Influence of dissolved oxygen and oxidation-reduction potential on phosphate release and uptake by activated sludge from sewage plants with enhanced biological phosphorus removal

Abstract

In wastewater treatment plants with enhanced biological phosphorus removal, the rate of aerobic phosphate uptake by activated sludge depends on previous phosphate release by polyphosphate-accumulating bacteria under anaerobic conditions. However, there has been disagreement as to whether the PO 4 release is caused either directly by low concentrations of dissolved oxygen (DO) or by the lowered redox potential (ORP) at zero DO. The results from experiments in a laboratory-scale treatment plant with activated sludge from different full-scale plants indicated a direct DO dependence of phosphate uptake and release. In contrast, the ORP modified the rate of phosphate release only under certain conditions. Phosphate release began at values between 0.1 and 0.5 mg O 2/l, depending on sludge conditions. At limiting DO concentrations, nitrate reduced the phosphate release in sludge from lowly loaded plants (with nitrification and denitrification zones) but not in sludge from highly loaded plants (without nitrification). In a pure culture of a polyphosphate-accumulating Acinetobacter strain (Br-2) phosphate release started only in completely anaerobic conditions. In sludge from sewage plants and in a pure culture of Acinetobacter, enhanced phosphate uptake began as soon as O 2 was available for respiration.