Biofiltration of methane emissions from a dairy farm effluent pond

Dairy farm effluent ponds are a source of methane (CH4), a potent greenhouse gas. Biofiltration, whereby CH4 is oxidised by methanotrophic bacteria, is a potentially cost-effective CH4 mitigation technology. We report on the performance of a field-scale biofilter treating dairy farm effluent pond CH4 emissions for 16 months. This study is the first to report on the feasibility of using biofiltration to mitigate dairy waste CH4 emissions. The 70-L filter comprised a 1:1 volumetric mixture of volcanic soil from a landfill and perlite. Biogas collected in a floating cover on a 4-m2 section of the pond was directed through the biofilterʹs base. Air was pumped through the filter to supply oxygen to the methanotrophs. The filterʹs maximum CH4 removal rate was 16 g m−3 h−1 (or 53 μg g−1 h−1), which is high compared with literature landfill soil oxidation rates (typically <1 to 40 μg g−1 h−1). At the trialʹs conclusion, the filter experienced acid accumulation, due to oxidation of H2S in the inlet biogas (evidenced by low pH [3.9] and high sulphate-S [1079 mg−1 kg−1] at the base of the filter compared with the top [pH = 4.6, sulphate-S = 369 mg−1 kg−1]). Nonetheless, the filterʹs oxidation rate peaked at the end of the experiment indicating negligible H2S impact on overall performance over the 16-month period. The results showed that a 50-m3 filter would be needed to offset CH4 emissions (approximately 720 g h−1) from a typical 1000-m2 dairy effluent pond. As this calculation is based on the efficiency of a single experimental filter, field testing of replicate biofilters is needed to accurately establish full-scale filter sizing. Nonetheless, this study has shown that biofilter technology is feasible to mitigate dairy effluent pond CH4 emissions. Current research is underway to make the filter more economically viable through design optimisation.