The objective was to evaluate anion exchange (IEX) resins to remove organic acids and thus reduce fouling in low-pressure membrane filtration. A secondary objective was to demonstrate the usefulness of a new fractionation strategy for identifying the organic components of wastewater effluent organic matter (EfOM). We previously showed that organic acids (14-60% of EfOM) caused almost all short-term fouling in polyethersulfone (PES) UF. Investigators using different fractionation strategies (without strict removal of colloids and with dramatic pH changes during fractionation) reported that the hydrophilic base/neutral fraction of EfOM was the most important foulant, but colloids were present or had formed in that fraction during processing. In this work we report that resistance to filtration was partially reduced with removal of colloids but nearly all short-term fouling was eliminated with removal of organic acids for PES-UF and for polyvinylidene fluoride (PVDF) MF. Critical flux was increased from 75 to 200 L m(-2) h(-1) after fixed-bed treatment with weak-base DEAE or strong-base MIEX. Strong-base IRA-958 or non-ionic DAX-8 in column operation, or MIEX with in-line addition, were less effective in reducing fouling and increasing critical flux. These results confirmed our earlier observation that organic acids were primarily responsible for short-term fouling from EfOM and also demonstrated that IEX was effective for removal of organic acids, reducing fouling, and increasing critical flux. (C) 2010 Elsevier B.V. All rights reserved.