A farm-scale grass biorefinery concept for combined pig feed and biogas production.

This study assessed the nutrient flows and the economic viability of a farm-scale grass biorefinery concept for a pig farm. Grass silage was separated into liquid and solid fractions; the liquid fraction was used as pig feed and the solid fraction was directed to biogas production together with pig slurry. The addition of grass to the farm's crop rotation and its use in feed and biogas production slightly increased the share of nitrogen (N) circulating within the farm (by 2%), thus decreasing phosphorus circulation by 3%. Despite the positive effect on N balance, the economy of the farm-scale concept had challenges. However, upgrading biogas to vehicle fuel and selling it on farm was more economically viable than combined heat and power production. The proposed concept could be economically viable with a slight increase in the price of vehicle fuel, a moderate increase in the price of the grass liquid fraction, or better optimization of the system, starting with grass cultivation and processing. Moreover, profitable production could also be reached by increasing the scale of production by increasing farm size or by two or more farms working together.