Chemical Precipation of Phosphate From Sewage At Low Lime Dosage

Abstract The use of lime, a much cheaper chemical than A1 and Fe salts, in physico-chemical phosphorus removal in sewage treatment, has one major disadvantage: the large amount of sludge produced. In fact precipitation with Ca(OH) 2 takes usually place at such pH values (≥10) as to give rise to the insolubization of CaCO 3 . However, the necessity to make precipitation take place at such high pH levels without considering the wastewater hardness, is not sufficiently analysed in scientific literature. The aim of this work is to demonstrate the technical-economical feasibility of a pre-precipitation process with lime at such a pH level (∼9) as to minimize CaCO 3 precipitation. It must also be taken into consideration that the pre-precipitation process does not require very high efficiencies in that the phosphorus left over from physicochemical treatment is removed in the biological stage.