Methane and nitrous oxide emission factors from full-scale garden waste composting facilities

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, define composting best practices and monitor mitigation strategies. Composting practices in Denmark rely on open windrow composting, with 90% of facilities solely treating garden waste. We measured CH₄ and N₂O emissions at three full-scale garden waste facilities and one farm in Denmark during four campaigns spread over the course of a whole year. Whole-site CH₄ emissions were measured using the tracer gas dispersion method, whereas N₂O emissions were derived based on CH₄ to N₂O ratios from flux chambers measurements and the whole-site CH₄ emissions. Whole-site CH₄ and N₂O emissions ranged from 0.065 to 29.5 kg CH₄ h^-1, and from ⩽10^-4 to 1.1 kg N₂O h^-1, respectively. Combining our measurements at 4 sites with measurements from previous studies at 8 other Danish sites, we derived emission factors for open windrow garden waste composting of 2.56 kg CH₄/Mg waste wet weight (ww) and 0.054 kg N₂O/Mg waste ww from 11 facilities and 1 farm storing shredded and sorted garden waste. This resulted in a notable improvement in the representativeness of these emission factors compared to those previously used in the Danish National Inventory Report (NIR), which were based on measurements performed 20 years ago at a single Danish composting facility. As a result of this study, the CH₄ emission factor was updated in the 2024 Danish NIR.