Production of high value-added carbon material precursors via direct hydrothermal conversion of municipal sewage sludge coupled with air flotation separation: A review

Municipal sewage sludge, as a byproduct of the sewage treatment process, has a large yield and needs to be properly disposed of and utilized as resources. Hydrothermal carbonization (HTC) is a green, efficient and clean production technology that converts sewage sludge into solid-phase products (including hydrochar and ash) and liquid-phase products. Hydrochar has potential as a precursor of high-value-added carbon materials, but ash residues limit its application. Air flotation technology uses the differences in the physical and chemical properties of mineral particles to separate them. Owing to the differences between hydrochar and ash, they are expected to be separated by flotation technology. Therefore, this systematic review examines the migration and transformation pathways of sewage sludge components during HTC and discusses the parameters affecting HTC. The optimization of operating parameters (including bubble size, flotation reagent type, and dosage) in the air flotation process is briefly introduced. The key is to propose a coupling technology of HTC and air flotation, which is used to separate hydrochar and ash. Coupling technology provides a new perspective and reference for future research on the high value-added use of sludge, provides innovative references for sustainable organic solid waste disposal and supports the transition to a carbon-neutral future.