Phosphorus recovery with acid and base from inorganic sewage sludge residues

Phosphate recovery from sludge incineration ash and sludge residues from Super Critical Water Oxidation, SCWO through leaching with acid HCl and base NaOH was studied. Leaching with base gives a selective dissolution of phosphorus but a low degree of recovery compared to leaching with acid. The high per cent of leached phosphorus compared to leached metals at leaching with acid, and the lower cost for acid makes leaching with acid less expensive. The cost for HCl is estimated to 0.32 Euro/kg P and upward. Since calcium may bind phosphorus at leaching with base, a two-step process was tested. A first leaching step with acid at pH-level 4 to dissolve calcium was followed by a second leaching step with base. Two-step leaching gave more leaching of phosphate, however, not as high as for leaching with acid. The cost for chemicals for two-step leaching was estimated to 1.2 Euro/kg P and upward.