The anaerobic (absence of oxygen) decomposition of organic matter contained in household waste is making a major contribution to global warming due to the emission of large quantities of methane, which is a powerful greenhouse effect gas.
The impact of waste on climate change is considerable and is at the same level as civil aviation – i.e. around 4% of the total. This impact originates from methane generated by the organic proportion of household waste that decomposes in anaerobic conditions in the landfill sites that are to be found in the majority of developing countries. Methane is one of the most powerful greenhouse effect gases. The official equivalence in the form of CO2: 1t CH4 21t CO2e, calculated for a horizon of 100 years is much undervalued when the considered horizon is at 30 or 40 years, with the latter corresponding to “critical years” with risk of a runaway of the warming process. This equivalence factor therefore becomes 80. The reduction of methane must therefore be a priority in order to limit climate change in the short term.
The composting process, whilst allowing the aerobic decomposition of the organic portion of household waste, prevents methane emissions. Furthermore, if this process remains only marginally mechanised, as in the Gevalor projects, it consumes very little energy and as a consequence emits very little CO2. The reductions in greenhouse gas effect emissions that result from this is of major importance.
The Clean Development Mechanism (CDM) enables these reductions in emissions to be used within the framework of a carbon market where they can be purchased by organisations wishing to compensate their own carbon emissions.
The CDM methods define two major approaches for calculating the reduction of emissions related to waste, which are:
- The avoidance of methane production: for example, by the composting or by the combustion of organic substances.
- The capture of methane, either at its discharge point or in bio-reactors, and possibly energy recovery with its combustion.
The calculation is based on the difference of the greenhouse effect gas (GES) emissions between two scenarios, which are:
The baseline scenario: this is what would occur in the absence of the project. In the case of composting it is often a matter of the simple dumping of waste in landfills in conformity with the local legislation. The conditions for the management of the landfill site, the composition of the household refuse and the climatic conditions have a strong influence on emission levels: locations where the rainfall level is greater than 1000 mm and the average annual temperature greater than 20°C are the most favourably located from this point of view.
The project scenario: this is happens thanks to the project. This allows the eradication of methane emissions thanks to aerobic composting, and is accompanied by a possible production of CO2 linked to the production of compost (electricity consumption, imperfections of the aerobic process, residual presence of organic substances within the composting refuse which are sent to landfill and methane leakages in the leachates).
* www.unep.org/Documents.Multilingual/Default.asp?DocumentID=653&ArticleID=6850&l=en: Action on World’s Waste Can Help Combat Climate Change While Increasing Access to Energy Says New UNEP Report “The waste management sector is contributing 3-5 per cent of global man-made greenhouse gas (GHG) emissions, equal to around the current emissions from international aviation and shipping, according to some estimates”.
**Document de travail AFD n° 68 : Réduire le méthane : l’autre défi du changement climatique.