METHOD FOR TREATING WASTE WATER SLUDGE

The invention relates to a method for treating waste water sludge comprising at least one metal originating from a waste water treatment coagulant, and phosphorus and heavy metals in order to recover said at least one metal and phosphorus and to discharge said heavy metals. In this method said waste wat er sludge is acidified to dissolve metals contained in the sludge thereby yielding an acidified sludge solution containing at least 1 % by weight of a t least one metal to be recovered. In a first precipitation stage the pH of sa id acidified sludge solution is raised to precipitate at least one metal to be recovered as a phosphate, and thereafter the phosphate precipitate is separated, thereby leaving a solution comprising heavy metals. In a second precipitation stage the pH of said solution comprising heavy metals is raise d and, if necessary, an appropriate chemical is added to precipitate heavy metals, and thereafter the precipitate is discharged.