Abstract: A pair of rollers define between them a narrow gap which is supplied with sludge to be treated. The sludge is conveyed by one of the pair of rollers to a third roller in contact with the conveying roller. The third roller is spaced a short distance from a rotating drum carrier to which is applied a thin film of the sludge. The sludge is applied to the carrier by the formation of a sludge bulge, which is an accumulation of sludge, in the area defined by the carrier, the third roller, and the conveying roller. The drum carrier picks up the sludge from the sludge bulge forming a film of sludge whose thickness is limited by the spacing of the third roller from the carrier.