Covering dairy slurry stores with hydrophobic fertilisers reduces greenhouse gases and other polluting gas emissions

The objectives of this study were to verify the effects of using various types of hydrophobic fertilisers to cover dairy slurry on polluting gas emissions and to evaluate the mechanisms by which the reduction of emissions occurred. The hydrophobic fertilisers were prepared by mixing finely ground chemical fertilisers (SP, superphosphate; CC, calcium carbonate; and AP + AS, a mixture of ammonium phosphate and ammonium sulfate) with hydrophobic silica (8% w/w) to make the cover moisture-proof and thus able to float on the slurry. Laboratory trials were carried out for 13 days using a pilot scale device. The cumulative volumes of the NH3 and CH4 gases emitted from the digested slurry were considerably higher than those from the raw slurry. In contrast, the volumes of CO2 and H2S gases emitted from the digested slurry were lower than those from the raw slurry. Nitrous oxide was not detected in the present study. The hydrophobic fertilisers composed of SP and AP + AS remarkably reduced the NH3 and CH4 emissions from the digested slurry. None of the fertilisers had an effect on the emissions of CO2 from the raw slurry, but when the digested slurry was covered with the SP and AP + AS fertilisers, an increase in the cumulative volumes of CO2 emissions was induced compared with the control slurry (uncovered), probably due to the relatively high activity of methane-oxidising bacteria. The CC and SP fertilisers were able to reduce the H2S emissions from the raw dairy slurry. The effect of the novel covers on greenhouse gas and other polluting gas emissions varied with the types of chemical fertilisers, depending on acidity or alkalinity, solubility, and the ability to adsorb these gases. The partial dissolution of the nutritional constituents under the hydrophobic fertiliser covering produced changes in the fermentation process of the dairy slurry during storage and consequent gas emissions.