Phosphorus removal and greenhouse gas N2O emission in a lime-induced aerobic sludge granule process

Aerobic sludge granulation was achieved in an activated sludge process continuously fed with lime (Ca2+ 100 mg l(-1) influent) every other day. Eighteen days after lime addition, activated sludge granules with the size of 0.5-2.2 mm were formed, which occupied 10-25% of total sludge volume. Sludge volume index (SVI) was reduced to an average of 50 ml g(-1), which increased average sludge concentration to 3.6-5.0 g VSS l(-1), 1.6-2.1 times of that of control. Greenhouse gas N2O emission was also significantly reduced: N2O concentration from the lime-addition reactor was 5-15 ppmv, 47-61% of that of control, Effluent PO4-P concentration was generally lower than 1 mg l(-1) when average influent PO4-P concentration was 6.07-6.37 mg l(-1). Total phosphorus (TP) and total nitrogen (TN) removal efficiencies were around 89.6% and 14.5-16.1%, over 3.5 and 1 times higher than those of control, respectively. COD removal rate in the lime-addition reactor was 2.05-2.48 kg COD m(-3) d(-1), higher than 1.34-1.61 kg COD m(-3) d(-1) in the control.