Carbon Footprint of Composting and Vermicomposting of Household Biowaste: A Decision-Making Factor for Regional Biowaste Recovery Policies?

Since 1 January 2024, French local authorities will be required to offer householders a means of recovering biowaste, either as a soil improver or as an energy source. Several criteria influence their choice: cost, availability of operators and equipment, social facilitation, etc. However, greenhouse gas (GHG) emissions are rarely taken into account in the decision-making process. This article compares the emissions of four biowaste recovery systems, differentiated by their process (composting or vermicomposting) and management type (community or industrial). It is based on the carbon footprint method defined by the French Agency for Ecological Transition (ADEME). The assumptions and emission factors come from two sources: a field survey of composting and vermicomposting companies and associations in the Lyon area and a review of the literature on GHG emissions from the decomposition of organic matter. The carbon footprint of the processes was determined by estimating the CO2 equivalent per ton of composted biowaste. The results show that industrial composting emits the most carbon (CO2). Depending on whether biogenic carbon is taken into account or not, the ranking of the other three processes changes. When biogenic CO2 is taken into account, it is the process that has the greatest influence on the result; on the other hand, when biogenic CO2 emissions are not taken into account, the type of management determines the ranking. These results are discussed in relation to the methodological limitations of the comparison, other biowaste management options and the reduction of biowaste-related emissions. For example, by studying the agricultural use of biowaste compost, the carbon balance could be refined by including the emissions avoided from the production of nitrogen fertiliser. However, environmental assessment is only one of a number of decision-making factors (social, economic, agricultural, etc.) in waste management.