Carbon capture in breweries using microalgae: Preliminary techno-economic insights for ruminant feed applications

Australian breweries produce large volumes of carbon dioxide (CO₂) and brewers' spent grain (BSG), presenting an opportunity for sustainable reuse. Microalgae can utilize brewery CO₂ and convert it into biomass desirable for ruminant feed applications. However, production costs remain a key limitation, largely influenced by cultivation system design. This techno-economic analysis (TEA) evaluated four systems, including bag photobioreactors (BPBR), vertical tubular photobioreactors (VPBR), thin-layer photobioreactors (TLPBR), and a helical thin-layer photobioreactor (HEPBR), for installation into a small to medium-sized brewery producing 3,000 hectoliters of beer annually and generating 10.2 tons CO₂ and 26.4 tons dry BSG. Productivity assumptions were derived from published ranges and applied as baseline values (BPBR: 0.25; VPBR: 1.2; TLPBR: 4.4  kg m^-3 d^-1). A "what if" scenario assessed three HEPBR productivity cases (5.7, 11.3, and 17  kg m^-3 d^-1), representing theoretical design potential. Equipment prices were sourced from supplier catalogues, direct quotes, and recent literature. Energy inputs assumed electricity was supplied from solar at 0.20 AUD kWh^-1. Results indicate that at this scale, ∼3.41 tons dry algae yr^-1 could be cultivated, with standalone algae cultivation economically unviable for feed applications (> $20 AUD kg^-1). Integrating algae with BSG reduced total feed costs to < $10 AUD kg^-1. The HEPBR scenarios demonstrated that improved reactor design may enhance feasibility, although economic outcomes remain sensitive to productivity assumptions. Overall, the integrated approach evaluated in this TEA aligns with circular bioeconomy principles and provides a pathway for breweries to valorize waste and contribute to more sustainable agricultural practices.