The effects of aluminium- and ferric-based chemical phosphorus removal on activated sludge digestibility and dewaterability

Abstract

The uses of Al3+ and Fe3+ salts in chemical phosphorus removal (CPR) in activated sludge plants have increased considerably in recent years and their full impacts on downstream processes such as dewaterability and digestibility are not fully understood. In this research, the effects of CPR on sludge digestibility and dewaterability were investigated in laboratory-scale experiments using sludge samples from a full-scale wastewater treatment plant. The results of the digestibility tests showed a 21% and 36% reduction in the biogas volume generated during anaerobic digestion of surplus activated sludge at 0.1 g/L doses of Al3+ and Fe3+ salts, respectively. This demonstrates that Al3+ dosing for CPR has less of a reduction effect compared with Fe3+ salts on biogas generation during anaerobic digestion of sludge. The dewaterability tests showed that primary sludge dewaterability was improved by up to 25% by Fe3+ and 16% by Al3+, while that of surplus activated sludge was reduced by 64% and 73%, respectively, at a metal salt dose of 50 mg/L. Consequently, a pre-precipitation process during CPR where phosphorus is removed in the primary tank would, therefore, enhance sludge dewaterability.