Upgrading biogas plants to produce microbial proteins for aquaculture feed

As the world population continues to grow, alternative protein sources must be sought to meet increased protein needs and avoid the detrimental effects of traditional protein production. Microbial proteins (MP) are a lowcarbon footprint alternative suitable for manufacturing food and feed. Bacterial-based MP can be produced at moderate costs using agro-waste, which is already utilised by anaerobic digestion (AD) plants to produce biogas and fertilisers. At present, there is the opportunity to turn existing AD plants into biorefineries for bio-based products like MP destined for fish feed. In this work, we evaluated the feasibility of biogas production coupled with MP production in the same medium-sized European biogas plant through mass and energy balances and basic economic assessment. The liquid phase rich in volatile fatty acids resulting from agro-waste fermentation was used to obtain MP by mixed cultures in a continuous stirred tank reactor, with the highest VSS productivity (1.21 g/L day) obtained at hydraulic retention times of 2-3 days. The obtained MPs were rich in proteins (74.0% of TVS), PHAs (22.5%) and essential amino acids, useable as feed ingredients in aquaculture. An initial economic analysis showed that the production of 590 tMP/year could yield a break-even price of 1300 <euro>/t of MP, taking into account the investments in the new technology and additional operating costs. This analysis illustrates the competitiveness of a combined plant when evaluated against established market benchmarks and highlights a promising prospect for the expansion of MP production capacity.