Key words: recycling, compost, composting, green waste, organic waste, recovery.
France produces annually around 600 million tons of waste of which more than 400 are organic waste. For the latter, composting is a fast-growing process of treatment, favored by the current regulatory and sociological context.
Another advantage, this process adapts to the volume to be treated. It makes it possible to process very large or smaller volumes by implementing a more or less advanced technology. Whether it is municipal, agricultural or agri-food waste, a double regulatory and sociological context helps to promote composting: the obligation to reduce 65% waste sent to landfill, the growing reluctance from the agricultural world to the spreading of sewage sludge and the obligation for waste producers to reduce their nitrogen load or to transform them into exportable products outside the "zone of structural surpluses".
What works for which objectives?
Faced with a growing social demand for this type of waste treatment, the objective of research conducted at Cemagref is to optimize composting processes both in terms of their technical performance and their environmental impacts. For example, gaseous emissions, odorous compounds or greenhouse gases, such as methane, carbon dioxide or nitrous oxide, are at the origin of the main environmental impacts.
Knowing the processes that govern the fate of nitrogen during composting is therefore essential to reduce these emissions. The research conducted at Cemagref on composting therefore has many objectives, among which are the global modeling of composting treatments, the diagnosis and control of gaseous emissions, the development of new processes, and the acquisition of experimental tools. simulation of composting treatment, methodological tools for monitoring these treatments, tools for qualifying the biodegradability of organic waste and its level of stabilization.
Evaluate the "compostability" of waste
The "formulation" of the waste mixture to be treated (or its pretreatment) as well as the treatment process and conditions are the main parameters involved in composting. These parameters involve different processes: biological processes, related to the nature of the waste, mass and heat transfers that depend on the process implemented and the treatment conditions.
By studying the biological processes, one can for example characterize the initial or residual biodegradability of a waste, before or during its treatment, and thus lead to a better control then to the optimization of the composting processes. In this perspective, a respirometric tool is being developed within the Sowaste team at Cemagref in Rennes. It makes it possible to measure the oxygen consumption associated with the biodegradation of an organic substrate. By modeling this consumption, we can quantify the different biodegradable organic fractions of the studied material. This method should make it possible to evaluate the "compostability" of a substrate, which value is then used to optimize the formulation of mixtures or to choose the pre-treatment to be applied. By using this tool at different stages of the composting process, it will be possible to evaluate the residual biodegradability of a material and thus to control or appraise a process, but also to quantify the biological stabilization of composts at the end of treatment.