The agglomeration community "Carcassonne Agglo" is responsible for collective sanitation throughout its territory. Since 1 January 2020, this territory consists of 83 municipalities with 116,837 inhabitants. The main wastewater treatment plant (Carcassonne St Jean) has a capacity of 156,667 Equivalent-Inhabitants. It processes approximately 5,600,000 m3 annually and produces 2,000 tonnes of dry sludge material discharged to an external composting platform. As part of the energy transition, in order to improve sludge treatment and after a feasibility study carried out in 2018, it was decided to set up an on-site sludge methanisation unit, with three objectives: - reduce the amount of sludge by about 30%; - stabilise sludge and reduce olfactory nuisances; - produce biomethane, renewable energy, re-injected into the GrDF network.