Détermination de l'intérêt environnemental via l'analyse du cycle de vie du traitement des effluents organiques par méthanisation au regard des contraintes territoriales

Résumé Fr En

ResumeSi la méthanisation est perçue comme une alternative aux énergies fossiles, il ne faut pas oublier que ses conséquences – positives ou négatives – varient beaucoup en fonction du contexte environnemental. L’analyse du cycle de vie menée sur un bassin versant breton les présente en fonction de différents critères et pose une question essentielle : à quelle échelle faut-il penser les impacts de la méthanisation ?

The management of organic waste is a territorial issue and especially in Brittany because of the existence of nitrogen excess inducing a significant deterioration of surface water used for drinking water production and a green algae bloom on the coast under certain conditions. Collective management of organic waste by anaerobic digestion allowing nitrogen export represents a possible way to locally reduce the nitrogen pollution. To provide evidence of the environmental performances of this alternative, a Life Cycle Assessment (LCA) was carried out for three scenarios of livestock manure and food industry waste management for a watershed in Brittany: a reference scenario representing current practice and two alternative scenarios with a digestat post-treatment which is either composting allowing the use of biogas from anaerobic digestion by a local industry or an evapo-concentration process using the biogas produced by anaerobic digestion. Only three relevant environmental impact categories were assessed: climate change, acidification and eutrophication. The scenario with composting highlights the interest of biogas recovery. The scenario with evapo-concentration is the least impacting but leads to a potential impact transfer for eutrophication, due to the export of a significant amount of nitrogen out of the studied watershed. LCA is insightful for collective thinking but also opens the debate on these complex territories that watersheds are: should we favour a local collaboration to move towards industrial ecology and probably facilitated social acceptance, or encourage the reduction of the nitrogen pressure into a watershed in litigation?