Process water from the hydrothermal carbonization of biomass: A review on the characterization, applications, and potential for future work

Hydrothermal carbonization (HTC) is a thermochemical process used to convert wet biomass into hydrochar, a carbon-rich solid fuel with high energy density. The reaction utilizes water as a reaction medium and can therefore utilize feedstocks with a high moisture content. The significant amount of water required may pose disposal concerns for an HTC plant on a large scale; hence, possible valorization options for this water are needed. As highlighted by different authors, there are many uncertainties in the composition of the process water. This review paper compares the physical and chemical properties of process water obtained under different HTC conditions, including organic compounds, heavy metals, color, nutrients, pH, and toxic compounds. Various valorization pathways of the process water and the companies actively investing in HTC with these valorization pathways in mind are reported in this review. Opportunities for resource recovery in process water and the potential for future work in process water valorization are also discussed. • Process water from the HTC of biomass contains many useful organic compounds. • Properties of process water depend on the feedstock and HTC process conditions. • HTC process water holds untapped potential for resource recovery. • Valorization of process water can enhance HTC process viability.