Phosphates recovery from iron phosphates sludge.

Recovery of phosphates from ferrous and/or ferric residues as a result of chemical precipitation was investigated. The product of biological reduction of sulphates--sulphides and free hydrogen sulphide--can react with iron ions forming ferrous sulphide which are less soluble then ferrous phosphates, releasing phosphates into solution. Laboratory scale experiments have confirmed the possibility of phosphates recovery. It was found that strict anaerobic conditions are required and that the process is COD/S ratio dependent.