Recovery of Added-Value Products from Biowaste by Subcritical and Supercritical Water Technologies—A Review

The introduction of sustainable practices into waste management can have favorable environment impact, increase resource value, and economic gains. Hydrothermal technologies have strong potential for the production of up-cycled ingredients from biowaste (amino acids, sugars, phenols, pharmacologically-active compounds, etc.), enabling additionally high energy recovery (50-80%) from biowaste with net-negative carbon emission. This review discusses the use of subcritical and supercritical water technologies for sustainable valorization of biowaste and conversion of biomass into high-value chemicals and biofuels. The potential for the extraction/generation of bioactive compounds from plant and animal waste is presented, emphasizing the efficiency, compound stability, and bioactivity of fractions obtained. The possibilities of simultaneous extraction of added-value compounds and hydrolysis of feedstock biopolymers by said technologies are elaborated. The review further addresses the production of biofuels through hydrothermal carbonization for solid fuels, hydrothermal waste liquefaction for liquid fuels, and supercritical water gasification for gaseous fuels. The paper highlights the environmental and economic advantages of technologies based on sub- and supercritical water, over conventional chemical and fermentative routes, emphasizing their contribution to circular bioeconomy by converting biowaste into value-added products and sustainable energy sources.