Role of Extracellular Polymeric Substances (EPS) in Biofouling of Reverse Osmosis Membranes MOSHE HERZBERG,* ,† SEOKTAE KANG, ‡ AND MENACHEM ELIMELECH ‡ Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Ben-Gurion University of the Negev, Sede Boqer Campus 84990, Israel, and Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, Connecticut 06520-8286 Received January 11, 2009. Revised manuscript received April 16, 2009. Accepted April 17, 2009. This study elucidates the mechanisms by which extracellular polymeric substances (EPS) impact permeate water ﬂux and salt rejection during biofouling of reverse osmosis (RO) membranes. RO fouling experiments were conducted with Pseudomonas aeruginosa PAO1, EPS extracted from PAO1 bioﬁlms, and dead PAO1 cells ﬁxed in formaldehyde. While a biofouling layer of dead bacterial cells decreases salt rejection and permeate ﬂux by a bioﬁlm-enhanced osmotic pressure mechanism, the EPS biofouling layer adversely impacts permeate ﬂux by increasing the hydraulic resistance to permeate ﬂow. During controlled fouling experiments with extracted EPS in a simulated wastewater solution, polysaccharides adsorbed on the RO membranes much more effectively than proteins (adsorption efﬁciencies of 61.2-88.7% and 11.6-12.4% for polysaccharides and proteins, respectively). Controlled fouling experiments with EPS in sodium chloride solutions supplemented with 0.5 mM calcium ions (total ionic strength of 15 mM) indicate that calcium increases the adsorption efﬁciency of polysaccharides and DNA by 2- and 3-fold, respectively. The increased adsorption of EPS onto the membrane resulted in a signiﬁcant decrease in permeate water ﬂux. Corroborating with these calcium effects, atomic force microscopy (AFM) measurements demonstrated that addition of calcium ions to the feed solution results in a marked increase in the adhesion forces between a carboxylated particle probe and the EPS layer. The increase in the interfacial adhesion forces is attributed to speciﬁc EPS- calcium interactions that play a major role in biofouling of RO membranes. Introduction The impairment of reverse osmosis (RO) membrane per- formance due to fouling represents a formidable challenge in advanced reclamation of wastewater efﬂuents and in seawater desalination (1-4). The most common adverse effects of biofouling include a decline in RO permeate ﬂux and a decrease in salt rejection (5, 6). Furthermore, frequent chemical cleaning for fouling control shortens membrane life and produces waste streams that need to be disposed of. Foulants of RO membranes typically comprise sparingly soluble inorganic salts, as well as organic, colloidal, and microbiological matter. While organic foulants and scaling on the membrane surface increase the hydraulic resistance to permeate ﬂux, colloidal fouling and bacterial cells reduce permeate ﬂux via a cake- and bioﬁlm-enhanced osmotic pressure (CEOP and BEOP) mechanism that leads to in- creased transmembrane osmotic pressure (7-11). Organic fouling of salt rejecting membranes (i.e., reverse osmosis and nanoﬁltration) has been studied extensively over the past decade. Among the factors that govern the fouling of such membranes are (i) membrane surface properties, including hydrophobicity, charge, and surface roughness (12-14); (ii) foulant properties, such as molecular weight and polarity (15, 16); (iii) source water chemistry (pH, divalent cations, and salinity) (17-21); and (iv) operating conditions (permeate ﬂux, applied pressure, and crossﬂow hydrody- namics) (21, 22). In general, surfaces that are most likely to resist organic fouling are characterized as being hydrophilic, H-bond acceptors, non-H-bond donors, and neutrally charged surfaces (3, 23, 24). Rougher and more hydrophobic mem- branes have a higher tendency for fouling by natural organic matter (NOM) and soluble microbial products (SMP) (25). In most cases, enhanced NOM fouling is observed in the presence of divalent cations, high ionic strength, and low pH (26). In addition, NOM and SMP components are differentially adsorbed to membranes during fouling. In experiments with both NOM and SMP as foulants, polysaccharides were reported to preferentially adsorb to nanoﬁltration (NF) membranes (27). As the adhesive and cohesive matrix of bioﬁlms, extra- cellular polymeric substances (EPS) play an important role in controlling the permeate water ﬂux decline in membrane processes. For example, Fonseca et al. (27) showed that NF permeate ﬂux decline during biofouling correlated to the membrane-associated EPS content. For RO membranes, it was proposed that EPS reduce turbulent ﬂow in close proximity to the membrane surface, leading to elevated concentration polarization and subsequent permeate ﬂux decline (28). A recent study modeling mass transfer in RO bioﬁlms suggested that both the void fraction in the bioﬁlm and the partitioning of solutes at the EPS-voids interface could hinder the back-diffusion of solutes and eventually increase the transmembrane osmotic pressure (29). In experiments examining fouling of RO membranes with alginate as a surrogate for EPS, calcium was found to severely decrease permeate ﬂux by the formation of a compact impermeable fouling layer (19). Fouling of RO and NF membranes with bovine serum albumin (BSA) and alginate, as model protein and polysaccharide, respectively, was recently reported to be affected by membrane surface roughness, with a synergistic fouling effect for a mixture of the two model compounds (17, 30). Alginate was found to foul the RO membrane more severely than BSA, exhibiting a much more pronounced effect in the presence of calcium. However, these studies with model biomacromolecules representing major EPS components cannot adequately simulate the complex nature of bacterial EPS and the overall process of biofouling of RO membranes. The mechanisms of biofouling and the speciﬁc contribu- tions of the various bioﬁlm components, especially EPS, to the deterioration of RO membrane performance are not well understood. In our previous study on RO biofouling, bacterial cells were found to enhance concentration polarization, thereby suggesting that EPS contribute mainly to increased hydraulic resistance (9). In this paper, we systematically investigate the contribution of EPS to the biofouling of RO * Corresponding author phone: +972-8-6563520; e-mail: herzberg@bgu.ac.il. † Ben-Gurion University of the Negev. ‡ Yale University. Environ. Sci. Technol. 2009, 43, 4393–4398 10.1021/es900087j CCC: $40.75  2009 American Chemical Society VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4393 Published on Web 05/07/2009 Downloaded by YALE UNIV on August 26, 2009 | http://pubs.acs.org Publication Date (Web): May 7, 2009 | doi: 10.1021/es900087j