Remaining methane potential in digestate from biogas plants Financed by Energiforsk, Region Skåne, IVL and participating biogas plants

Background The digestate that leaves the biogas plant is not completely digested. residual biogas potential in the digestate The amount of degradation depends on: Type of substrates Retention time The biogas process Secondary digester The degree of degradation affects The amount of produced biogas that can be utilised Methane formation in the subsequent storage of the digestate The mineralization degree of the organially bound nitrogen Lack of data on the biogas potential in the outgoing digestate from co-digestion plants in Sweden.

Purpose The project aims to evaluate the residual biogas potential in the digestate after the primary digester and after the secondary digester. This knowledge will provide a basis for both more environmentally and economically optimized biogas plants.

Goal Determine BMP (Biochemical Methane Potential) in outbound biomass after Primary digester and after Secondary digester. Carry out economic calculations that show the profitability of increased degradation of the biomass through various additions in the process solution. Determine the mineralization of organic bound nitrogen in the withdrawn samples and also after the BMP tests.

Method Sampling from biogas plants BMP test Analysis Primary digester Secondary digester BMP test The same temperature as the biogas plants Three occasions Analysis Before and after BMP test Economic assessment Retention time Biogas potential Payback period

Substrate distribution in percent

Results BMP A considerable part of the biogas production takes place in the Secondary digester

Results BMP Highest 5.6 m3 methane/ m3 digestate Lowest 2.5 m3 methane/ m3 digestate. total of 42-100 GWh remaining in the digestate in Sweden. 5-12% of the energy in the biogas production in Sweden.

Environmental consideration The BMP tests the digestion has occurred at a favorable temperature for the methane producing bacteria. The gas production obtained in the trials should therefore not be equated with methane emissions from the biogas storage, but is a measure of what can still be recovered in the biogas plant. Environmental permit Applicants should when applying explain how the biogas plant ensures that a adequate degradation is achieved.

Increased retention time After 20 days about 2/3 of methane production has been obtained. New Secondary digester

Average methane potential

Pay back period The calculation does not include site-specific costs, land works, pipelines and other things required.

Mineralization of organic bound nitrogen

Slutsatser BMP tests Economic perspective The value of the digestate In general the digestate from the biogas plants is well digested, which is very important for the risk of methane emissions. Biogaspotential of 42-100 GWh remaining in the digestate in Sweden. Economic perspective In order to digest the remaining biogas potential, retention times and digester volumes are required which are not always practical and economically reasonable. For some of the biogas plants it may be profitable with additional digester to increase retention time by 5-20 days. The value of the digestate An increased digester volume also causes the proportion of nitrogen converted to ammonium nitrogen to increase

Thank you for your attention! www.ivl.se Anders Hjort Anders.hjort@ivl.se 0761111643

Participating Biogas Plants Borås Energi och Miljö - Sobacken Brogas AB Falkenbergs Biogas AB Kristianstad Biogas AB OX2 - Helsingborg/NSR SBI Jordberga AB SBI Västerås AB Söderåsens Bioenergi AB Södra Hallands Kraft Biogas AB Uppsala Vatten och Avfall AB Vårgårda Herrljunga Biogas AB