The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal Part 1: Literaturereview

Simultaneous chemical precipitation of phosphate (P) is commonly used in ac tivated sludge systems to supplement biological excess P removal (BEPR). Th is paper briefly reviews the use of metal salts (typically iron or aluminiu m) for this purpose and focuses on the question of possible interference wi th the BEPR mechanism arising from the addition of chemical precipitant. So me evidence of weakened BEPR has emerged in activated sludge systems in Sou th Africa, based partly on observations from full-scale plants in which con trolled studies were not satisfactorily carried out. In some cases, extrapo lations have been made either from laboratory-scale systems in which unreal istically large doses of metal precipitant were used, or from systems which were not operated close to steady-state conditions over extended periods. In other cases where simultaneous precipitation has been applied, the syste ms studied were not designed for BEPR. It was concluded that there is room for further investigation of:the reported negative effect of simultaneous c hemical precipitation on BEPR. To this end, a review of methods:far fractio nating the phosphorus compounds in activated sludge is presented. It does n ot appear to be possible to tailor a crude fractionation procedure to suit specifically the extraction of biologically-formed polyphosphate (polyP) se parately from Chemically-formed phosphorus precipitates in a complex medium such as activated sludge. More powerful analytical techniques are required to determine the nature, chain-length and mass of stored polyP in activate d sludge. Similarly, there is a need to carry out further fundamental resea rch into the interaction between the biological polymers in the sludge matr ix and chemical removal mechanisms. Nevertheless, the available basic chemi cal fractionation procedures do make it possible to obtain a broad classifi cation:and :measurement of chemical vs. biologically accumulated forms of P in activated sludge.