Procedures are disclosed for preven

Procedures are disclosed for preventing fouling of reverse osmosis and ultrafiltration membranes by relatively large molecules and particulate matter. In particular, absorbents are used, with and without finely divided activated carbon, to form a "precoat" on the membrane. The "precoat" is laid down on the membrane at the beginning of a reverse osmosis or ultrafiltration cycle. It is removed at the end of a cycle by flushing, preferably followed by a cleaning. The "precoat" process is applied to the ultrafiltration of raw sewage as a preferred example.