Influence of an Intermediate Thermal Hydrolysis Process (ITHP) on the Kinetics of Anaerobic Digestion of Sewage Sludge

Over the last 16 years, different sludge pre-treatment processes have been retrofitted to existing sewage sludge treatment plants in order to improve anaerobic digestion treatment process efficiency. Some of these pre-treatment technologies, notably the thermal hydrolysis process (THP), have greatly increased the sludge throughput and allowed more efficient utilisation of treatment assets without adversely impacting on the biology of sewage sludge anaerobic digestion process. Whilst the digester throughput is increased by utilising the THP process, the biogas yield has not increased significantly or the expected increase in volatile solid reduction (VSR) has not been realised. In an attempt to improve the impact of THP on the process performance, a new sludge treatment process configuration was developed which consists of an intermediate step of thermal hydrolysis (ITHP). The results obtained from laboratory scale digestion investigations showed marked improvements in the sludge digestion process efficiency during ITHP process in comparison with conventional mesophilic anaerobic digestion (CMAD) and THP digestion process configurations. To establish the reasons for these differences, the sludge organic matter kinetics was investigated by quantifying the degradation rates of the main sludge constituents; protein, carbohydrates and lipids as well as volatile fatty acids (VFA) for the ITHP process configuration.