SRT contributes significantly to sludge reduction in the OSA-based activated sludge process

Though activated sludge systems have contributed significantly to the control of hygiene of our society, the wastewater treatment generates large amount of excess sludge. The oxic-settling-anaerobic (OSA)-based biological processes have been shown to be promising approaches for sludge reduction during wastewater treatment. However, the sludge reduction mechanism is still unclear. Four conditions were examined to clarify the sludge reduction mechanism in the OSA-based process. Sludge retention time (SRT) was the main 'contributor' to sludge reduction. The sludge reduction percentage of the process with side hydrolysis and acidification was 42%, with the contribution by long SRT of 33%, energy uncoupling of 7.7%, and hydrolysis/acidification of 1.1%. In addition, the sludge reduction in the OSA-based process had no obvious impact on the efficiency of nutrient removal. The clarified mechanism for sludge reduction in the OSA-based process could provide valuable clue for future system optimization.