They therefore use a layer of filtration aid, fossil or wood flour, as a filtering medium, which is applied on top of a special cloth mounted on a rotating support called a drum. The vacuum between upstream and downstream of the drum keeps the precoat on the fabric and allows the suction of the oil tobe filtered.

The precoat layer retains the solid while allowing the liquid to pass through. A steel blade slices through a light layer of precoat at each turn of the drum, knocking off the dried-out dirt and keeping the filter surface clean.