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Methane Emission Leakage from Co-Digestion on Dairy Farms

September 24, 2015

Anaerobic digestion of manure is an excellent process for reducing the potential pathogens and antimicrobial resistance associated with manure management. It is also an excellent process for producing methane, but science and experience suggests that anaerobic digestion will not reduce greenhouse gas emission. We can expect that methane emission will not decrease, and nitrous oxide emissions may increase unless further processing of the liquid and solid digestate occurs following digestion. This post will focus on methane emission.

Anaerobic digestion is not cost effective using a farm’s manure alone (BC Bioprducts Association 2007, Mallon and Weersink 2007, Gregersen et al. 2007, Shumway and Bischop 2008, ECOregon 2009).  Cornwall Agri-Food Council Development Team (undated) provided modelling of anaerobic digestion in one area in the UK, and determined that anaerobic digestion was not economically viable without grants, or non-manure wastes such as potatoes or other off-farm waste.

There are three implications for methane production and emission when considering input of additional off-farm waste:

  1. methane emission reduction credit for the off-farm waste
  2. increased methane production when including off-farm waste
  3. increased “leakage” (methane emission from the digestate after it comes out of the digester).

Methane emission to the atmosphere from a dairy farm importing off-farm wastes may be higher than if the farm had no anaerobic digester. If the off-farm waste has not been going to landfill and has not been producing methane in its previous management, there are no emission reduction credits available by importing the material to the farm. With the increased methane production during anaerobic digestion, additional “leakage” can be expected.

Methane Emission Credits When Co-digesting Manure with Other Solid Wastes

If the imported off-farm waste was previously disposed of at a landfill, it is possible to claim diversion credits because this waste would have emitted methane at the landfill. Washington State’s greenhouse gas emission protocol (Cook and Kruger 2008) includes two options for off-farm waste. The first is to assume that the off-farm waste has no CO2 emissions impact and therefore provides no carbon offset credit. The second is to assume that all off-farm waste was destined to go to landfill and provide the appropriate offset credit. This would have to take into consideration any methane emission reduction strategies such as landfill gas capture already in place at the landfill.

For off-farm waste in British Columbia, unless it was residential foodwaste, it is unlikely that it has been going to landfill. Most organic waste has already been diverted for animal feeding, composting, or deposits on agricultural land. BC BioProducts (2007) was not able to confirm that any of the readily available organic wastes for co-digestion were going to landfill.  Methane emission reduction credits from off-farm waste would require verification that it was being diverted from landfill.

Methane Production Increases When Including Off-farm Waste

Off-farm organic waste can dramatically increase methane production during anaerobic digestion. The figure below shows that the methane production potential of dairy cattle manure alone is relatively low compared to the potential methane production from other organic wastes.

Potential methane production from various organic wastes (from BC Bioproducts Association 2007)

Potential methane production from various organic wastes (from BC Bioproducts Association 2007)

Frear (2010) reported that adding 15-20% food processing waste to a dairy manure digester more than doubled biogas production and increased the percentage of methane in the biogas.  Ontario Ministry of Agriculture and Food (2007) reports that addition of up to 25% off-farm waste may double or triple biogas yield. Mang (2009) summarized anaerobic digestion projects in Europe and stated that increased biogas production from co-digestion with off-farm wastes may actually make some projects economically self-sustaining. (Taglia 2010) reported that adding off-farm waste could increase the biogas production five-fold. Jepsen (undated) stated that in Denmark, adding off-farm waste to manure digesters provides up to 60% of the digester’s methane production. Crolla et al. (undated) determined that biogas yields doubled when adding off-farm substrates to anaerobic digesters in Ontario.

Increased Potential for Methane Emission From the Digestate

The BC Bioproducts Association (2007) stated that: “It is important to note that up to 15% of biogas may be produced in the digestate storage. It is paramount that digestate storage be covered to limit direct emission to the atmosphere.”  The Leipzig Institute for Energy (undated) concluded that enhanced production of methane during digestate storage results from the higher amounts of biodegradable organic matter added for co-digestion with off-farm waste.

IPCC (2006) estimated that 5-15% of the potential methane production was emitted as “leakage”.  The UNFCCC CDM (2012) then further estimates ”leakage”  from the digestate after it has been removed from the digester and distinguishes between liquid and solid digestate. This ranges from 0.05 for two stage digesters, 0.10 for covered anaerobic lagoons, 0.15 for UASB type digesters, and 0.20 for conventional digesters.  The previous protocol developed by the UNFCCC CDM which was adopted by the US EPA (2011) was 10% unless local data is available to support an alternative.

Liebetrau (2011) measured “leakage” from 10 anaerobic digesters in Germany. He measured negligible CH4 leakage from the digesters themselves, 0.4 to 2.4% CH4 leakage during gas utilization, and 0.2 to 11% of the total CH4 produced during storage of the digestate. The amount of methane emitted during storage of digestate depended on whether the storage was covered or not. Lukehurst et al. (2010) showed the residual methane yield from digestate storage following varying retention times in three different types of anaerobic digesters. They also stated that “In European countries with a developed biogas sector (e.g. Germany, Denmark and Austria) there are now financial incentives to establish covered digestate stores, with the main objective of reducing emissions.” Linke et al. (2013) developed a model for estimating methane emission from digestate following anaerobic digestion and concluded that digestate storage tanks need to be covered to reduce CH4 emission and improve CH4 recovery.

In a review of GHG emissions from agriculture in Germany, Lengers (2011) concluded that “looking at the development of methane emissions resulting from manure management, agricultural soils and enteric fermentation, abatement efforts in methane emissions failed to have major impacts.”

There are two reasons why significant methane emission following the digestion process is expected when adding off-farm waste:

  1. Increased carbon, which increases the potential of methane emission because the digestion process continues after removal from an anaerobic digester,
  2. The digestate has been inoculated with methane producing bacteria

Based on the science, the EPA recommendation and the IPPC protocols, the expected methane “leakage” following anaerobic digestion is approximately 10% of the total methane production.


Importing off-farm organic waste is critical for economic viability of anaerobic digestion in British Columbia, unless it is fully supported by taxpayer money. In BC, there is negligible methane emission reduction potential associated with including off-farm wastes because most of these wastes have not been going to landfill.

Based on the leakage estimate of 10% of the methane production, and considering that methane production has doubled, it is reasonable to expect that the net methane emission to the atmosphere from implementing anaerobic digestion is higher than the baseline emissions from manure storage alone.

This suggests that implementation of anaerobic digestion for animal manure in British Columbia will not reduce greenhouse gas emissions resulting from methane.


BC Bioproducts Association. 2007. Feasibility Study – Anaerobic Digester and Gas Processing Facility in the Fraser Valley, British Columbia.

Cook, K., and C. Kruger. 2008. Recommendations for the Development of Agricultural Sector Carbon Offsets in Washington State. Agriculture Sector Carbon Market Workgroup.

Cornwall Agri-food Council Development Team. undated. Economic Modelling of Anaerobic Digestion / Biogas Installations in a Range of Rural Scenarios in Cornwall.

Crolla, A., C. Kinsley, T. Sauvez and K. Kennedy. undated. Anaerobic Digestion of Manure with Various Co-substrates. Research Note. Ontario Rural Wastewater Center, University of Guelph.

ECOregon 2009. Dairy Manure Anaerobic Digester Feasibility Study Report. Prepared for Volbeda Dairy, Oregon.

Frear, C. 2010. Co-digestion: Opportunity and Risk – A Washington State Perspective. Fifth AgStar National Conference, Green Bay, WI April 27-28, 2010.

Gregersen, K.H., H.B. Moller, S.G. Sommer, T. Birkmose and L.H. Nielsen. 2007. Promotion of biogas for electricity and heat production in EU countries. Economic and environmental benefits of biogas from centralized co-digestion. PROBIOGAS. An EIE/Altener project, co-funded by the EU Commission.

IPCC. 2006. 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Volume 4. Agriculture, Forestry and Other Land Use. Chapter 10. Emissions from Livestock and Manure Management.

Jepsen, S-E. undated. Co-digestion of animal manure and organic household waste – the Danish experience. Ministry of Environment and Energy. Danish EPA.

Lengers, B. 2011. GHG survey of German agriculture – specific view on dairy production systems. Technical paper 2011. Institute for Food and Resource Economics, University of Bonn.

Liebetrau, J. 2011. Analysis of greenhouse gas emissions from 10 biogas plants within the agricultural sector. Deutsches Biomasse Forshungs Zentrum.

Liepzig Institute for Energy. undated. GHG Mitigation by Anaerobic Digestion.


Linke, B., I. Muha, G. Wittum and V. Plogsties. 2013. Mesophilic anaerobic c0-digestion of cow manure and biogas crops in full scale German biogas plants: A model for calculating the effect of hydraulic retention time and VS crop proportion in the mixture on methane yield from digester and from digestate storage at different temperatures. Bioresource Technology 130: 689-695.

Lukehurst, C.T., P. Frost and T. Al Seadi. 2010. Utilisation of digestate from biogas plants as biofertiliser. Task 37. IEA Bioenergy.

Mallon, S., and A. Weersink. 2007. The Financial Feasibility of Anaerobic Digestion for Ontario’s Livestock Industries. University of Guelph.

Mang, H-P. 2009. Co-digestion: Some European Experiences.  German Society for Sustainable Biogas and Bioenergy Utilization (GERBIO). 2009 AgSTAR National Conference. Baltimore, USA.

Ontario Ministry of Agriculture and Food. 2007. Anaerobic Digestion Basics. Agdex Factsheet 720/400.

Shumway, C.R., and C.P. Bishop. 2008. The Economics of Anaerobic Digestion with Co-Product Marketing. Northwest Dairy Digester Workshop. November 2008.

Taglia, P. 2010. Biogas – Rethinking the Midwest’s Potential. Clean Wisconsin.

UNFCCC/CDM. 2012. United Nations Framework Convention on Climate Change/Clean Development Mechanism. Methological Tool: Project and Leakage Emissions from Anaerobic Digesters. EB 66 Report Annex 32 (


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