Greenhouse Gas and Air Pollutant Emissions from Composting

Composting can divert organic waste from landfills, reduce landfill methane emissions, and recycle nutrients back to soils. However, the composting process is also a source of greenhouse gas and air pollutant emissions. Researchers, regulators, and policy decision-makers all rely on emissions estimates to develop local emissions inventories and weigh competing waste diversion options, yet reported emission factors are difficult to interpret and highly variable. This review explores the impacts of waste characteristics, pretreatment processes, and composting conditions on CO₂, CH₄, N₂O, NH₃, and VOC emissions by critically reviewing and analyzing 388 emission factors from 46 studies. The values reported to date suggest that CH₄ is the single largest contributor to 100-year global warming potential (GWP₁₀₀) for yard waste composting, comprising approximately 80% of the total GWP₁₀₀. For nitrogen-rich wastes including manure, mixed municipal organic waste, and wastewater treatment sludge, N₂O is the largest contributor to GWP₁₀₀, accounting for half to as much as 90% of the total GWP₁₀₀. If waste is anaerobically digested prior to composting, N₂O, NH₃, and VOC emissions tend to decrease relative to composting the untreated waste. Effective pile management and aeration are key to minimizing CH₄ emissions. However, forced aeration can increase NH₃ emissions in some cases.