Wood waste decomposition in landfills: An assessment of current knowledge and implications for emissions reporting

Large quantities of wood products have historically been disposed of in landfills. The fate of this vast pool of carbon plays an important role in national carbon balances and accurate emission reporting. The Republic of Ireland, like many EU countries, utilises the 2006 Intergovernmental Panel on Climate Change (IPCC) guidelines for greenhouse gas reporting in the waste sector, which provides default factors for emissions estimation. For wood products, the release of carbon is directly proportional to the decomposition of the degradable organic carbon fraction of the product, for which the IPCC provides a value of 0.5 (50%). However, in situ analytic results of the decomposition rates of carbon in landfilled wood do not corroborate this figure; suggesting that carbon emissions are likely overestimated. To assess the impact of this overestimation on emission reporting, carbon decomposition values obtained from literature and the IPCC default factor were applied to the Irish wood fraction of landfilled waste for the years 1957-2016 and compared. Univariate analysis found a statistically significant difference between carbon (methane) emissions calculated using the IPCC default factor and decomposition factors from direct measurements for softwoods (F = 45.362, p = <.001), hardwoods (F = 20.691, p = <.001) and engineered wood products (U = 4.726, p = <.001). However, there was no significant difference between emissions calculated using only the in situ analytic decomposition factors, regardless of time in landfill, location or subsequently, climate. This suggests that methane emissions from the wood fraction of landfilled waste in Ireland could be drastically overestimated; potentially by a factor of 56. The results of this study highlight the implications of emission reporting at a lower tier and prompts further research into the decomposition of wood products in landfills at a national level.