A review on hydrothermal pretreatment of sewage sludge: Energy recovery options and major challenges

Disposal of sewage sludge is a major management issue due to its inherent high moisture content (≈ 98-99 percent). The major drawbacks of conventional anaerobic digestion process AD for sewage sludge are insufficient solid reduction, lesser energy recovery, and long retention times. Hydrothermal pretreatment is a potential option for improving resource recovery from sewage sludge. The hydrothermal process involves thermal disintegration of biomass-rich waste in hot compressed water. In this chapter, three hydrothermal processes, namely, thermal hydrolysis (TH), wet oxidation (WO), and hydrothermal carbonization (HTC) are selected for review due to the requirements of less severe reaction conditions. Among these processes, WO is performed in the oxidative environment whereas TH and HTC are conducted in non-oxidative medium. In the review, major reaction pathway and some recent studies are discussed. In addition, the major challenges associated with these processes are identified. Finally, the future scope of work in this field is also suggested.