To ask the Minister of Agriculture, Environment and Rural Affairs for his assessment of the utilisation of anaerobic digestors as a method of reducing carbon emissions.

Anaerobic digestion (AD) of organic feedstocks to produce renewable energy is considered to be carbon neutral as the carbon contained in the feedstock is derived from non-fossil carbon sources. The energy generated by AD is a direct replacement for fossil fuels (natural gas, oil and coal) and the other product of the digestion process, anaerobic digestate, is a valuable fertiliser.

One of the approaches to assessing carbon emission reduction potential of AD is Life Cycle Analysis (LCA). In a recent peer –reviewed study undertaken at Queen’s University Belfast, Pérez-Camacho et al (2019) considered the LCA of biogas production and utilisation in three scenarios: substituting for grid electricity; natural gas grid; and transport fuels, with a focus on Greenhouse Gas (GHG) emissions.

Their study suggests significant GHG emission reductions for a ‘biogas as a fuel’ substitution scenario due to the displacement of fossil petrol and diesel fuels with savings of between 524 and 477 kg of CO2 equivalent (per MWh of energy provided by the fuels). The study also suggested lower theoretical GHG emissions when biogas was substituted for current fuels used for electricity generation, or if substituted for natural gas (after being upgraded to biomethane).

When considering the whole LCA, it is also important to note anaerobic digestate is a valuable fertiliser, which when applied to agricultural land provides nutrients required for crop growth. Application of digestate also results in the incorporation of carbon in to soil where it increases the soil carbon levels and is stored for take up by crops over subsequent growing seasons.

Mezzullo et al (2013) found significant GHG savings from anaerobic digestate application when the effect was displacing inorganic fertiliser use, though did also note the importance of considering consequential increases in ammonia emissions and the potential acidification/eutrophication impacts on ecosystems. However, these can largely be resolved with best practice management, in the main by gas tight storage of the digestate, adjustment of pH, as well as by the timing and method of application of the digestate to land as a biofertiliser.