Abstract
Biogas production from manure is of particular value in regard of lowering greenhouse gas emissions and enhancing nutrient re-circulation. However, the relatively low energy content and the characteristics of manure often result in low degradation efficiency, and the development of operating strategies is required to improve the biogas yield and the economic benefits. In this study, the potential to enhance the performance of two full-scale biogas plants operating with cattle manure, in mono-digestion or combined with poultry manure, was investigated. Four continuously fed laboratory-scale reactors were operated in sets of two, in which the temperature in one reactor in each set was increased from 37–42°C to 52°C. The potential to increase the capacity was thereafter assessed by increasing the organic loading rate (OLR), from ca 3 to 5 kg volatile solids (VS)/ m3 and day. The processes were evaluated with both chemical and microbiological parameters, and in addition, the residual methane potential (RMP) was measured to evaluate the risk of increased methane emissions from the digestate. The results showed that both processes could be changed from mesophilic to themophilic temperature without major problems and with a similar shift in the microbial community profile to a typical thermophilic community, e.g., an increase in the relative abundance of the phylum Firmicutes. However, the temperature increase in the reactor co-digesting cattle and poultry manure caused a slight accumulation of fatty acids (2 g/l) and reduced the specific methane production, most likely due to ammonia inhibition (0.4–0.7 g NH3/l). Still, during operation at higher OLR, thermophilic as compared to mesophilic temperature slightly increased the methane yield and specific methane production, in both investigated processes. However, the higher OLR decreased the overall degree of degradation in all processes, and this showed a positive correlation with increased RMP values. Chemical analyses suggested that high RMP values (40–98 Nml gVS−1) were related to the degradation of cellulose, hemicellulose, and volatile fatty acid enriched in the digestate. Conclusively, increased temperature and load can increase the methane yield from manure but can result in less efficient degradation and increased risks for methane emissions during storage and handling of the digestate.
Highlights
Manure, if poorly managed, represent a source of air and water pollution; if well integrated into the management chain, manure can be instead be a resource (Burg et al, 2018)
Chemical analysis revealed that the concentrations of cellulose, hemicellulose, and lignin were higher in the substrate mix used in biogas plant B compared with plant A (Supplementary Tables S1–S3)
This study showed that changing the operating temperature from mesophilic to thermophilic conditions was possible for both investigated processes
Summary
If poorly managed, represent a source of air and water pollution; if well integrated into the management chain, manure can be instead be a resource (Burg et al, 2018). The potential for biogas production from cattle manure is generally high in Europe, and in Sweden the potential is estimated to ca 3–6 TWh yearly (Achinas et al, 2020; Swedish Energy Agency, 2020a). The production of biogas from manure is slowly increasing, but the development is slow due to difficulties in profitability (Ahlberg-Eliasson, 2018). Cattle manure has high water and fiber contents, resulting in low biogas yields and degradation efficiency, which hamper an increased use of the biogas technology in agriculture (Ahlberg-Eliasson, 2018). To reach economically feasible biogas production from manure, it is essential to find ways to improve the degradation and methane yield as well as reduce risks for pathogen spreading
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