Observations Supporting Phosphate Removal by Biologically Mediated Chemical Precipitation – A Review

Abstract

Chemical precipitation of phosphorus in biological treatment plants may be mediated biologically in at least two ways. First the elevated P-concentrations created by anaerobic phosphate release from bacteria can initiate and accelerate calcium phosphate precipitation. Secondly biological denitrification in fixed biofilms and possibly also in bacterial floes can lead to phosphate precipitation due to the elevated pH-conditions inside the biofilms. In both cases the bulk precipitation conditions must be generally favourable, i.e. the calcium concentration should be reasonably high, roughly above 50 mg/ℓ, and the concentration of precipitation inhibitors low: magnesium, pyrophosphates and bicarbonate (alkalinity). With respect to accelerated bulk precipitation it is also essential that the pH is relatively high, preferably above 7.5. Biologically mediated phosphate precipitation is only two P-removal mechanisms out of several mechanisms. Apart from normal biological P-assimilation phosphate can be removed through polyphosphate accumulation in bacteria and through normal chemical bulk precipitation. The paper gives a survey of the mechanisms. Virtually all the published literature on P-removal in biological treatment plants contain insufficient data to evaluate the alternative P-removal mechanisms. In particular there is a need for agreement on procedures with which to characterize the P-fractions in the sludge in order to determine how much P is removed by the bacteria and by mineral phases. Still much qualitative and quantitative knowledge is required on both the biological and chemical P-removal mechanisms before reasonable optimization strategies and design criteria can be outlined.