From slurry to biomethane: a systems assessment of a centralised anaerobic digestion plant in Northern Ireland

The generation of excess slurry from livestock farming poses a significant environmental challenge in Northern Ireland (NI), where high livestock densities have led to nutrient surpluses, particularly phosphorus (P), and consequently contributing to water pollution. The environmental impacts of inadequately managed livestock slurry highlight the need for solutions to mitigate these effects while enhancing resource efficiency. Anaerobic digestion (AD) of livestock slurry offers a promising strategy to address these challenges. This study evaluates the role of a slurry-based biogas system centred on a 5 MW biomethane plant, assessing its potential to support regional nutrient management <br/>and decarbonisation objectives in NI. A multi-dimensional assessment was undertaken to compare scenarios involving the AD of slurry and grass silage. Environmental performance was evaluated using a system-boundary life cycle assessment, evaluating trade-offs associated with fossil natural gas displacement and mineral fertiliser substitution via digestate utilisation. The economic aspect was assessed using techno-economic modelling to determine the break-even biomethane price. Key technical considerations included on-farm slurry processing, biogas production and upgrading, digestate management, and ammonia recovery. Processing (mechanical separation) of slurry on-farm produces solid and liquid fractions: the P-rich separated solid fraction is transported to the AD plant, while the nitrogen-rich liquid fraction is retained on-farm for agronomic use. This integrated approach enhances nutrient recovery, supports phosphorus redistribution, and strengthens carbon cycling within the agri-food system, while generating renewable energy. Results show the potential annual removal of 29 tonnes of phosphorus from farm slurry, greenhouse gas savings of approximately 6,000 t CO₂e, and a breakeven biomethane price of 13 p/kWh. These findings demonstrate that integrated slurry based biogas systems can support nutrient circularity, renewable energy generation, and climate mitigation simultaneously. With appropriate policy support and market incentives, livestock slurry <br/>valorisation can play a strategic role in advancing a circular bioeconomy and achieving net-zero targets in NI.