Composting Emission Calculator

Composting is aerobic, so most carbon leaves as carbon dioxide. That CO2 is biogenic, fixed from the atmosphere by plants within the past growing season, and is excluded from greenhouse gas inventories. Counted instead are the two trace gases produced when conditions depart from ideal. Methane forms in anaerobic pockets where oxygen is consumed faster than it diffuses in, typically in wet, dense or infrequently turned material. Nitrous oxide forms during nitrification and denitrification of feedstock nitrogen, particularly in transition zones between aerobic and anaerobic conditions. Each is converted using 100-year global warming potentials from the IPCC Fifth Assessment Report: 28 for methane and 265 for nitrous oxide. Because of that multiplier, a nitrous oxide factor of just 0.3 kg per tonne contributes comparably to a methane factor of 4 kg per tonne. IPCC default factors for biological treatment are 4 kg methane and 0.3 kg nitrous oxide per tonne on a wet weight basis, with reported methane ranges of roughly 0.08 to 20. Process control decides where a facility sits: a carbon to nitrogen ratio of 25:1 to 35:1, moisture near 50 to 60 percent, and maintained pore-space oxygen keep both gases low, with enclosed in-vessel systems best. The avoided landfill methane credit usually dominates the net result and deserves scrutiny, since it depends on the assumed counterfactual and on gas collection efficiency, commonly taken as 75 percent but ranging from near zero at uncontrolled sites to above 90 percent. This is an emissions calculation, not a life cycle assessment, and excludes transport, aeration electricity and post-application soil carbon.

Returning organic residues to soil is an ancient practice, but composting as a controlled engineering process dates to the twentieth century. Albert Howard, working in India between 1924 and 1931, developed the Indore process with Yeshwant Wad, specifying layered ratios of plant residue to manure, controlled moisture and scheduled turning to keep the mass aerobic. Howard set the method out in An Agricultural Testament in 1940, and it became the template for windrow composting. Forced aeration arrived much later, when the US Department of Agriculture research station at Beltsville, Maryland developed the aerated static pile method during the mid-1970s for sewage sludge treatment, removing the need for repeated mechanical turning. The greenhouse gas accounting layer is far more recent. The IPCC issued its first national inventory guidelines in 1996, but the emission factors for biological treatment of solid waste used here come from Volume 5 of the 2006 IPCC Guidelines, updated in the 2019 Refinement, while the global warming potentials come from the Fifth Assessment Report published in 2013. The US Environmental Protection Agency released its Waste Reduction Model, WARM, in 1998 to give comparable life cycle factors across waste pathways. Regulation supplied the demand for such numbers: the European Landfill Directive of 1999 required member states to cut biodegradable municipal waste sent to landfill to 35 percent of 1995 levels, and California enacted SB 1383 in 2016, mandating a 75 percent reduction in organic waste disposal by 2025.