Thermochemical methods for the treatment of municipal sludge

Rapid economic growth is making the disposal of municipal sludge non-negligible during the acceleration of urbanization. Thermochemical treatment has been proved to be of superiority with regard to volume reduction and product conversion of municipal sludge. This work provides a comprehensive review on thermochemical treatment methods of municipal sludge, including combustion, pyrolysis, hydrothermal carbonization, hydrothermal liquefaction, wet oxidation, supercritical water oxidation and gasification. Comparisons among these thermochemical methods are clarified from technological process, environmental risks, use of products and economic cost. Combustion, more specifically mono-incineration, is preferred for its rapid and obvious volume reduction of municipal sludge. While other methods are relatively uncompetitive now owing to difficulties on system operation and technological development. The combination of additives with atomized municipal sludge which is ejected by high-pressure air, and the coupling of different processes (e.g., pyrolysis) are potential optimization methods of combustion. Gasification provides an option for sustainable disposal of municipal sludge due to acceptable disposal costs and low environmental risks. It exhibits considerable emission decrease and energy saving effect. To compare thoroughly all treatment methods, effective evaluation like life cycle assessment methodology is necessary.