**Economic and Environmental Analysis of Small-Scale Anaerobic Digestion Plants on Irish Dairy Farms**

### **Sean O'Connor 1,\*, Ehiaze Ehimen 1, Suresh C. Pillai 1, Gary Lyons 2 and John Bartlett 1**


Received: 29 December 2019; Accepted: 26 January 2020; Published: 3 February 2020

**Abstract:** The European Union's (EU) climate and energy package requires all EU countries to reduce their greenhouse gas (GHG) emissions by 20% by 2020. Based on current trends, Ireland is on track to miss this target with a projected reduction of only 5% to 6%. The agriculture sector has consistently been the single largest contributor to Irish GHG emissions, representing 33% of all emissions in 2017. Small-scale anaerobic digestion (SSAD) holds promise as an attractive technology for the treatment of livestock manure and the organic fraction of municipal wastes, especially in low population communities or standalone waste treatment facilities. This study assesses the viability of SSAD in Ireland, by modelling the technical, economic, and environmental considerations of operating such plants on commercial Irish dairy farms. The study examines the integration of SSAD on dairy farms with various herd sizes ranging from 50 to 250 dairy cows, with co-digestion a fforded by grass grown on available land. Results demonstrate feedstock quantities available on-farm to be su fficient to meet the farm's energy needs with surplus energy exported, representing between 73% and 79% of the total energy generated. All scenarios investigated demonstrate a net CO2 reduction ranging between 2059–173,237 kg CO2-eq. yr<sup>−</sup>1. The study found SSAD systems to be profitable within the plant's lifespan on farms with dairy herds sizes of >100 cows (with payback periods of 8–13 years). The simulated introduction of capital subvention grants similar to other EU countries was seen to significantly lower the plant payback periods. The insights generated from this study show SSAD to be an economically sustainable method for the mitigation of GHG emissions in the Irish agriculture sector.

**Keywords:** anaerobic digestion; methane production; co-digestion; combined heat and power; farm-scale; technical-economic analysis; life cycle assessment; greenhouse gas emission; Ireland
