Nutrient released from BPR sludge during anaerobic digestion was refixed with Ca2+ and Mg2+ ions as calcium phosphate and struvite. The higher contents of Ca and Mg in primary sludge and nightsoil showed potentialities to control the nutrient returning back to liquid stream when these sludges were mixed with BPR sludge. Addition of Mg2+ to BPR sludge was also effective. The amount of TP removed from a sludge could be computed from the amount of P refixed by the amount of Ca and Mg removed during digestion. About 40 to 45% of TP and 35 to 39% of TN removal efficiencies could be expected when primary sludge or nightsoil was added to BPR sludge with a volumetric ratio of 0.67/1. With 100mg/L of Mg2+ addition (Ca/Mg molar ratio of 0.81) to BPR sludge, TN and TP removal efficiencies were 52.3% and 58.4%, respectively.

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