Greenhouse gas emissions from windrow composting of organic wastes: Patterns and emissions factors

Abstract

Direct emissions from commercial-scale composting are uncertain. We used micrometeorological methods to continuously measure greenhouse gas (CO2, CH4, N2O) emissions from full composting of green waste and manure. We measured oxygen (O2), moisture, and temperature continuously inside the composting pile, and analyzed chemical and physical characteristics of the feedstock weekly as potential drivers of emissions. Temperature, moisture, and O2 all varied significantly by week. Feedstock porosity, C:N, and potential N mineralization all declined significantly over time. Potential net nitrification remained near zero throughout. CH4 and CO2 fluxes, indicators of feedstock lability, were variable, and most emissions (75% and 50% respectively) occurred during the first three weeks of composting. Total CH4 emitted was 1.7 ± 0.32 g CH4 kg−1 feedstock, near the median literature value using different approaches (1.4 g CH4 kg−1). N2O concentrations remained below the instrument detection. Oxygen, moisture and temperature exhibited threshold effects on CH4 emissions. Net lifecycle emissions were negative (−690 g CO2-e kg−1), however, after considering avoided emissions and sinks. Managing composting piles to minimize methanogenesis—by maintaining sufficient O2 concentrations, and focusing on the first three weeks—could reduce emissions, contributing to the climate change mitigation benefit of composting.