Abstract: A process for dewatering sewage sludges of which the sludge solids contain less than 70% of organic matter, and whose dewatering characteristics have been improved by adding organic flocculants, on a filter press, wherein the sewage sludges are concentrated by adding an organic flocculant during thickening by gravity sedimentation from 0.5-5.0% by weight of sludge solids to 6-14, and especially 8-12, % by weight of sludge solids, the sewage sludges are mixed with additives, such as finely divided coal or ash or mixtures thereof, in an amount of from 0.5 to 1.5 parts by weight per part by weight of sludge solids, and the sludges are treated, before, during or after admixture of the additives, with a further 2-8 kg or organic flocculant/tonne of sludge solids and are then dewatered on the filter press.