This paper compares full scale conventional pretreatment to full scale microfiltration pretreatment at a water recycling plant in El Segundo, California. The West Basin Water Recycling Plant includs three 2.5 mgd reverse osmosis trains. Each train uses cellulose acetate membranes. Feedwater to Train Nos. 1 and 2 is pretreated using a conventional process consisting of lime clarification, recarbonation, and filtration. Feedwater to Train No. 3 is pretreated using microfiltration. The two pretreatment processes will be evaluated with regard to life cycle costs and pretreated water quality. Run times between reverse osmosis chemical cleanings will also be evaluated as a proxy for the impacts of the two pretreatment processes on the reverse osmosis membranes. Costs that will be considered include: capital costs; operation and maintenance labor; chemical costs; sludge handling and disposal; power costs; replacement parts and supplies. Pretreated water quality parameters that will be considered are: turbidity; SDI; conductivity; and, magnesium. Includes tables, figures.