Swedish farm-scale biogas production-substrates and operating parameters

Abstract: Biogas production from agricultural waste streams provides three value streams: production of fossil-free energy, reduced greenhouse gas (GHG) emissions and production of nutrient-rich digestate that can be used as fertiliser. However, farm-based biogas production is expanding rather slowly. One limitation is the low energy content and restricted degradation of manure. Previous research has identified strategies for improving gas production, but the other two value streams have received less attention. Therefore Swedish farm-scale biogas plants (FSBP) were evaluated in this thesis with the aim of identifying ways to improve overall efficiency in terms of high gas yields, high degree of degradation and high digestate nutrient concentration. The results revealed large variations in the efficiency of Swedish FSBP, but with consistent correlations between gas production, degree of degradation and retention time. Co-digestion was found to be a commonly used strategy to improve biogas yield and digestate nutrient content. Detailed laboratory studies of different operating strategies showed that co-digesting manure with rapeseed oil, starch, albumin or cultivated energy crops had positive effects on volumetric gas yield and/or plant nutrient levels in digestate. Increased digester temperature improved biogas yield from cattle manure, whereas for poultry manure it instead resulted in decreased gas yield and instability, due to high ammonia levels. Residual methane potential in digestate was found to correlate positively with organic load and negatively with retention time, illustrating the importance of sufficient duration of degradation in reducing GHG emissions. Based on these findings, it can be concluded that measurement and evaluation of residual methane potential is a promising tool in understanding FSBP processes and assessing their efficiency.