Methane and nitrous oxide emission from garden waste composting facilities – emission factors and mitigation potential

Bio-waste composting is environmentally favourable to landfilling; however, this practice has been shown to emit methane (CH₄) and nitrous oxide (N₂O), both of which contribute to climate change. Measurement-based studies are necessary to quantify these emissions accurately, revise emission factors, define composting best practices and monitor mitigation strategies. This study investigated CH₄ and N₂O emissions, as well as associated parameters (i.e., gas composition, temperature, material age and composition), at garden waste windrow composting facilities in Denmark. We report measured CH₄ andN₂O emissions and emissions factors at 11 full-scale composting facilities and one farm in Denmark. In addition, gas concentrations and temperatures were measured inside material piles present, including windrows and stored mounds of untreated garden waste, biofuel and compost products. Methane and N₂O fluxes on the surface of the windrows and material piles were measured using flux chambers. Total facility emissions were quantified using a tracer gas-based method. Finally, large scale experimental studies were performed to investigate if CH₄ and N₂O emissions and CO₂ emissions from energy consumption could be reduced from composting facilities by improvement in operating conditions such as reducing windrow size, increase turning frequency, implement active aeration and pre-treatment of the garden waste. The outcomes include the establishment of a revised national emission baseline for current composting practices in Denmark; an improvement in the robustness and representativeness of emission factors, thereby enabling an update of the values applied in the Danish National Inventory Report (NIR); and the development of evidence‑based best‑practice recommendations for composting, directed at the Danish Environmental Protection Agency, municipal authorities, and the waste management sector. Collectively, these outputs are anticipated to support forthcoming revisions of the national composting guidelines and to strengthen the scientific foundation for emissions reporting and regulatory decision‑making.