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

2. 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.

3. 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.

4. 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Determine the mineralization of organic bound nitrogen in the withdrawn samples and also after the BMP tests.

5. 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

6. Substrate distribution in percent

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

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

9. 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.

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

11. Average methane potential

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

13. Mineralization of organic bound nitrogen

14. 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 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

15. Thank you for your attention!
Anders Hjort

16. 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