Analysis: anaerobic digestion is a natural biological process which converts animal slurries and other wet organic residues to biogas

By Dónal Ó Céileachair, David Wall, Richard O'Shea and Jerry Murphy, UCC

Ireland recently set a target to cut greenhouse gas emissions in agriculture by 25%, which set off bitter exchanges between farmers and environmentalists. This highlighted the difficulty in reaching a zero-carbon economy; agriculture already accounts for over 37% of total emissions in Ireland.

If Ireland struggles with a 25% reduction, how will we cut emissions by 100%? Intense pressure has been placed on farmers to quickly become more sustainable and reduce methane emissions generated from belching cattle and slurry management.

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From RTÉ News, Climate Change Advisory Council concerned about dairy sector emissions

To complicate matters, the Russian invasion of Ukraine has raised serious concerns about energy security and the availability of natural gas. Increasing costs of fertiliser due to the scarcity of natural gas creates further cost challenges for farmers producing grass for cattle. The rising costs of all forms of energy (diesel, electricity, natural gas) impacts the farmer's balance sheet. Society at large is at risk of electricity 'blackouts’ this winter. Are there solutions to these challenges?

Anaerobic digestion is a natural biological process which converts animal slurries and other wet organic residues to biogas, comprised of 60% methane and 40% carbon dioxide (CO2). This occurs in large tanks (digesters), where microbes, in the absence of oxygen, consume organic material to produce the biogas and a biofertiliser. It's a mature technology and has been used widely to treat a variety of wastes (dairy washings, crop residues, and food and beverage by-products) in countries such as France and Denmark.

The model in Denmark is to remove the CO2 from the biogas to have pure methane (termed biomethane). This is subsequently injected into the natural gas grid where it can be used by the food and beverage industry (cheese, infant formula, alcohol), home heating, or as fuel for heavy transport including tractors, buses, and trucks. Denmark now has a production capacity equal to about 25% of their natural gas.

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From EnviTech Biogas, inside a biogas plant in Flemløse in Denmark

Ireland has not yet embraced this technology, but it offers a unique opportunity to reduce emissions from agriculture and provide a natural gas substitute to the agri-food industry. Biogas can also provide a foil for intermittent renewable electricity, as it can be burned "as is" in a generator to produce heat and electricity. Unlike wind turbines or PV solar panels, which rely on specific weather conditions, biogas is a ‘dispatchable’ resource that can produce electricity constantly.

Even better, biogas can be stored and used to produce increased amounts of electricity at periods of high demand. When the wind is not blowing, we can ‘turn on’ biogas facilities to generate renewable electricity and reduce dependency on fossil fuels. This could play a vital role in reducing the likelihood of winter blackouts and add to the smart energy systems required in the future.

But anaerobic digestion encompasses so much more than just renewable energy. The CO2 in biogas, when separated and cleaned, can be food-grade and used in hospitals, beer production, and in food packaging. The market in Denmark for CO2 is 65,000 tonnes per year, and one large digester system could supply 25% of this market.

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From RTÉ Radio 1's Countrywide, Billy Costello from the Costello Group on how anaerobic digesters work

Similarly, the effluent from the digester, termed ‘digestate’, is a biofertiliser that can partially replace expensive imported synthetic nitrogen fertilisers in the form of ammonia, which is sourced from natural gas and has rapidly increased in price due to the Russia-Ukraine conflict. Farmers now face a major challenge in obtaining fertiliser, which may lead to reduced crop yields and more expensive food.

The use of digestate as fertiliser, instead of spreading raw animal slurries and manures on land, would reduce the pollution of our water courses, as almost all pathogens are destroyed in the digestion process; 70% of Irish wells currently suffer from contamination. Anaerobic digesters can also double as enhanced slurry storage systems over the winter months, and with sufficient nutrient management plans, overcome potential for eutrophication of waterways.

In terms of greenhouse gas emissions, anaerobic digestion reduces the methane emitted from open slurry tanks, as the process captures this methane in the form of biogas. Such emissions from slurry management practices account for 12% of total agricultural emissions. Methane has a global warming potential 28 times greater than CO2. In essence, when methane is captured in biogas and ultimately combusted, it releases CO2, which has 28 times less impact on global warming than the original methane.

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From RTÉ Radio 1's Countrywide, Donal Dennehy from Nutricia on plans to harvest biogas from farm waste including silage and slurry with the use of anaerobic digesters

On a whole life-cycle balance, combustion of biogas as an alternative fuel can result in less emissions compared to the open slurry holding tanks. Furthermore, the increased nutrient availability in digestate as compared to raw slurry reduces the need for synthetic nitrogen fertiliser, and further reduces emissions associated with fertiliser use.

The potential for anaerobic digestion in Ireland is considerable. Such systems can form an essential element of the circular economy, enabling a reduction in environmental impact as well as producing fuel, food grade CO2 and fertiliser. Researchers in the MaREI centre at UCC have provided quantitative evidence of the role of biogas in decarbonising industries in Ireland which at present are reliant on natural gas.

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Reducing emissions in agriculture is not an easy task. Nonetheless, anaerobic digestion presents an opportunity for farmers to diversify their revenue stream whilst protecting the environment and becoming more sustainable. Much can be learned from other countries in their historic development of anaerobic digestion systems. For instance, evidence would not necessarily support the digestion of crops alone from a sustainability perspective.

Co-operative approaches should be encouraged whereby local communities’ avail of local resources to reduce emissions across the agricultural, industrial, and transportation sectors, and enhance energy security by switching to locally produced biogas and thereby create jobs to keep rural communities viable.

Dónal Ó Céileachair is a PhD student in the Circular Economy, Energy and Environmental Systems (CEEES) group and researcher at the MaREI Centre at UCC. Dr David Wall is Lecturer in Transportation in the School of Engineering and Architecture at UCC. He is a Funded Investigator at the MaREI Centre and Principal Investigator in the Circular Economy, Energy and Environmental Systems (CEEES) group. Dr Richard O'Shea is Lecturer in Sustainability in Enterprise with the School of Engineering and Architecture at UCC. He is a Funded Investigator at the MaREI Centre and Principal Investigator in the Circular Economy, Energy and Environmental Systems (CEEES) group. Prof Jerry Murphy is Professor of Civil Engineering at UCC. He is Director of the MaREI Centre and Principal Investigator in the Circular Economy, Energy and Environmental Systems (CEEES) group.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ