The City of Tucson, Arizona operates a 150-mgd ozone, direct filtration plant,commissioned in 1993 and designed to treat surface water originating from theColorado River and delivered via the Central Arizona Project (CAP). CAP water ischaracterized by moderate turbidity (2-3 NTU) and total organic carbon (2-3mg/L), and high hardness (200-250 mg/L as calcium carbonate) and total dissolvedsolids (600-700 mg/L). In 1995, the City and the U.S. Bureau of Reclamation (BOR)began studying methods to enhance the quality of CAP water to meet localrequirements related to matching finished CAP water quality with that of theCity's high quality and historical water source, deep groundwater. This requiredthat the CAP water meet the following parameters as stipulated in the 1995 WaterConsumer Protection Act: TOC <0.4 mg/L, TDS <210 mg/L and hardness <83 mg/L ascalcium carbonate. The Tucson/BOR study indicated that the most cost effectivemethod for meeting such requirements would be with membranes, either reverseosmosis (RO) or nanofiltration (NF), following appropriate preliminary treatment.NF and RO use semi-permeable, non-porous membranes with molecular mass cutoffs of100 to 500 Daltons. RO is designed primarily for TDS reduction (demineralization)while NF is employed for softening. Both provide excellent removal of TOC anddisinfection byproduct (DBP) precursors. Good turbidity and pathogen removal isalso achieved, however neither is designed to serve as a primary barrier to thesecontaminants. NF and RO have been used almost exclusively for the treatment ofgroundwaters, typically having low levels of particulate and biological matter.When used to treat surface waters, such as CAP water, control of membrane foulingis the major challenge for the cost-effective application of these processes.Surface waters contain high concentrations of particles, non-biodegradable andbiodegradable natural organic matter (NBOM and BOM, respectively) that can causerapid and often irreversible fouling of the NF/RO membranes. Membrane foulingcauses reduced water production and, in some cases, decreased product water(permeate) quality, thereby necessitating frequent cleaning and premature elementreplacement. Fouling can be caused by physical, biological and chemicalcontaminants, usually in combination. Physical fouling can be controlled byreduction in particulate content of the feed water (clarification and/orfiltration). Biological fouling can be minimized by controlling both bacterialand BOM levels. Chemical fouling, both from NBOM and oxidizable metals, can beminimized through optimized chemical clarification upstream of the membraneprocess. The purpose of this study is to evaluate the effectiveness of twobiological-based preliminary treatment processes (slow sand filtration (SSF) andbiologically optimized rapid sand filtration (BORSF)) and one physical process(microfiltration (MF)) for the control of the different classes of NF/RO membranefoulants in CAP water. Operation of NF/RO on the effluent from each pretreatmentprovides a means to determine the relative contribution of each foulant class toperformance impacts on the NF/RO membranes. This paper reports on the results ofthe pretreatment and NF/RO pilot systems during testing conducted in 2000. Includes 3 references, tables, figures.