Method of converting biological waste for agricultural use

A method of treating a metals-containing biological mass to produce a solid material which can be used as a agricultural fertilizer or soil conditioner, comprising the steps of:

• subjecting the biological mass, which is in a fluid state comprising a solid phase and an aqueous phase, to a combined thermal and chemical treatment comprising heating, acidification and chlorinating oxidation and under acidic conditions by means of an oxidizing agent selected from the group consisting of chlorinating oxidizing agents and persulfuric acids,
• separating and recovering the solid phase and the aqueous phase said aqueous phase containing metals and ammonia;
• and recovering solid phase and aqueous phase;
• purifying the aqueous phase containing metals and ammonia by heat treatment with the addition of at least one compound selected from the group consisting of calcium hydroxide, calcium oxide and sodium sulphide so as to make the metals insoluble and to remove ammonia;
• absorbing the ammonia in an aqueous solution, and
• mixing the ammonia-containing solution with the recovered solid phase to neutralize the solid phase and to enrich it in nitrogen.