Hydrothermal carbonization for energy-efficient processing of sewage sludge: A review

Hydrothermal carbonization is an important thermochemical conversion process that can be used as an energy-efficient alternative to enhance the dewaterability of sewage sludge and meanwhile to convert sewage sludge into high value-added products, such as clean biofuel, organic fertilizer and precursors of functional materials. This paper presents an overview of the latest development of hydrothermal carbonization in the field of sewage sludge treatment, with a particular focus on critical hydrothermal parameters, physicochemical characteristics of products streams, current understanding on hydrochar formation mechanisms, sewage sludge dewaterability improvement and techno-economic advantages. Recent advances have shown that hydrothermal carbonization of sewage sludge is an exothermal process, which is governed by temperature to a large extent. Both polymerizations of highly reactive intermediates derived from degradation of biopolymers in sewage sludge and solid-solid conversion of their undissolved fractions are regarded as the major mechanisms of hydrochar formation. The high ash content of hydrochar is probably the limiting factor for its potential applications in energy and functional materials. The chemistry in hydrothermal carbonization of sewage sludge, closely related to the process parameters and the chemical composition of sewage sludge, offers huge potential to influence the products distribution and characteristics and the process energetics as desired, which provides a promising opportunity to construct a high-efficiency industrial chain for energy and resources recovery from sewage sludge by a controlled hydrothermal process. This review identifies the current challenges and knowledge gaps, and provides new perspectives for future research efforts targeting at sustainable treatment of sewage sludge by hydrothermal carbonization.