Advancing the circular economy: A zero-waste valorisation approach for the de-oiled fish waste through anaerobic co-digestion and fertilisers production

This study explores an innovative biorefinery approach to achieving a zero-waste objective for tuna (e.g., Thunnus albacares, Thunnus thynnus) waste. The process integrates fish oil extraction using bio-based solvents (2-MeOx, ethanol, and d-Limonene) with anaerobic co-digestion (co-AD) and struvite precipitation to produce biomethane and fertilisers (struvite and digestate). Onion stalks were used to balance carbon and nitrogen content during co-AD. Co-AD with 30 % (Volatile Solids based) raw fish waste increased methane yield by 25 %, while co-AD with de-oiled fish residues treated with 2-MeOx and ethanol enhanced methane yield by 46 % and 58 %. However, residual ethanol in the substrate negatively affected process stability, causing acidification, and d-Limonene inhibited microbial metabolism. The digestate was used to produce struvite, a fertiliser, with a 90 % recovery of NH4+-N at pH 9. Phosphorus sourced from fish bones was also successfully integrated into struvite precipitation, offering an additional valorisation route for solid residues. Pot tests with Capsicum annuum plants demonstrated successful fertiliser use even for raw digestate, with plant growth comparable to that achieved with conventional chemical fertilisers. This strategy holds promise for a more sustainable blue economy. This integrated cascade biorefinery model thus demonstrates a feasible pathway for transforming tuna processing waste into multiple high-value products, contributing to a more resilient and sustainable blue economy, while promoting waste minimisation and resource circularity.