Effects of moisture on emissions of methane, nitrous oxide and carbon dioxide from food and garden waste composting

Globally, waste generation is continually increasing, with landfill as the main destination for biological waste. Composting is a simple alternative for handling waste, but when poorly managed poses a risk of greenhouse gas emissions. The moisture content of substrate affects emissions of methane (CH4) and nitrous oxide (N2O) from composting, but the scale and mechanisms behind these effects are poorly understood. This study examined effects of different moisture levels (44–66%) on CH4, N2O and carbon dioxide (CO2) emissions during 20 days of composting food and garden waste under controlled conditions (55 °C, 16% oxygen) in a 200-L reactor. Total CO2 emissions were 400–500 g CO2-C kg−1 initial C. Total CH4 emissions were highest, 35 g CH4-C kg−1 initial C, for the wettest substrate (66% moisture) and decreased exponentially with declining moisture content, with the lowest total emissions, 0.04 g CH4-C kg−1 initial C, observed with the driest substrate. Total N2O emissions were negatively correlated with moisture content, decreasing from 1.2 g N2O-N kg−1 initial N at 44% moisture to 0.3 g N2O-N kg−1 initial N at 59%, but the wettest substrate (66% moisture) had the highest N2O emissions, 1.4 g N2O-N kg−1 initial N. NH4-N accumulated in the wettest material, suggesting that the increased N2O emissions were due to reduced oxygen availability. The results indicate potential to reduce greenhouse gas emissions from large-scale composting by adjusting the moisture content at different stages of composting, thus lowering its overall environmental impact. This finding can be used in guidelines for large-scale composting process to avoid moisture conditions causing large greenhouse gas emissions.