Effect of manure co-digestion on methane production, carbon retention, and fertilizer value of digestate.

Anaerobic digestion can provide benefits not only from the perspective of renewable energy production but also in the form of fertilization effect and increased retention of C in soils after digestate application. This study consisted of two phases, where the first phase assessed the suitability of carbon-rich co-feedstocks for methane production via laboratory testing. The second phase assessed the balance and stability of C before and after anaerobic digestion by systematic digestate characterization, and by evaluating its carbon retention potential using a modeling approach. The results indicated that pyrolysis chars had a negligible effect on the methane production potential of cattle manure, while wheat straw expectedly increased methane production. Thus, a mixture of cattle manure and wheat straw was digested in pilot-scale leach-bed reactors and compared with undigested manure and straw. Although the total amount of C in the digestate was lower than in the untreated feedstocks, the digestion process stabilized C and was modeled to be more effective in retaining C in the soil than untreated cattle manure and wheat straw. In addition, digestion converted 23-27 % of the C into valuable methane, increasing the valorization of the total C in the feedstock. Considering anaerobic digestion processes as a strategy to optimize both carbon and nutrient valorization provides a more holistic approach to addressing climate change and improving soil health.

Application of mathematical optimization to exploit regional nutrient recycling potential of biogas plant digestate.

Nutrients can be circulated back to agriculture from waste streams through anaerobic digestion and digestate processing. Digestate processing, however, is making slow progress as circulated nutrient products have not been cost competitive compared to fossil fertilizers and not designed from the farmer's perspective to truly match with the regional nutrient need. In this study, the aim is to assess apply mathematical optimization to the design of a cost-optimal processing route for a biogas plant's digestate to produce fertilizer products based on specified regional needs. Another aim is to analyze whether such a cost-optimal solution can fully exploit the nutrient recycling potential, that is, the efficiency of such a solution in returning nutrients to agriculture. The results indicate that mathematical optimization allows the design of a cost-optimal digestate production routes based on the region's nutrient need and characteristics. The true cost optimum was found for a design combining three processing technologies and producing four nutrient products, which when mixed, would fulfil farmer's fertilization needs. However, there seems to be a conflict between an optimal economic design and a full exploitation of recycling potential as only 25% of the digestate's phosphorus was utilized within the case region. This is because only 29% of the digestate mass was used and processed as fertilizer, as the concentration of required nutrients was deemed too low for economic use. The proposed mathematical model could be implemented as tool to assist in biogas plant investment decisions.

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.