A comprehensive review on sewage sludge as sustainable feedstock for bioenergy production

The greatly rising amounts of sewage sludge from different sources are causing severe concerns around the globe. Disposal of sewage sludge is exceptionally difficult and poses serious environmental problems due to the significant quantity of toxic heavy metals and organic contaminants in its constituents. This review provides an overview of different conversion methods of sewage sludge into energy, that is, pyrolysis, hydrothermal gasification, hydrothermal liquefaction, hydrothermal carbonization, anaerobic digestion, and transesterification. It also focuses on each conversion route's state-of-the-art research advances, features, and drawbacks to ensure financial and ecological responsibility. The detailed descriptions of the methods and respective energy generation could help scholars and industrialists produce energy from sewage sludge to keep the environment harmless.