Managing global sewage sludge production remains a critical environmental challenge, as traditional wastewater treatment practices reach their functional limits. Conventional thermal conversion technologies, such as pyrolysis and gasification, are often rendered energetically unviable for sludge due to the energy-intensive pre-drying. This research investigates Hydrothermal Liquefaction (HTL) as a superior thermochemical pathway that utilises the moisture within digested sewage sludge as a reaction medium. Therefore, HTL offers the route to ‘extract’ and densify the energy contents of wet organic wastes. 

To enhance resource recovery and overcome "end of waste" barriers, this study explores the integration of HTL with Aqueous Phase Reforming (APR) to produce biofuel gases, including hydrogen and methane. A key innovation of this work is the characterisation of the mixed aqueous and biocrude fraction as a direct APR feedstock, addressing a significant knowledge gap in coupled hydrothermal processing. 

HTL experiments on digested sewage sludge at temperatures up to 330 °C demonstrate effective energy densification, increasing the higher heating value from 14.5 MJ kg-1 in the feedstock to 30.7 MJ kg-1, giving an energy yield of 27.5 %. This integrated approach supports the water sector by providing a scalable, sustainable route for biosolids valorisation and negative carbon emissions.