Assessment of whole-site methane emissions from anaerobic digestion plants: Towards establishing emission factors for various plant configurations

This study examines methane (CH₄) emission factors from biogas and wastewater treatment plants, based on primary and secondary data collected from 109 facilities. Primary emission data were measured at 19 facilities representing prevalent plant configurations across Europe. Statistical analysis highlights two categorical variables, namely primary feedstock and plant size, expressed as CH₄ production (≤ 250 kg h⁻¹: small and medium-sized plants, > 250 kg h⁻¹: large plants), each of which has a significant impact on whole-site CH₄ emissions. Additionally, digestate storage (gastight vs. not-gastight) has a meaningful effect when considering CH₄ production as a continuous variable in the statistical analysis.

Our results indicate that wastewater treatment plants have the highest average CH₄ losses (7.0 % of CH₄ produced, n = 31 or 0.10 kg population equivalent (PE)^-1 yr⁻¹, n = 28), followed by manure-based plants (3.7 %, n = 49), biowaste treatment facilities (2.8 %, n = 11) and energy crop-processing plants (1.9 %, n = 14). Furthermore, small and medium-sized plants have elevated emissions (5.6 %, n = 67) compared to larger counterparts (2.2 %, n = 42), primarily attributed to the absence of gastight digestate storage.

Emissions tend to be lower with gastight digestate storage (2.7 %, n = 61) than not-gastight storage options (6.2 %, n = 48). Emission factors were determined for normal operating conditions, with a further investigation into other-than-normal operating conditions revealing temporal or constant emission peaks in eight out of 19 facilities. These peaks, suggesting potential areas for targeted mitigation strategies, were attributed to pressure relief valves, flare ignition problems and major leakages.