Integration of Anaerobic Digestion and Hydrothermal Liquefaction for Resource Recovery from Fish Waste Digestate

In Taiwan, over 400,000 tonnes of fish are consumed annually through commercial fish markets, with approximately 45% converted into low-value fish waste (FW), including viscera, bones, and scales, resulting in more than 200,000 tonnes of fish waste each year. These fish wastes are characterized by high moisture content and high biodegradability, particularly proteins and lipids. They are predominantly managed through low-efficiency practices such as landfilling, composting, or feed conversion, which often attract environmental concerns.

Anaerobic digestion (AD) is a conventional technology for converting organic waste into renewable energy. Previous studies have shown that AD of fish waste is frequently inhibited by long-chain fatty acids and high nitrogen content, leading to limited energy recovery and the generation of large volumes of digestate that require further treatment. Meanwhile, hydrothermal liquefaction (HTL) can effectively convert high-moisture and heterogeneous biomass materials into bio-crude oil without the need for energy-intensive drying, making it particularly suitable for anaerobic digestates.

This study investigates the integration of anaerobic digestion as a pre-treatment step with HTL to explore the resource recovery potential of liquid and solid digestate derived from fish waste. Fish waste was subjected to anaerobic digestion under five different substrate-to-inoculum (S/I) ratios, and HTL subsequently treated the resulting digestate under subcritical water conditions at temperatures ranging from 275 to 325 °C with a residence time of 30-60 min. The distribution and characteristics of the main HTL products, including bio-crude oil, solid residue, and aqueous phase, were analyzed.

The anaerobic digestion results showed variable biomethane production across different S/I ratios (1.84 ± 0.27-93.45 ± 8.84 mL CH₄ g^-1 VS). Notably, the AD process was utilized to partially degrade macromolecules while preserving sufficient organic carbon for subsequent HTL. Under HTL conditions at 325 °C for 60 min, bio-crude oil yields reached over 50 wt%. Gas chromatography–mass spectrometry (GC–MS) analysis indicated that the bio-crude oil was dominated by lipid-derived aliphatic amides, alongside protein-derived nitrogen-containing heterocyclic compounds, including lactams and fatty acid pyrrolidides. This suggests that the organic constituents of fish waste were effectively transformed into the oil phase with promising potential for further upgrading and valorization, while partially incorporating nitrogen into stable oil-phase compounds.

Furthermore, a life cycle assessment (LCA) framework was applied to compare this integrated AD–HTL pathway with traditional fish waste management practices to evaluate its potential environmental benefits in terms of resource recovery and pollution mitigation. Environmental benefits (such as carbon negativity and resource circularity) can be expected compared with conventional treatments for fish waste. Overall, the findings demonstrated that integrating biological and thermochemical processes could contribute to more sustainable approaches for managing biomass wastes, enabling the recovery of high-value secondary energy carriers and material resources.