Effects of return sludge alkaline treatment on sludge reduction in laboratory-scale anaerobic-anoxic-oxic process.

•Effects of alkaline treatments of return sludge on sludge reduction were studied.•Alkaline treatments at pH 10 and 11 led to sludge reduction of 18.8%–31.7%.•Alkaline treatment at pH 12 led the system to break down.•Alkaline treatment at pH 10 did not affect removals of COD and NH4+-N and SVI.•Alkaline treatments at pH 10 and 11 increased SOUR of aerobic activated sludge.