Foderbetor och kogödsel som substrat för biogasproduktion; anaerob mesofil samrötning i labbskala

One of Sweden’s sixteen national environmental objectives strives to decrease the impact on the climate. By 2020, green house gas emissions should be 40 % less compared to the levels of 1990 and a minimum of 50 % of the energy consumption should come from renewable energy sources. Because of this there is a great need of increasing the production of renewable energy. This is where biogas comes in as a competitive alternative. However, there is a problem. Substrates for biogas production have become a scarcity due to the increasing demand for biogas that has been expressed lately. The objective is to co-digest fodder beet and cow manure in order to find the best methane yielding mixture(s), and based on these results be able suggest whether fodder beets are a suitable substrate to use in a greater extent. This might prove to be one possible way in dealing with today’s substrate shortage. The mesophilic co-digestion has been performed in lab scale, where seven glass bottles (800ml) have served as digesters. Several analyses of important parameters i.e. gas production and composition, carbon/nitrogen-ratio, total solids and pH have been performed during and after the digestion tests. The best results (25-28 % methane) were obtained when 5-15 % beets were added to the digester. This shows that fodder beets is best used as a supplement in a smaller extent. Results also show that, when using fodder beets as substrate, two of the most important process parameters to control are pH and buffer capacity (alkalinity/volatile organic acid ratio). This is because the digester easily gets acidic. Based on these results it’s hard to suggest fodder beet usage in a greater extent when being mixed with only cow manure for biogas production. This is especially true for smaller biogas plants where the possibility of observation often is limited or deficient. However, because of the rapid initial degradation and gas production, fodder beets would make a good complement in digesters where more persistent substrates with slower gas production are being digested. Also, since fodder beets are rich in carbon and poor in nitrogen they would also make a good complement when one desire to increase the carbon/nitrogen-ratio in the digeste