Development and optimisation of an oxygen-based amendment for cattle slurry designed to reduce methane emissions during storage

Abstract

Methane (CH4) is a potent greenhouse gas (GHG) that is produced via anaerobic digestion in slurry storage facilities, which are ubiquitous in farming systems with animal husbandry. Globally, CH4 emissions from agriculture produce significant radiative forcing (change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change), increasing global temperatures which contribute to climate change. Reduction of this key GHG from agriculture therefore must be a priority for governments. In Ireland, development of GHG mitigating measures in agriculture, including slurry amendments and/or anaerobic digestion, has been mandated as part of the national response to climate change. Uptake of such measures is currently very low among the farming community; however if these measures do come to prominence in future, a wide range of options must be available to satisfy local and national needs.

Due to the incompatibility of most acidic slurry amendments with anaerobic digestion, an oxygen-based treatment that inhibits methanogenesis was considered in this thesis. The objective of this thesis was to develop and evaluate the impact of an oxygen-based slurry amendment on cattle slurry during storage. Furthermore, this thesis aimed to provide increased understanding of the conditions in which this amendment can be used and ultimately provide a foundation of knowledge for further exploration into this field.

Cattle slurry, which is the dominant agricultural waste in Ireland, was treated at a range of scales with different formulations of hydrogen peroxide and potassium iodide in order to gauge how effective the treatment was in primarily reducing CH4 emissions. CH4 emissions were captured using the static chamber methodology and examined using a gas chromatograph. However, the addition of this slurry amendment increased ammonia (NH3) emissions considerably. Therefore, further development of the amendment was required, which led to experimentation with a number of peroxide-based treatments.

A total GHG profile of the improved treatment, which only contained hydrogen peroxide and potassium iodide, was developed. Based on this profile including CH4, CO2 and N2O, it was found that the treatment did not significantly reduce total GHG emissions, although high temperatures encountered during the experimentation likely reduced treatment efficacy. Ammonia emissions still increased due to the agitation effect of the treatment, however

addition of CaCl2, which is compatible with anaerobic digestion, reduced NH3 emissions during its window of efficacy.

The effect of the treatment on hydrogen sulphide emissions upon agitation was also examined and were found to be reduced, which may provide a safer environment for farm workers and animals, if the treatment was to be applied nationally.

Lastly, the effect of temperature on the treatment inhibition of CH4 was investigated and it was found that with increasing temperature, inhibition of CH4 reduced. This reduction in efficacy was assigned to the increased methanogenic activity in slurries at higher temperatures. Therefore, a higher frequency of treatment may be required to reduce emissions more effectively if conditions require it.

Overall, the results from this thesis show that an oxygen-based amendment may work on an Irish farm mainly under winter temperatures and if the mode of addition is heavily modified so as not to increase NH3 emissions from the addition of the amendment. Coupling this amendment with another manure management technique in anaerobic digestion will require further study and, ultimately, strict guidelines in the treatment of slurry during storage, transport and rapid integration into the anaerobic digestion pipeline. Without which would risk increasing GHG emissions even further.