Abstract
Over the last 18 years, different sludge pre-treatment processes have been used to improve the performance of sewage sludge anaerobic digestion efficiency. Some of these pre-treatment technologies, notably the Thermal Hydrolysis Process (THP), has significantly increased the sludge throughput and allowed more efficient utilisation of treatment assets without adversely impacting the biology of the anaerobic digestion process. However, the expected increase in Volatile Solid reduction (VSr) and the consequent increase of biogas production have not been fully realised. Specifically, to address this poor performance when the THP process is used and to overcome its limitations, its application as an Intermediate Thermal Hydrolysis Process (ITHP) was studied. The ITHP process configuration consists of a first stage conventional Mesophilic Anaerobic Digestion (MAD) followed by THP and then a second stage MAD (i.e. MAD+THP+MAD). The main aims of this research were therefore to evaluate the impact of the ITHP configuration on an already digested sludge constituents, namely, carbohydrates, proteins and lipids degradation and the extent of their conversion to biogas. The sludge constituents’ degradation as a result of thermal hydrolysis and Anaerobic Digestion (AD) followed a stepwise process where the initial faster degradation was followed by a second stage slower degradation process. The sludge constituents’ degradation kinetic rate constants showed that the use of ITHP can further enhance the already digested sludge degradation reducing the sludge mass and increasing its conversion to biogas. Furthermore, the ITHP configuration showed a significant impact on sludge Extracellular Polymeric Substance (EPS) content. The results obtained from laboratory scale experiments showed that the ITHP process configuration resulted in an overall average VSr of 62% in comparison with the THP configuration which provided a VSr of 47%. As a result, the overall biogas production from the ITHP process was found to be in excess of 478 m3/tonne dry solids (TDS) fed, compared with 345 m3/tds feed from the THP digestion configuration.