Management of biological nutrient removal plant sludges — Change the paradigms?

Experience with the management of BNR sludges in Johannesburg has shown that accumulated N and P can readily be released from these sludges during sludge thickening, dewatering and digestion. Therefore, primary and waste activated sludges should be treated in as aerobic a state as possible in separate process streams. All sludge dewatering liquors should be pretreated for P; suspended solids and N removal before recycle to the liquid phase. Composting provides a viable alternative to anaerobic digestion for the stabilization of BNR sludges, as use of the latter leads to the maximum solubilization of N and P.