Application of microfiltration (MF) and ultrafiltration (UF) membrane systemsfor municipal potable water treatment is becoming increasingly common. Typically, the capacity of these plants is less than 10 million gallons per day (mgd). However, several largecapacity UF plants are in design or under construction utilizing immersed membranes. For example, the Olivenhain Municipal Drinking Water Treatment Facility near San Diego, California, scheduled for completion in September 2001, will have an initial and ultimate rated capacity of 25 mgd and 30 mgd, respectively. This paper introduces basic design concepts used to retrofit existing rapid gravity sandfilters and sedimentation basins using UF immersed membranes. Particular emphasis is given to conversion of large treatment facilities. An example of the retrofit of an existing filtration basin is included. Additional information is presented for the layout of new treatment facilities. Includes 2 references, figures.