Methane and nitrous oxide formation and emissions at a full-scale garden waste windrow composting facility in Denmark

Although composting is widely regarded as a more environmentally sustainable alternative to landfilling, it remains a source of greenhouse gas emissions. Methane (CH₄) and nitrous oxide (N₂O) emissions from a full-scale Danish central garden waste composting facility were quantified during four measurement campaigns using flux chambers and the tracer gas dispersion method (TDM). Surface fluxes, pore gas composition, and temperature were measured in all on-site material piles, including active composting windrows and storage piles of mature compost and biofuel (the coarse fraction of shredded garden waste destined for energy recovery through incineration). The highest CH₄ surface fluxes (2.1 g CH₄ h^-1 m^-2) and pore gas CH₄ concentrations (18 % v/v) were observed in composting windrows containing shredded and sorted material. The highest N₂O fluxes (0.045 g N₂O h^-1 m^-2) were measured in windrows composed of shredded but unsorted material. Although emissions from storage piles of shredded garden waste, biofuel, and mature compost were lower, they were not negligible. Pore gas CH₄ concentrations increased with windrow age as oxygen availability declined. However, CH₄ surface fluxes remained relatively constant, suggesting enhanced CH₄ oxidation in the windrow surface layer. Average facility-scale emissions measured by TDM were 8.3 kg CH₄ h^-1 (range: 4.6-13 kg h^-1) and 0.037 kg N₂O h^-1 (range: 0.019-0.054 kg h^-1). These results demonstrate that both composting windrows and storage piles contribute to overall facility emissions. Comprehensive, sub-facility-scale measurements are therefore needed to identify key emission sources, support mitigation efforts, and optimize composting operations.