Hydrothermal Carbonization of Fish Waste: A Sustainable Pathway for Valorization and Resource Recovery

Fisheries and aquaculture residues pose escalating environmental challenges due to their high moisture content, nutrient loads, and pollutant potential when improperly managed. Conventional valorization routes, such as fishmeal, fish oil, and silage, offer partial mitigation but remain limited in scalability, conversion efficiency, and environmental performance. In this study, fish processing residues were subjected to hydrothermal carbonization (HTC) under controlled subcritical conditions (180–220 °C), along with a high-severity catalytic run (325 °C) using sodium bicarbonate (NaHCO3) as an additive. The latter condition exceeded the typical HTC range and entered the subcritical hydrothermal liquefaction (HTL) regime. The resulting solid, liquid, and gaseous fractions were comprehensively characterized to assess their energy potential, chemical composition, and reactivity. Hydrochars achieved higher heating values (HHVs) ranging from 14.2 to 25.7 MJ/kg. These results underscore their suitability as renewable solid fuels. The gas products were dominated by CO2 under standard HTC conditions. In contrast, the catalytic run in the subcritical HTL regime achieved a hydrogen enrichment of up to 30 vol.%, demonstrating the efficacy of NaHCO3 in promoting the water-gas shift reaction. Subsequent air gasification confirmed the high reactivity of the hydrochars, producing syngas enriched in H2 and CO at elevated temperatures. Overall, this study demonstrates a scalable multiproduct valorization route for fishery residues, supporting circular bioeconomy strategies and contributing to the achievement of UN Sustainable Development Goals (SDGs 7, 12, and 13).