Desalination 187 (2006) 313–321 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved Presented at the International Conference on Integrated Concepts on Water Recycling, Wollongong, NSW Australia, 14–17 February 2005. *Corresponding author. Fouling of reverse osmosis membranes by hydrophilic organic matter: implications for water reuse Sangyoup Lee, Wui Seng Ang, Menachem Elimelech* Department of Chemical Engineering, Environmental Engineering Program, P.O. Box 208286, Yale University, New Haven, CT 06520-8286, USA Tel. +1 (203) 432-2789; Fax +1 (203) 432-2881; email: menachem.elimelech@yale.edu Received 14 January 2005; accepted 29 April 2005 Abstract Effluent organic matter (EfOM) is suspected as a major cause of fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. Among the main constituents in EfOM, polysaccharides are the most ubiquitous. The influence of solution chemistry and hydrodynamics on RO membrane fouling with alginate — a model for polysaccharides in secondary wastewater effluent — was systematically investigated. Results of fouling runs with alginate demonstrate that RO membrane fouling increases with decreasing pH, increasing ionic strength, and addition of calcium ions. At fixed solution ionic strength and pH, the presence of divalent calcium ions, at concentrations typical of those found in secondary wastewater effluent, had a dramatic effect on membrane fouling. However, for similar concentrations of divalent magnesium ions, fouling was negligible. The severe fouling in the presence of calcium is attributed to the formation of a thick, dense alginate gel layer on the membrane surface via calcium- alginate complexation and crosslinking (bridging) of alginate macromolecules by calcium. In addition to solution chemistry, hydrodynamic operating conditions — initial permeate flux and crossflow velocity — were also shown to influence RO membrane fouling with alginate. Keywords: Alginate; Polysaccharide; Wastewater reclamation; RO fouling; Divalent cations; Organic fouling 1. Introduction In recent years, membrane processes have emerged as a viable technology for water and wastewater treatment. A successfully implemented membrane technology can produce water of superior quality from a variety of source waters compared to conventional technologies. Of par- ticular interest is the use of reverse osmosis (RO)