Effect of Sludge Retention Time on the Efficiency of Excess Sludge Reduction by Ultrasonic Disintegration

Excess sludge reduction in activated sludge plants can, inter alia, be achieved by the integration of sludge disintegration technology in the recycle stream of activated sludge treatment plants. In previous research by Lambert et al. (2016), a long-term experimental study of 120 days at a biodiesel production plant demonstrated that ultrasonic sludge disintegration can result in a substantial reduction of waste sludge of about 45%, and this at a relatively low specific energy. Moreover, it was revealed from this pilot experiment that the efficiency of the excess sludge reduction, indicated as SRE (Sludge Reduction Efficiency), increases when the activated sludge plant is operated at a higher sludge retention time (SRT). This is an important finding, because this would mean that the ultrasonic technology can be operated more cost efficiently at a higher sludge retention time. To confirm this finding and to give a deeper insight into the underlying mechanisms and long-term effects that promote the excess sludge reduction, lab-scale aerobic digestion experiments were performed. Both the endogenous respiration rate and the VSS concentration were monitored during the 30 days’ experimental period, and could give more information about the fate and the biodegradation of the particulate COD in activated sludge. Modeling the batch tests indicated a higher endogenous residue decay rate (b_XE) and a clear instantaneous change in the active biomass concentration, of nearly 50%, which can be directly assigned to the ultrasonic pre-treatment. This has ultimately led to a more thorough VSS reduction at the end of the digestion period.