Combined Chemical and Biological Removal of Phosphate in Activated Sludge Plants

Abstract

Biological phosphate removal has been achieved in nutrient removal plants in Johannesburg by generating the required substrate for the process. Substrate generation has taken place by primary fermentation and subsequent elutriation. This technique, coupled with control of nitrate return to the anaerobic zone, has allowed phosphorus to be removed almost exclusively by biological means in plants designed for nutrient removal. However, operational problems with primary sedimentation tanks being used for fermentation, for which they were not designed, necessitated the occasional use of chemicals in the Northern Works plant. Contrary to current literature, the addition of ferric sulphate to this plant resulted in inhibition of polyphosphate storage; the cornerstone of the biological process. Laboratory batch studies revealed that iron-treated activated sludge showed a lower propensity for ferric phosphate precipitation than an untreated sludge. These laboratory studies were borne out by practical experence on two extended aeration plants, not designed for nutrient removal. The performance of these plants with different chemicals is discussed and possible reasons are advanced for some of the plant observations.