In recovering a fertiliser, free from heavy metals, from sewage sludge, the dewatered sludge is heated to 70-75 deg C by waste heat from an associated drying process, to destroy the cell structures comprising protein cpds. and surrounding the heavy metals, and is mixed with an acid acting to transfer the heavy metals from the solid to the liq. phase, and the heavy metals are pptd. by ion-exchange after sepn. of the phases. Novelty is as follows. The pH of the sludge is lowered to below 2 by mixing with mineral acids, pref. HNO3, or a mixt. of acids. The dissolution of the heavy metals adhering to the surface of the sludge particles and of the heavy metal cpds. is brought about by prolonged intensive mixing of the heated and acidified sludge, opt. with addn. of an anti-foam, in a closed vessel, and the gas formed is withdrawn through a pressure-regulating valve and burned in the furnace of the drier. The filtrate withdrawn in the phase sepn. is subjected to reverse osmosis, and the acid phase is recycled to the mixer, while the purified water is used to wash the solid, to improve the degree of de-metallisation. The content of the matter forming humus in the solid remaining after phase sepn. is raised by mixing with compost from kitchen and garden waste, and the mixt. is granulated and is dried to a dust-free material with 95% solids content, and is used in agriculture and forestry as a fertiliser, opt. after mixing with plant nutrients not present in the sewage sludge.
