Ultrasonic solubilization of return activated sludge to reduce excess sludge production

Conventional biological wastewater treatment results in the generation of a considerable amount of excess sludge that has to be wasted. Technologies that are able to reduce sludge quantities are, therefore, of large interest. Sludge disintegration methods, like ultrasonic treatment, are considered to be very promising for reducing the amount of sludge to be disposed when applied in the sludge recycle line of the wastewater treatment plant. This PhD research aims to determine the main factors influencing the ultrasonic disintegration process by multivariate data analysis and to develop a mechanistically inspired model for ultrasonic sludge disintegration that is compatible with existing Activated Sludge Models (ASMs). For this purpose, experimental data were collected up to the level of the state variables, which are used in the existing ASM's. The general approach is the translation of data from lab-scale and long term pilot-scale experiments into a mechanistically inspired model for ultrasonic sludge disintegration, which includes the following processes:

► The conversion of volatile suspended solids (VSS) in readily biodegradable COD, slowly biodegradable (or colloidal) COD and inert COD, nitrogen (TKN and NH4-N) and phosphorus (PO4-P);

► The reduction of the active biomass concentration;

► The loss of soluble COD during the ultrasonic process itself as a direct result of the ultrasonic sludge disintegration and solubilization;

► Aerobic and anoxic cryptic growth on the ultrasonic lysate (=sludge yield).

Additionally, special attention is paid to the long-term effects of ultrasonic sludge disintegration towards excess sludge reduction via onsite pilot scale experiments, using industrial wastewater with (i) a high organic loading and (ii) nutrient deficient characteristics. By using two parallel sequencing batch reactors (SBRs) in each case, the differences between a non-treated pilot SBR and a SBR equipped with a 1 KW ultrasonic treatment system, will explicitly be discerned in terms of excess sludge reduction, effluent quality and settleabilty of the activated sludge flocs.