Increased Baseline Methane Concentrations at a Commercial Dairy Farm Associated with Anaerobic Digestion.

Methane-based biogas production from anaerobic digestion of manure and food waste has the potential to offset energy costs and mitigate the generation of greenhouse gases (GHGs). However, this offset may be reduced, or a net increase in GHG production may occur, through unintentional releases of biogas (fugitive emissions). Due to relatively recent adoption of farm-scale anaerobic digestion technology, the rate and frequency of fugitive methane emissions by such biogas systems is not well realized. This study challenges the assumption that anaerobic digestion represents a closed system and attempts to quantify the magnitude of fugitive emissions from a biogas production system operating on a dairy farm. A 3-tower micrometeorological setup around a manure storage tank measured methane concentrations alongside atmospheric conditions at a commercial dairy farm. Baseline concentrations measured at 4.5 m height, windward of the storage tank, were used in conjunction with a Gaussian dispersion model to infer methane emission rates from individual components of the biogas system. The methods employed here offer a distinct approach to the problem of multi-source emissions from agricultural systems. Emission rates prior to AD operation on the farm were contrasted against four years of biogas production under two distinct regimes: one using only dairy manure as an input feedstock, and another adding off-farm food waste materials to dairy manure feedstock. Under the second regime, average baseline methane concentrations were seen to increase by 0.43 ± 0.03 ppm, indicating that the occurrence of fugitive emissions is related to the appropriate pairing of production capacity with feedstock materials and loading rate. When correctly incorporated, biogas production as a manure management practice and energy source is anticipated to provide a net reduction in global warming potential.