SUEZ & Carcassonne Agglo inaugurate anaerobic digestion unit at Saint-Jean WWTP
On April 30th, Christian Pouget, Prefect of the Aude department; Régis Banquet, President of Carcassonne Agglo, Vice-President of Intercommunalités de France and Mayor of Alzonne; and Pierre Pauliac, Co-CEO of SUEZ and Chief Operating Officer Water, officially inaugurated the anaerobic digestion unit at the Saint-Jean wastewater treatment plant.
The facility generates 4,500 MWh/year of biogas annually from 60,000 tonnes of sewage sludge. The biogas is injected into the public natural gas grid, providing an alternative to fossil fuels and helping to reduce emissions by an estimated 1,200 tonnes of CO₂ equivalent per year. The project is aligned with Carcassonne Agglo’s Green Pact, aimed at advancing regional environmental goals.
Pierre Pauliac said: “The Carcassonne Agglo’s anaerobic digestion benefits from SUEZ’s expertise, which has contributed to build more than 85% of France's sewage sludge methanation capacity. This new project draws on our capacity for innovation – integrating wastewater treatment and methanation technologies – to meet customer needs for sanitation and local energy. Through this long-term partnership with Carcassonne Agglo, which is committed to promoting the ecological transition of its region, we are pleased to provide innovative and sustainable solutions to preserve resources and the environment.”
Renewable energy and regional planning
Operational since late 2024, the biogas unit forms part of Carcassonne Agglo’s Territorial Climate, Air and Energy Plan (PCAET). It is designed to promote local energy production through the conversion of wastewater residues. The output of 4,500 MWh per year is comparable to the energy needs of approximately 750 households. Emissions reductions achieved through this process are estimated to be equivalent to avoiding 700 round-trip flights between Paris and New York.
The facility also includes photovoltaic panels on the roof, producing around 10,000 kWh annually to help power the site and enhance its energy resilience.
Operational and environmental considerations
In addition to renewable energy production, the on-site digestion process reduces the volume of sludge by 30%, decreasing the need for transportation and associated impacts. The sludge is stabilised to reduce environmental risk, and odour control measures are implemented through the use of an Azurair® deodorisation unit.
Architectural efforts were made to ensure the facility blends with the existing landscape, with new structures designed to reflect the style of surrounding buildings and remain visually unobtrusive from the Aude riverbank.
Technology and industry collaboration
Constructed by SUEZ in collaboration with BONNERY, TOUJA, and other local firms, the facility incorporates technologies developed through SUEZ’s experience in wastewater treatment and sludge methanation. The company, which has built a significant portion of France’s methanation infrastructure, is also responsible for operating the plant.
Key technologies used at the site include:
- A Digelis® Simplex metal digester with a capacity of 3,300 m3. This steel biogas digester is based on innovative technology developed by our partner LIPP, which is seamless and bolt-free. It significantly reduces the methanation time while guaranteeing the same durability as a concrete digester.
- A Methanis® Grid biogas treatment unit to separate methane and carbon dioxide. The biomethane obtained is then injected into the local urban gas network.
