Quantifying the greenhouse impacts of landfill, composting and incineration: a review of IPCC and US EPA models, and implications for climate change policy

This study  examines the net greenhouse impacts of landfill, composting or incineration of municipal waste. Landfill methane (CH4) emissions are widely believed to contribute significantly to global warming. This has led to policies such as the European Union (EU) Landfill Directive (1999) to divert organic waste away from landfill. In 2018 the EU increased its target to 90% diversion of waste from landfill. EU nations must incinerate or pre-compost waste, landfilling only the residue. Several nations also ban landfilling of plastic. Incineration of mixed waste including plastic has high CO2-equivalent (CO2-e) greenhouse gas (GHG) emissions. In developed countries most landfill sites that generate significant CH4 now have landfill gas recovery systems. Field studies in France and the USA show that about 70 to 90% of landfill methane (CH4) can be captured. In anaerobic landfill conditions, material protected by lignin does not rot, ¬due to lack of oxygen. It is stored long-term. This avoids 3.667 tonnes of CO2 emissions per tonne of carbon that would be emitted by   burning or aerobic decomposition. The Intergovernmental Panel on Climate Change (IPCC) and US EPA provide default landfill emission factors (EFs) to calculate CH4 emissions and convert them to CO2-e units. Some CO2 emissions are avoided due to landfill carbon storage. Captured landfill biogas used to generate electricity may replace electricity from fossil fuels such as coal, reducing fossil CO2 emissions. The net climate warming impact of landfill is calculated by deducting these CO2 offsets from CO2-equivalent methane emissions. Based on IPCC default factors, a tonne of landfilled municipal waste would ultimately form some 40 to 60 kg of CH4 (assuming organic waste is not diverted from landfill). At about 64-75% gas recovery, CO2-e emissions would be balanced by offsets. At 80% recovery, the net climate warming impact of landfill is likely to be negative, depending on electricity output and waste composition. In addition landfill stores the carbon in plastic waste that would probably otherwise be burned. The IPCC Intergovernmental Panel on Climate Change) and US EPA (Environment Protection Agency) waste emission models are then assessed for accuracy. Other researchers have reviewed the oxidation factor but this is a systematic assessment of all factors, checking cited references, scientific assumptions and comparing more recent data. The IPCC default landfill gas emission factors were found to be conservative, overstating landfill gas generation and emissions. Even the Global Warming Potential (GWP) for anthropogenic CH4 emissions, based on an outdated 1985 study, appears to overstate its impact. The IPCC and US EPA assume that landfill gas contains equal volumes (or molar masses) of CH4 and CO2. The US EPA landfill CH4 factors were increased in this belief. A extensive literature search found no evidence to support this assumption. A new factor for the IPCC landfill gas model, allowing for reduced CH4 due to loss of carbon and hydrogen in fermentation and leachate, and oxygen penetration at unsealed managed sites was proposed and validated against data from an Australian landfill, producing a significantly better fit to measured gas recovery data. US EPA default emissions factors for composting and Waste-to-Energy (WTE) incineration were also assessed. Municipal waste contains wet or non-combustible material. It was found that mass burn WTE requires fossil-based plastics, synthetic wastes and natural gas to produce enough heat to generate electricity and destroy dioxins: US EPA data showed fossil CO2 emissions per MWh of electricity were higher than from modern coal-fired electricity generators. The IPCC composting emission factors (EFs) were found to be reasonably accurate, but several nations use EFs  based on a study by Amlinger et al. (2008). These were found to be unrealistically low. Claims that compost stores carbon in soil for more than 100 years (Smith et al. 2001) were found to be based on a misunderstanding of earlier research.  Compost emits CH4 and N2O. Based on IPCC default factors, GHG impacts of composting were found to be about 164kg CO2-e/t green waste (including energy used to make the compost and a small offset for carbon storage).  About 60% of the green waste decomposes during composting. CO2-e emissions are estimated at about 452 kg/t finished compost, including energy used to dig the compost into the soil. Several offsets for composting and WTE incineration proposed in other LCAs and in the US EPA Waste and Resource Recovery Model, WARM documentation, could not be substantiated. These findings are used in a Life Cycle Assessment. Finally, the thesis proposes a new Waste Hierarchy, specifically targeting greenhouse gas emissions.