Low-pressure (LP) membrane use has increased dramatically over the last decade in response to more stringent pathogen-related drinking water regulations, water reclamation and the need for more effective reverse osmosis pretreatment, and from dramatically reduced membrane costs. More cost-effective and reliable operation of LP membrane systems is constrained, however, by fouling, in particular fouling by NOM. NOM fouling is poorly understood because of both the complexity and types of NOM that exists in natural sources and wastewater effluent and NOM-membrane interactions. This report is available as a Pay-Per-View item only. NOM exists in three primary forms (allochthonous, autochthonous, and effluent-derived), with a variety of components having differing fouling tendencies. LP membranes comprise hollow fibers of differing polymeric materials, with a range of properties that likewise influence fouling propensity. Fouling management strategies (backwash, air scrub, chemical cleaning) employed with LP membrane systems differ from supplier to supplier. This, combined with a number of the methods used to reduce NOM levels prior to membrane treatment (e.g., coagulation, clarification), further complicate the understanding of NOM fouling. The overall goal of this project was to investigate the specific contributions of the different types of natural organic matter (NOM) to microfiltration/ultrafiltration (MF/UF) fouling. The intent was to develop a surrogate test or index that could be used to predict NOM fouling at low cost through a combination of source water characterization and rapid bench-scale testing. The research incorporated bench-, pilot- and full-scale investigations. Testing was conducted with four source waters, selected to capture the fouling characteristics of the three primary types of NOM. Bench testing use a stirred, cell apparatus and three flat sheet membrane types, representing commercially dominant MF and UF hollow fiber membranes of the same polymer types. Hollow fiber bench testing used two PVDF and two PES membranes operated in both sequential and alternating filtration/backwash mode. Pilot testing included PVDF MF and UF and PES UF systems operated on three of the four source waters and incorporated a host of fouling management strategies. Full-scale investigations captured operating data from several plants having differing levels and types of NOM.