Benefits and drawbacks of thermal pre-hydrolysis for operational performance of wastewater treatment plants.

This paper presents benefits and potential drawbacks of thermal pre-hydrolysis of sewage sludge from an operator's prospective. The innovative continuous Thermo-Pressure-Hydrolysis Process (TDH) has been tested in full-scale at Zirl wastewater treatment plant (WWTP), Austria, and its influence on sludge digestion and dewatering has been evaluated. A mathematical plant-wide model with application of the IWA Activated Sludge Model No.1 (ASM1) and the Anaerobic Digestion Model No.1 (ADM1) has been used for a systematic comparison of both scenarios--operational plant performance with and without thermal pre-hydrolysis. The impacts of TDH pre-hydrolysis on biogas potential, dewatering performance and return load in terms of ammonia and inert organic compounds (Si) have been simulated by the calibrated model and are displayed by Sankey mass flow figures. Implementation of full scale TDH process provided higher anaerobic degradation efficiency with subsequent increased biogas production (+75-80%) from waste activated sludge (WAS). Both effects--enhanced degradation of organic matter and improved cake's solids content from 25.2 to 32.7% TSS--promise a reduction in sludge disposal costs of about 25%. However, increased ammonia release and generation of soluble inerts Si was observed when TDH process was introduced.