Recent developments on thermal municipal sludge pretreatment technologies for enhanced anaerobic digestion

Sludge management is still one of the most challenging issues in wastewater treatment plants due to a dramatic increase in sludge production, high sludge disposal costs, legal constraints as well as social and environmental concerns. There is a great effort to develop more environmentally friendly and economical technologies for minimization of excess sludge production and converting wastewater treatment sludge from waste into a renewable resource for bioenergy recovery. Recently, among these technologies, pretreatment processes applied before anaerobic sludge digestion have received a growing attention with several advantages over conventional digestion process. The main goal of the present paper is to present a state-of-the-art review of recent developments on advanced anaerobic digestion employed in municipal wastewater treatment plants. Thermal pretreatment technologies documented in the literature are presented extensively. The effectiveness of thermal pretreatment methods, namely conventional, microwave and radio frequency heatings, are discussed and compared in terms of heating principles, sludge disintegration, digester performance, and sludge rheology. The effectiveness and practicality of the aforementioned methods at industrial-scale and some challenges associated with the implementation at full-scale are also reviewed. Particular attention is paid to integration of combined heat and power systems with thermal hydrolysis for achieving energy self-sufficiency in full-scale plants. Furthermore, the municipal sludge production around the world as well as current sludge disposal and reuse options are addressed.