Phosphorus recovery from acidic wastewater by hydroxyapatite precipitation

Phosphorus recovery from acidic wastewater was carried out using lime milk at various Ca/P molar ratios in the range of 0.6-1.8. A molar ratio of Ca/P of 1.4 and above gives an 80% recovery efficiency of phosphorus from the wastewater. The chemical and phase composition of the precipitated products was analyzed. The phase composition was determined qualitatively and quantitatively by X-ray analysis. Depending on the conditions of the precipitation reaction, impurities and additives (e.g. flocculants), various phase compositions and different mass fractions of the crystalline phase were obtained. The main phase component of the samples obtained by precipitation of phosphorus from effluent at a pH below 10 is brushite, above 10 it is hydroxyapatite. The high Ca/P molar ratio and long reaction time favour the formation of crystalline fractions and larger crystallites. The flocculant aid used in the study, to shorten the solid phase sedimentation time, has adversely affected the degree of crystallinity. A higher degree of crystallinity of the precipitate is beneficial as it promotes the effective separation of the precipitation reaction product from the liquid phase.