From wastewater treatment to resources recovery through hydrothermal treatments of municipal sewage sludge: A critical review

The treatment of wastewater not only helps to alleviate the scarcity of water and environmental challenges but also generates a by-product called Municipal Sewage Sludge (MSS). The MSS is a high moisture precipitant with plentiful organic and inorganic components. However, some pollutants in MSS components and its high-water content complicate its valorization, and thus, special attention is required for its valorization. Hydrothermal treatments (HTs) can be applied to feedstocks with high moisture content without pre-drying. These processes are gaining more attention to processing MSS into fuels and value-added materials. This work systematically reviews the findings recently emerged from HTs of MSS. The related chemical reactions and the effect of reaction parameters are highlighted based on most recent works. The problems addressed and those not yet addressed are also discussed. Some suggestions are made for the plausible applications of end products with the aim to optimize the whole process. Finally, insight is given into the future of MSS HTs and the concerns that might be the focus of substantial studies.