Experimental results are presented for a two-fiber, bench-scale test that was designed to evaluate the effect of setpoint flux, backwash frequency, and backwash time on long-term fouling rate of low-pressure membrane systems. An ultrafiltration (UF) membrane was used to treat granular-media filtered, secondary wastewater effluent. The fouling rate was measured by the decline in specific flux over many backwash cycles. The long-term fouling rate was decreased substantially by a modest lowering of setpoint flux from 65 to 50 L/(m(2) h) and by an increase in backwash frequency from 15 to 10 min. Increasing the backwash time from 30 s to 1 min did not decrease the fouling rate substantially. Microscopic observation and chemical analysis suggest that the extracellular polymeric substances (EPS) play an important role in fouling resistance for UF membranes treating secondary wastewater effluent. Both adsorption inside membrane pores and deposition on the membrane surface may occur. The comparison of bench-scale and pilot-scale test also shows that the two-fiber, bench-scale test is a very useful, first-cut approach for acquiring process design information for pilot-scale and full-scale operation. (c) 2005 Elsevier B.V. All rights reserved.