These systems use a layer (precoat) of fossil flour or wood flour as a filter medium, which is applied on top of a special cloth mounted on a rotating support called the drum. The depression between the upstream and downstream of the drum keeps the precoat on the canvas and allows the suction of the liquid to be filtered.

By passing the product through the precoat, the dirt is retained by the coating while the clean liquid passes through the porosity thereof. A steel blade cuts a light layer of precoat at each turn of the drum, dropping the dehydrated dirt and keeping the filtering surface clean.
This product is mainly suitable for the recovery of the liquid part.