In most cases, the first step is formation of the precoat, a thin layer (1.5 to 3.0 mm or 1/16 - 1/8 inch) that
protects the septum and ensures clarity by stopping the solids at the surface. Careful selection of the
precoat filter aid grade allows the fastest possible flow yet traps the solids. A slurry is made from filtered
liquid, or sometimes water, and filter aid. The concentration should be low as possible, 0.5 % is typical.
Agitation in the precoat tank should be sufficient to keep the filter aid in suspension. Excessive agitation
for extended time may break down the particles. Filter aids should be added at 500 to 1200 g/m² (10 - 25
lbs/100 ft²) of filter area. The precoat is formed by recirculating the filter aid slurry through the filter. The
coarser particles deposit themselves first on the screen followed by smaller ones. Precoating rate of 40
litre/m²/min. (1 GPM/ft²) is normal. Much lower rates are used with higher viscosity liquids. There should
be at least 0.07 kg/cm² (1 psi) differential pressure during the precoat process. Precoating liquor should
clear up within 10 to 15 minutes.