Anaerobic digestion (AD) is the most widely applied technology for the treatment of sludge produced in WWTPs. At present, thermal hydrolysis (TH) is becoming the technology of choice to improve anaerobic digestion due to its techno-economic advantages. The drivers behind the decision to install this technology, however, differ widely as they depend on local legislation and prevailing pricing structure. Thermal hydrolysis can be applied with different objectives (increase biogas yield, reduce digestion volume, reduce digestate quantity, pathogens removal) that can result in different optimum configurations. In this context, the same TH process can be placed as pre-, inter- and post-treatment to AD, each with distinct advantages and drawbacks. For all cases analysed there is a solid business case, with attractive payback, and the site-specific conditions will be key to determine the optimum location of the TH technology within the WWTP. Pre-treatment of mixed sludge is the configuration showing the lowest capex and is attractive for grassroots designs, but presents a poor energy balance. Inter-treatment incurs in higher capex, but in return this energy self-sufficient configuration yields the best operating numbers. Post-treatment is to be considered only attractive at relatively high biosolid prices and demands careful consideration of odour issues.
