracellular polymeric substances (EPS).5,6 This microbial consortium features a multilayer architecture composed of live and dead cells encased in EPS. Once established, biofilms are notoriously resistant to removal by treatment with oxidizing agents, biocides, or antibiotics because of the protection provided by the EPS matrix.7 Therefore, much current research is aimed at developing methods for inhibiting either the growth of biofilm-forming bacteria or the adhesion of bacteria to a wide range of surfaces. Thin-film composite polyamide membranes are at the core of RO membrane desalination.4,8 Because of the chemical sensitivity of the RO membrane to oxidizing agents, such as chlorine, new and effective cleaning approaches must be developed.4,9 Osmotic backwashing is a potentially novel, chemical-free, RO cleaning technique that works by reversing the water flux through the RO membrane via forward © 2014 American Chemical Society

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MATERIALS AND METHODS To test the efficiency of osmotic back-flushing (OBF) in dispersing biofilm and reclaiming RO membrane performance, Received: Revised: Accepted: Published: 162

December 10, 2013 January 4, 2014 January 6, 2014 January 6, 2014 dx.doi.org/10.1021/ez400183d | Environ. Sci. Technol. Lett. 2014, 1, 162−166

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Figure 1. (A) Normalized water flux reduction (top panel) during P. aeruginosa biofilm development (triangles) compared to a wastewater (WW) solution baseline, i.e., clean membrane with no biofouling (circles). (B) Water flux recovery (top panel) after an osmotic back-flushing (OBF) cycle (squares) with respect to a reference clean membrane (circles). Bottom panels show the corresponding concentrations of planktonic P. aeruginosa cells measured in two artificial feed wastewater [WWI (white background) and WWII (blue background)] solutions (described in the Supporting Information). Experimental conditions: initial water flux of 36 L m−2 h−1, cross-flow velocity of 8.5 cm s−1, pH of 7.6−8.2, temperature of 25 °C, and density of 4 × 106 cells mL−1.

OBF experiments were initiated by biofouling the RO membrane for ∼15 h as described above. The OBF procedure was then applied by passing a series of hypersaline solution pulses (1.5 M NaCl) on the feedwater side in a flow-through, open-loop mode. The hypersaline flush was conducted for 50− 60 s while maintaining the hydraulic pressure (13.8 bar) and increasing the cross-flow velocity to 17 cm s−1 (Figure S2 of the Supporting Information). Deionized (DI) water was simultaneously directed to the support layer from a small DI water reservoir (400 mL) placed on a digital balance interfaced with a PC, allowing real-time determination of reverse water flux during OBF. Back-flushing was terminated by switching the hypersaline feedwater to a DI water wash for 2 min (Figure S3 of the Supporting Information). To complete the OBF procedure, we performed the hypersaline−DI water cycle described above five times. The feedwater (WWI) was then replaced with a new sterile WWII, a step similar to that in the controlled biofouling experiments. Ten hours later, a second OBF procedure was executed before the experiment was terminated. Feedwater and permeate subsamples (50 mL) were routinely collected for determining salt rejection (based on electric conductivity) and bacterial concentration. At the end of each experiment, membrane sections were sacrificed and immediately imaged by confocal laser scanning microscopy (CLSM) under in vivo conditions to determine biofilm architecture and dimensions. Membrane subsections were stored in 50 mL tubes at −80 °C for total organic carbon and protein analyses. Additional details about the experimental procedures and analytical approaches are given in the Supporting Information.

two independent sets of biofouling and OBF experiments were conducted. Each set of experiments was conducted for 25 h by passing artificial wastewater (WW) with Pseudomonas aeruginosa (ATCC 27853) as a model bacterial strain through a custombuilt cross-flow RO setup (Figures S1−S3 of the Supporting Information). A thin-film composite polyamide RO membrane (SW30XLE, Dow Filmtec) with a water permeability coefficient, A, of 3.3 L m−2 h−1 bar−1 and a salt (NaCl) permeability coefficient, B, of 0.184 L m−2 h−1 was used. Membrane salt rejection was between 99.0 and 99.6% during the experiments. Additional details about the WW composition, bacterial growth conditions, and membrane properties are given in the Supporting Information. The hydraulic pressure was held constant at 13.8 bar (200 psi) throughout the entire biofouling or OBF experiments by a high-pressure pump (Hydra-Cell, Wanner Engineering Inc.), yielding an initial water flux of 36 L m−2 h−1. During the experiments, the cross-flow velocity was 8.5 cm s−1, the temperature was held constant at 25 ± 0.2 °C, and the pH was fixed at 7.9 ± 0.4. The feedwater temperature was controlled by a high-capacity chiller unit (Polyscience), and the feed tank was further isolated in a fabricated refrigerator for better temperature control. A digital flow meter (Humonics 1000) was interfaced with a personal computer (PC) to acquire real-time permeate water flux. Controlled biofouling experiments were conducted in the closed-loop cross-flow RO setup for 25 h (Figure S1 of the Supporting Information). A midexponential growth phase P. aeruginosa culture was added to a 10 L feed tank (denoted WWI) to produce an initial concentration of 4 × 106 cells mL−1. After ∼15 h and an ∼55% reduction in permeate water flux, WWI was replaced (without changing the transmembrane pressure) with a new sterile WW solution (denoted WWII) to prevent a substantial increase in the planktonic bacterial concentration during the biofouling experiment. Permeate water flux was then monitored for an additional 10 h.

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RESULTS AND DISCUSSION

Osmotic Back-Flushing Reclaims RO Membrane Performance. Reverse osmosis (RO) membranes were biofouled over 15−16 h at an initial water flux of 36 L m−2 h−1. Biofouling resulted in a 54 ± 0.7% decline in permeate water flux (Figure 1A, top panel) and a concomitant increase in 163

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flux of 25 ± 3 L m−2 h−1 that rapidly decreased until OBF termination at 13 ± 4 L m−2 h−1 after 60 s (Figure 2A). The rapid decline in back-flush water flux was attributed to the diffusion of salt across the membrane active layer and subsequent accumulation within the membrane support layer, which results in significant concentrative internal concentration polarization, as well as to dilutive external concentration polarization on the active layer side caused by the permeating pure water.18−20 Applying a 2 minute DI water wash at a crossflow velocity of 17 cm s−1 immediately after the hypersaline solution flush removed these detrimental concentration polarizations and restored the system for the next OBF cycle. As a result, each OBF cycle was initiated at a similar water flux [22− 30 L m−2 h−1 (Figure 2A)]. This dynamic OBF procedure resulted in 63% permeate water flux recovery [OBFI (Figure 1B)]. Exchanging with a new WWII solution substantially reduced the bacterial concentration (1.2 × 106 ± 1 × 105 cells mL−1) in the feedwater (Figure 1, bottom panel). A moderate (6−10%) reduction in water flux was observed for the subsequent 10 h. A second OBF procedure was then conducted [OBFII (Figure 1B)], recovering an additional 52% of the water flux and yielding an overall 70% water flux recovery (Table 1). Osmotic Back-Flushing Perforates the Biofilm Layer. Membrane subsamples were sectioned and imaged by CLSM at the end of each experiment to examine biofilm characteristics before and after OBF. Biofilm was composed of live and dead cells embedded in an EPS matrix, with an overall thickness of 72 ± 7 μm (Figure 3A). Under the applied hydraulic pressure, biofilm structure comprised two layers (Figures S4 and S5A of the Supporting Information): a highly organized packed film (PF) beneath a loose film (LF). The thin PF layer (27 ± 6 μm) was characterized by a compact cell assembly separated by numerous small size vertical pores (few micrometers in diameter). While the sample was being prepared for staining, the overlying LF layer occasionally detached from the PF layer as a biofilm sheet (Figure S5A of the Supporting Information). The thicker LF layer (44 ± 9 μm) had a spongelike pliable structure embedded in a thick layer of EPS (Figures S4A and S5A of the Supporting Information). While both layers contribute to the decline in membrane performance, the relative contribution of each of these layers to the sharp reduction in permeate water flux (Figure 1A) is unknown. Following the OBF procedure, membrane coupons appeared to be clean (Figure S5B of the Supporting Information). Membrane autopsy using CLSM image analysis indicated that the LF layer was removed, while a thin and perforated biofilm layer (24 ± 4 μm) was left attached to the active surface

the planktonic bacterial concentration (Figure 1A, bottom panel). The reduction in membrane water flux is attributed to biofilm growth2 as well as deposition of planktonic bacteria and organic matter (such as transparent exopolymer particles, TEP) on the RO membrane surface.14,15 Replacing the feedwater after 16 h with sterile wastewater (WWII) minimized bacterial deposition, favoring mainly biofilm development. Ten hours later, an only negligible (∼5%) reduction in water flux was observed (Figure 1A). We suggest that at the working hydraulic pressure of 13.8 bar and a cross-flow velocity of 8.5 cm s−1, a critical water flux was reached because of the equilibrium between biofilm growth and detachment.16,17 Additional RO biofouling runs, which resulted in similar reductions in water flux (∼55%), were conducted before applying osmotic back-flushing (OBF) (Figure 1B). The OBF procedure (Figure 2A) was applied by passing five sequential

Figure 2. (A) Water flux behavior during an osmotic back-flushing procedure composed of a sequence of five ∼1 min hypersaline flushes (orange symbols) with 2 min DI water washes (blue symbols) between the hypersaline flushes. (B) Back-flush water flux dynamics during a single osmotic back-flush cycle. Each cycle comprises a hypersaline back-flush followed by a DI water wash.

hypersaline solution pulses, with a DI water wash applied after each hypersaline solution pulse [OBF cycle (Figure 2B)]. OBF cycles were characterized by an average initial back-flush water Table 1. Biofilm Removal Efficiency by Osmotic Back-Flushinga

biofilm layer after OBF recovery (%) removal (%)

permeate flux reduction (%)

total protein biomass (pg/μm2)

TOC biomass (pg/μm2)

live cell biovolume (μm3/μm2)

dead cell biovolume (μm3/μm2)

EPS biovolume (μm3/μm2)

average biofilm thickness (μm)

54 ± 0.7

0.96 ± 0.16

2.8 ± 0.1

38 ± 5

41 ± 6

73 ± 11

72 ± 7

20,b 12c 63,b 70c

0.21 ± 0.04

0.9 ± 0.2

9±3

12 ± 6

15 ± 3

13 ± 4

78d

66d

76d

70d

79d

82d

Permeate flux reduction is presented as the difference between experimental water flux and the baseline run. Permeate flux recovery was determined as the difference in water fluxes after and before OBF divided by the difference between the baseline and biofouled membrane water fluxes. Biovolume and thickness were averaged, and the standard deviation (SD) was calculated from 9−12 randomly sampled images. Biovolume and thickness removal are given as the percent of biofilm layer before and after OBF. bAfter first osmotic back-flushing (OBFI). cAfter second osmotic back-flushing (OBFII). dSignificant (P < 0.01) difference before and after OBF. a

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between OBF procedures, future osmotic-based cleaning techniques must target this remaining attached PF layer.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed descriptions of experimental media and the bacterial strain, reverse osmosis membrane properties, biofouling and osmotic back-flush procedure, biofilm evaluation with confocal laser scanning microscopy (CLSM), protein assay, total organic carbon (TOC) analysis, and statistical analysis; schematic diagrams of the RO setup for biofouling experiments, overview of the system during the hypersaline flush, and system during the DI water wash (Figures S1−S3); CLSM images of the biofilm layers (Figure S4); and RO membranes before and after OBF (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 3. (A) CLSM orthogonal views of P. aeruginosa biofilm structures developed on the RO membrane after it had been biofouled for 24 h. (B) Biofilm architecture after an osmotic back-flushing procedure. Top insets are matching enlargements of the biofilm layer before and after OBF (A and B, respectively) with a schematic illustration of the flow of water through the membrane and/or biofilm. Biofilms were stained with Con A (blue), SYTO 9 (green), and PI (red) dyes specific for EPS (polysaccharides), live cells, and dead cells, respectively. All axis units are micrometers.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (203) 4322789. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was made possible by the postdoctoral fellowship (to E.B.-Z.) provided by the United States-Israel Binational Agricultural Research and Development (BARD, Fellowship FI-474-12) fund. We also thank Evyatar Shaulsky for technical assistance in the fabrication of the reverse osmosis system.

(Figure 3B). This remaining layer was observed as a mat of dead cells with uplifted clusters of live cells, all encased in EPS (Figure 3B). Most of the cavities in the remaining biofilm layer were between 15 and 50 μm in diameter, although some cavities were as large as 137 μm in diameter (Figure 3B). The remaining live colonies around these cavities were likely the remnants of the LF layer. Cleaning Efficiency via Osmotic Back-Flushing. OBF resulted in substantial biomass removal that was reflected by a marked permeate flux recovery (Table 1). Prior to OBF, the total biovolume was composed of both live and dead cells, and a thick EPS matrix (Figure 3A, Figure S4 of the Supporting Information, and Table 1). Following OBF, all live, dead, and EPS biovolumes were reduced relative to that of the control biofouling experiment (Figure S5B of the Supporting Information and Table 1), as indicated by a 74% reduction in the total biovolume (calculated as the sum of live, dead, and EPS biovolumes). Correspondingly, total organic carbon (TOC) and total proteins were significantly removed after OBF (66 and 74%, respectively). Because much of the PF layer remained attached (Figure 3B), biomass removal was predominantly attributed to the dispersal of the LF top layer. Although both live and dead cell biovolumes were reduced, there were 21% more dead cells than live cells after OBF. We attribute the relative increase in the level of dead cells mainly to the hypertonic shock caused by contact with the hypersaline solution.21 Our proposed biofilm dispersal scenario posits that once an OBF cycle was applied, water percolated back through the entire active membrane surface (25−13 L m−2 h−1), resulting in a buildup of pressure underneath the biofilm layer. Weak points within the biofilm structure were then breached, resulting in water streams that disperse the above LF layer, leaving a craterlike structure in the remaining attached PF layer. To achieve high water flux recovery and extend the intervals

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