Answer: it is a tax for sewerage.
The connection to the collective sanitation network by the sewerage system makes it compulsory to pay the sanitation fee to the municipality or to your EPCI (Public establishment of inter-municipal cooperation: community of municipalities, community of agglomeration…) ). It includes a fixed part used to cover the costs of the collective sanitation service and a variable part, defined according to the volume of water taken by each household from the public water distribution network. This charge appears on your water bill under the name "wastewater collection and treatment". If you use a non-collective sanitation system, you will also have to pay a sanitation fee since your sanitation system is subject to control by the administration. You too, send us your DIY question.