Air Pollutant Emission Rates for Dry Anaerobic Digestion and Composting of Organic Municipal Solid Waste

Dry anaerobic digestion (AD) of organic municipal solid waste (MSW) followed by composting of the residual digestate is a waste diversion strategy that generates biogas and soil amendment products. The AD-composting process avoids methane (CH₄) emissions from landfilling, but emissions of other greenhouse gases, odorous/toxic species, and reactive compounds can affect net climate and air quality impacts. In situ measurements of key sources at two large-scale industrial facilities in California were conducted to quantify pollutant emission rates across the AD-composting process. These measurements established a strong relationship between flared biogas ammonia (NH₃) content and emitted nitrogen oxides (NO), indicating that fuel NO formation is significant and dominates over the thermal or prompt NO pathways when biogas NH₃ concentration exceeds ∼200 ppm. Composting is the largest source of CH₄, carbon dioxide (CO₂), nitrous oxide (N₂O), and carbon monoxide (CO) emissions (∼60-70%), and dominate NH₃, hydrogen sulfide (H₂S), and volatile organic compounds (VOC) emissions (>90%). The high CH₄ contribution to CO₂-equivalent emissions demonstrates that composting can be an important CH₄ source, which could be reduced with improved aeration. Controlling greenhouse gas and toxic/odorous emissions from composting offers the greatest mitigation opportunities for reducing the climate and air quality impacts of the AD-composting process.