On-farm anaerobic digestion viability: policy perspectives and operational opportunities

Abstract

This thesis by published works examines on-farm anaerobic digestion (AD) viability under agricultural, economic and policy constraints. It argues that farm-scale AD cannot be assessed solely through energy yield or plant economics, because viability depends on interactions between feedstock availability, farm operations, costs, energy prices, heat use, regulation, incentives, nutrient recycling, methane mitigation and emerging technologies. The original contribution lies in integrating technical, economic and policy evidence to identify conditions under which on-farm AD can become more viable, particularly for slurry- and manure-based systems. The submitted works examine viability across progressively wider system boundaries. The first work considers small-scale livestock AD in a European policy context and frames slurry-led AD as advanced manure management and nutrient recycling rather than simply renewable electricity generation. The second develops this through a UK mixed-farm study, testing capital cost, energy prices, heat valorisation, policy support and financial return. These studies show that slurry-led AD can be technically feasible but financially sensitive, and that energy-output policy does not adequately value methane mitigation, nutrient recycling, slurry management or farm energy resilience. The third work applies experimental cattle slurry/food waste co-digestion data to UK dairy-farm scenarios. It shows that co-digestion could improve energy output and seasonal performance of slurry-based AD, while indicating that targeted intervention at the largest dairy holdings could affect a disproportionate share of UK dairy slurry treatment, subject to local food-waste availability, regulatory compliance and nutrient-balance constraints. The fourth work assesses CO₂ biomethanisation as a UK-wide operational opportunity. Literature-derived values applied to public UK AD datasets produced estimated biomethane uplifts of 57–61%, while analysis of national AD and sewage-gas energy data indicated a potential increase of 12,954 GWh/year. Overall, the thesis reframes on-farm AD as a multifunctional system for slurry management, nutrient recycling, rural waste valorisation, methane mitigation and renewable gas production.

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ORCID for Sonia Heaven: orcid.org/0000-0001-7798-4683

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