Pathway development for brewer's spent grain valorization using multi-objective optimization

Brewer's spent grain (BSG) is a major biowaste stream in the European beer industry used for animal feed, biogas and landfilling. Its composition offers potential for producing a variety of value-added products, making it crucial to benchmark valorization technologies for better management. In response to this sectorial need, the study applies a methodological framework previously developed open-source OUTDOOR software to the brewing industry context. A multi-objective optimization was performed for a superstructure that includes all possible BSG valorization pathways, guiding decision-making in biorefinery design, and assessing the economic and environmental sustainability of the process. Beyond traditional valorization options, a comprehensive review of potential products and conversion pathways-including biological, thermochemical and extraction routes-was conducted based on biomass characterization. A Pareto efficiency analysis was applied to integrate the two optimization objectives, using earnings before interest and taxes (EBIT) and minimum selling price (MSP) to evaluate economic performance, and the carbon footprint analysis compliant with ISO 14067 to assess environmental impact. Results indicate animal feed as the most profitable route, with an EBIT approximately 10 % higher than the best second alternative (compost). Environmentally, compost exhibits the smallest carbon footprint, around 91 % lower than the next best option (biogas). Ethanol and hydrochar stood out for their environmental value and proximity to market competitiveness, based on MSP. An evaluation of cascade conversion strategies suggested that the combination of multiple valorization routes is only viable through synergies between pathways, i.e. if obtaining one product facilitates the remaining routes through increased yields or resource efficiencies.