Abstract: In the treatment of solid residues containing heavy metals from combustion plants, the heavy-metal compounds dissolved after washing-out and subsequently precipitated are separated from the solvent, usually water or slightly acid aqueous solutions by cross-current diaphragm filtration. The sludge obtained during filtration and comprising precipitated insoluble heavy-metal compounds is dewatered together with the remaining solid residues and made suitable for dumping, e.g. by using binders. The cross-current diaphragm filter improves the washing of heavy metals out of the washing or waste water, and the method and the associated plant are simplified by the joint processing of the sludges containing solids.