Integrated Modeling of Thermal Hydrolysis Pretreatment and Anaerobic Digestion of Wastewater Treatment Sludges

A combination of lab testing and model application was conducted to develop an approach to model TH pretreatment of sludges prior to anaerobic digestion. The anaerobic degradability of a variety of raw sludges and sludges that were pretreated by thermal hydrolysis were evaluated in batch digestion tests. The raw sludge characteristics were estimated by simulation of the full scale wastewater treatment plant and then employed in simulation of the batch digestion tests. TH-treatment did not enhance the ultimate digestibility of the sludges tested in this study. The existing model was capable of simulating the observed overall COD removal and CH4 generation responses of both the raw and TH-treated sludges in the batch tests. Hence, TH treatment did not appear to convert any non-biodegradable COD fractions to biodegradable fractions. However, examination of the simulated fractions suggested that the particular characteristics of the sludges tested in this study was the reason for the negligible increase in biodegradability for the Thtreated samples. This result reinforces the need to consider the characteristics of the sludge source when assessing biosolids treatment. The use of modeling is essential in this regard. Soluble COD concentrations for raw sludge digestions were adequately simulated with the existing model. Simulated residual soluble COD concentrations for TH-treated sludges were consistently less than the observed values. The results suggest that TH-treatment of sludges formed non-biodegradable soluble COD that would be removed from the sludge in subsequent dewatering processes. The generation of non-biodegradable soluble COD during TH treatment will enhance solids removal when sludge digestion is integrated with dewatering however this would increase the loading of non-biodegradable COD in return streams to the wastewater treatment process.