Discovery of a new method to reduce methane emissions from farm dairy effluent

Purpose: The New Zealand Government requires gross emissions of biogenic methane (CH₄) to be reduced to 10% below 2017 levels by 2030. However, the amount of CH₄ emissions reported in the ‘Manure Management’ category of New Zealand’s Greenhouse Gas Inventory has increased by 123% since 1990. The purpose of this research was to determine the effect of treating farm dairy effluent (FDE) with polyferric sulphate (PFS) on CH₄ emissions. Methods: The effect of treating FDE with PFS on CH4 emissions was measured at four scales: (i) 1-L gas jars in the laboratory, (ii) 1.1-m-deep × 150-mm-diameter pipe microcosms in the laboratory, (iii) large 3.4-m-deep × 0.47-m-diameter pipes on-farm, and (iv) 2-m-deep × 8.4-m-diameter (100,000 L) commercial effluent storage tanks on a farm. Gas emissions were captured by repeated discrete sampling and CH₄ concentrations were determined by gas chromatography. Results: We discovered that treating FDE with PFS at an average rate of 220 mg Fe L¯¹ of FDE reduced CH₄ emissions by up to 99% and that this effect continued for an extended period of time (up to 2 months) after treatment. The PFS treatment also reduced CO₂ emissions by approximately 50% and reduced hydrogen sulphide emissions. PFS treatment resulted in a small increase in nitrous oxide (N₂O) emissions, but these emissions were very low and only represented < 3% of the total CO₂-e greenhouse gas emissions from the treated FDE. Conclusions: A new method to reduce CH₄ emissions from farm dairy effluent by up to 99% has been discovered