Article L3451-1 of the French General Code of Local Authorities
The départements of Paris, Hauts-de-Seine, Seine-Saint-Denis and Val-de-Marne, as well as the interdepartmental institution they have created between them, provide collective wastewater treatment, which includes its collection and transport, when the communes, their public cooperation establishments or their mixed syndicates do not provide this, its treatment and the elimination of the sludge produced. They may also, in the same circumstances, collect, transport, store and treat rainwater.