Precoat filtration is a US Environmental Protection Agency- (USEPA-) accepted filtration technique for potable water treatment. When the Surface Water Treatment Rule (SWTR) went into effect in 1989, it provoked a renewed interest in this filtration process as well as other accepted filtration methods. This manual provides general guidelines for the use of precoat filtration, commonly called diatomaceous earth (DE or diatomite) filtration, in potable water treatment. It includes an evaluation of appropriate applications for precoat filtration, discusses the design of economical filtration units, and presents an overview of operating practices.