Greenhouse gas emissions from food and garden waste composting

Composting is a robust waste treatment technology. Use of finished compost enables plant nutrient recycling, carbon sequestration, soil structure improvement and mineral fertiliser replacement. However, composting also emits greenhouse gases (GHG) such as methane (CH₄) and nitrous oxide (N₂O) with high global warming potential (GWP).This thesis analysed emissions of CH₄ and N₂O during composting as influenced by management and process conditions and examined how these emissions could be reduced. The GHG emissions from home-scale, large-scale and reactor composting were determined. At small scale, 18 home compost units were analysed over one year. At large scale, the effects of aeration strategies on emissions were studied in 10-20 m long windrow composts. Finally, reactor composting was studied in two sets of experiments under a controlled laboratory environment to investigate the influence of specific process conditions on CH₄ and N₂O emissions. Methane emissions increased almost 1000-fold when moisture content in the compost substrate increased from about 44% to 66% in the reactor. Moreover, CH₄ emissions increased 100-fold as a result of poor aeration. In home composting CH₄ emissions were low, but increased with temperature, mixing frequency and moisture. In windrow composting, high CH₄ emissions were associated with thermophilic temperatures and large moisture gradients. Moisture content from about 44% to 59% significantly affected N₂O emissions in the reactor studies, with N₂O emissions increasing at lower moisture. The presence of nitrate in the initial substrate resulted in an early N₂O emissions peak in reactor composting. Extended composting period during some reactor runs resulted in higher total GHG emissions due to continued production of both CH₄ and N₂O late in the process, after 50% of initial carbon had been mineralised. Total direct GHG emissions from home and windrow composting systems, assessed based on their GWP, were similar to or lower than those reported by others, while emissions from most composting reactor runs were lower. To reduce CH₄ emissions, composting at a combination of high moisture and high temperature should be avoided. To reduce N₂O emissions, extremes of moisture content in the compost matrix should be avoided.