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

Highlights • 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 NH 4 +-N and SVI. • Alkaline treatments at pH 10 and 11 increased SOUR of aerobic activated sludge. Abstract Three alkaline treatments (pH 10, 11, and 12 for 1 h) were used to treat return sludge alone to reduce sludge production in laboratory-scale anaerobic–anoxic–oxic processes. After 99 days of operation, alkaline treatments at pH 10 and 11 led to accumulative excess sludge production and sludge yield reduction of 18.8%–31.7% and 14.7%–27.8%, respectively. However, alkaline treatment at pH 12 led to system breakdown because of sludge bulking. The alkaline treatment at pH 10 did not affect the chemical oxygen demand and NH 4 +-N removals of the system and sludge volume index (SVI) of aerobic activated sludge. However, alkaline treatments at pH 11 and 12 obviously deteriorated the wastewater treatment efficiencies and sludge SVI. Although the three treatments increased the effluent pH by 0.08 to 0.38, the effluent pH of three systems were all lower than 9.00. The treatments at pH 10 and 11 increased the specific oxygen uptake rate of activated sludge, whereas the treatment at pH 12 decreased this rate. [ABSTRACT FROM AUTHOR]