Despite Ireland having "lagged behind" in the area of anaerobic digestion, "there are great lessons to be learned from how it has been applied in Europe".

With a “consensus” that biogas production itself may result in greenhouse gas emissions, experts warn there is a need for the environmental risk to be assessed in determining the overall policy approach.

During a recent Oireachtas joint committee on environment and climate action session, researcher Ciara Beausang said that when the environmental sustainability of biogas is discussed, it is usually focused on greenhouse gas mitigation. Ms Beausang said that one of the most important factors to consider is the raw material from which biogas will be produced.

“In Ireland, the most readily available resource for biogas is grass silage, as outlined in reports from the Sustainable Energy Authority of Ireland and Gas Networks Ireland,” she told the committee.

Co-digestion of grass silage with animal manures such as cattle slurry is likely to be the main way biogas is produced nationally. There is a risk that using high shares of grass silage may have negative environmental impacts.

She said that “conventional thinking” was that additional grass silage for biogas could be produced by increasing fertiliser application.

“However, in my research, I modelled the environmental impacts of digesting different proportions of grass silage and cattle slurry for biogas production and found that using high shares of grass silage may have negative environmental impacts," she continued.

Ms Beausang explained that while this approach avoids competition with feed production, the results showed that this can lead to an increase in greenhouse gas emissions when higher proportions of grass silage are digested due to the additional fertiliser that is required.

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“The impact arises from both the production of the fertiliser itself and the emissions that are released when it is spread on land," she said.

“As a result, alternative approaches need to be considered to provide surplus grass silage without relying on additional inorganic nitrogen fertiliser.” 

She said that preliminary research has shown that incorporating legumes such as red closer or the use of multi-species swards “has the potential to increase yields without the addition of inorganic nitrogen”.

“This would improve the sustainability of biogas produced from silage and should be promoted, provided that other potential impacts, such as carbon loss and herbicide use during reseeding, are minimised.” 

Ms Beausang also raised concerns about methane leakage from biogas plants.

Methane, the main component of biogas, is a “powerful, short-lived climate pollutant” she said.

“When methane leakages occur due to increased biogas production, these can contribute to global warming and reduce the environmental benefits,” she explained.

“The rate of methane loss assumed in life-cycle assessments of biogas is relatively conservative, with many studies assuming a loss of 1% by default. 

"In my research, I used a value of 2.4%, which was the average rate of emissions for 13 agricultural biogas plants in Denmark.

“As methane loss may be the largest contributor to the carbon footprint of biogas production, it would be important that biogas plants in Ireland monitor, report and address methane losses.” 

She also told the committee that there is a “potential risk of burden-shifting occurring” if other environmental impacts are not taken into account.

“The material remaining after anaerobic digestion is known as digestate and can be used as a fertiliser. Digestate has been shown to have higher levels of ammonium compared with the organic substrate going into the AD process,” she said.

“As a result, there is a risk of increased environmental impacts from acidification due to the emissions from digestate application.

“This is a concern, especially as Ireland continues to breach its emissions target for ammonia under the national emissions ceilings directive.

“Appropriate digestate management will be critical, including the use of low emission slurry spreading [LESS] for digestate application.”

Fianna Fáil TD Christopher O’Sullivan has drafted a bill to allow for the State to come up with a national strategy for recycling organic waste and turning it into biogas.

He told the meeting that a "blind eye" cannot be turned to the potential environmental risks associated with these technologies, and asked Ms Beausang to confirm if AD is a "potentially sustainable technology that can lead to the decarbonisation of agriculture" if the risks are mitigated against by displacing the leakage in the process.

"There are potential solutions to the risks," she said. 

"It has been mentioned that Ireland has lagged behind in this area compared with Europe, but there are great lessons to be learned from how it has been applied in Europe. 

"If all of these areas are addressed, sustainability could be enhanced."

Ms Beausang also told the meeting that biomethane will form a "relatively small percentage" of the decarbonisation of energy in Ireland.

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