Treatment of manure methane emissions in a full-scale biofilter by microbial oxidation

Efficient collection and treatment of methane (CH₄) from a pig manure tank was demonstrated in a full-scale biofilter designed for optimal biological CH₄ oxidation. The 400 m² biofilter consisted of a gas distribution layer and a compost-based oxidation layer. The biofilter removed on average 93 ± 6.5 % of the CH₄ loaded to the biofilter, corresponding to an annual reduction of 39 tons of CH₄. The CH₄ load to the biofilter was lowest during early spring (0.5 kg h^-1), when a small amount of manure was stored in the tank and the air temperature was reduced, and highest during summer (12.9 kg h^-1), when the air temperature was high and the tank contained more manure. The biofilter CH₄ oxidation rate ranged between 32 and 650 g CH₄ m^-2 d^-1, with the maximum rate observed in August when the biofilter load was at its highest (774 g CH₄ m^-2 d^-1). Most of the time, CH₄ was completely oxidised in the gas distribution layer and the deepest part of the biofilter. When the CH₄ load exceeded 9 kg h^-1, oxidation efficiency decreased to below 90 %. The optimal CH₄ oxidation temperature of the CH₄ oxidising community was 50 °C, which is close to the annual average temperature of 49 ± 1.7 °C. With a CO₂ shadow price of 9.90 € for reducing one ton of CO₂ equivalent, the biofilter was revealed as a cost-efficient mitigation technology for reducing CH₄ emissions from manure storage tanks.