Anaerobic co-digestion of combined sludges from a BNR wastewater treatment plant

The fate of phosphorus contained in waste activated sludges from a biological nutrient removal plant was investigated in pilot-scale, co-digestion studies at. full-scale wastewater treatment plant In these studies, co-digestion of primary fermenter and waste activated sludges resulted in a significant release of the biologically-bound sludge phosphorus. As much as 80% of phosphate removed during treatment was released to the supernatant during anaerobic digestion. Phosphate release was found to be independent of digester hydraulic retention time at the values tested (20 vs. 10 day). Co-digestion also impacted other aspects of digester operation, including volatile solids,reduction and total gas and methane production. Jar tests, conducted to evaluate the efficiency of various chemical treatment methods for removing phosphate from digester supernatants, revealed that lime, alum and ferric chloride all proved equally effective. Overall, ferric chloride at a dosage of 2 g l(-1) was found to be the most effective method for combined removal of phosphorus, COD and TSS.