Journal of Membrane Science 295 (2007) 11–20 Biofouling of reverse osmosis membranes: Role of biofilm-enhanced osmotic pressure Moshe Herzberg, Menachem Elimelech ∗ Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, CT 06520-8286, USA Received 11 November 2006; received in revised form 11 February 2007; accepted 13 February 2007 Available online 20 February 2007 Abstract A bench-scale investigation of RO biofouling with Pseudomonas aeruginosa PA01 was conducted in order to elucidate the mechanisms governing the decline in RO membrane performance caused by cell deposition and biofilm growth. A sharp decline in permeate water flux and a concomitant increase in salt passage were observed following the inoculation of the RO test unit with a late exponential culture of P. aeruginosa PA01 under enhanced biofouling conditions. The decrease in permeate flux and salt rejection is attributed to the growth of a biofilm comprised of bacterial cells and their self-produced extracellular polymeric substances (EPS). Biofilm growth dynamics on the RO membrane surface are observed using confocal microscopy, where active cells, dead cells, and EPS are monitored. We propose that the biofilm deteriorates membrane performance by increasing both the trans-membrane osmotic pressure and hydraulic resistance. By comparing the decrease in permeate flux and salt rejection upon fouling with dead cells of P. aeruginosa PA01 and upon biofilm growth on the membrane surface, we can distinguish between these two fouling mechanisms. Bacterial cells on the membrane hinder the back diffusion of salt, which results in elevated osmotic pressure on the membrane surface, and therefore a decrease in permeate flux and salt rejection. On the other hand, EPS contributes to the decline in membrane water flux by increasing the hydraulic resistance to permeate flow. Scanning electron microscope (SEM) images of dead cells and biofilm further support these proposed mechanisms. Biofilm imaging reveals an opaque EPS matrix surrounding P. aeruginosa PA01 cells that could provide hydraulic resistance to permeate flux. In contrast, SEM images taken after fouling runs with dead cells reveal a porous cake layer comprised of EPS-free individual cells that is likely to provide negligible resistance to permeate flow compared to the RO membrane resistance. We conclude that “biofilm-enhanced osmotic pressure” plays a dominant role in RO biofouling. © 2007 Elsevier B.V. All rights reserved. Keywords: Biofouling; EPS; P. aeruginosa; Biofilm; Fouling; Biofilm-enhanced osmotic pressure; Cake-enhanced osmotic 1. Introduction The decrease in performance of reverse osmosis (RO) mem- branes in water reuse and purification systems due to fouling is a major concern [1–5]. Fouling requires frequent chemical cleaning and ultimately shortens membrane life, thus impos- ing a large economic burden on RO membrane plant operation. The major types of fouling in RO membranes are inorganic salt precipitation (contributed by sparingly soluble salts), organic (mostly natural organic matter or effluent organic matter), col- loidal (caused by accumulation of a colloidal cake layer on the membrane surface), and microbiological (usually governed by bacterial biofilm formation). ∗ Corresponding author. Tel.: +1 203 432 2789; fax: +1 203 432 2881. E-mail address: menachem.elimelech@yale.edu (M. Elimelech). In natural and engineered aquatic systems, bacteria are often found as biofilms—structured communities of bacterial cells enclosed in self-produced extracellular polymeric substances (EPS), irreversibly associated with solid surfaces [6,7]. Bacteria in RO systems for water and wastewater reuse are no exception. The combination of the inevitable presence of microorganisms in a non-sterile system, the relative abundance of nutrients, and the convective permeate flow through the membrane, will eventually lead to biofilm growth on the RO membrane surface [8,9]. The transport and attachment of suspended bacterial cells to a solid–liquid interface is the first step in biofilm formation. The approach and attachment of bacteria to a surface are medi- ated by physical, chemical, and biological factors. As bacteria approach the surface, surface–bacteria interactions (such as elec- trostatic and hydrophobic interactions) start to play an important role [8,10–13], with attachment being generally more favorable with hydrophobic, non-polar surfaces [6]. The hydrophobicity 0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.02.024