This manual explains applications and operation of precoat filtration, also called diatomaceous earth (DE) filtration, for potable water treatment. In precoat filtration, unclarified water contaning foreign particles is forced, under pressure or by vacuum, through a layer of DE filtering media that has been deposited (precoated) on a septum. As the water passes through, suspended particles are captured and removed. Average DE pore sizes are much smaller than the pore sizes of conventional filter media, such as sand, allowing precoat filters to capture smaller particles than conventional filters. Perlite is also used as a precoat filter media.