Paris is one of the very few cities in the world equipped with a dual network system: the drinking water network is duplicated by a totally independent non-potable water network which possesses its own means of production, storage and distribution pipes. In this second network there circulates untreated water characterized by pressure lower than that observed in the drinking water network. The fact that the constraints governing non-potable water are less than for drinking water is reflected in lower margins in relation to comparative production and storage capacity.
Non-potable water in Paris has existed for two centuries; it is very inexpensive and its use in large quantities forms part of the landscape to which Parisians are accustomed. 98% of non-potable water consumption are restricted to the uses of water linked to urban concentration for the comfort or safety of town dwellers: hydrants, fountains, street cleaning, watering of public gardens, flushing of the sewers etc. Therefore, the main consumer being the city of Paris, the operation of the non-potable water network differs in many ways from that applied to the drinking water network.
The economic context, the technical implications and the ecological repercussions of the use of non-potable water provide an alternative solution to the exclusive use of drinking water which seems to be efficient and attractive. This exceptional situation does not only offer advantages especially when one analyses the consequences for the network of waste water or the cost of maintaining a dual pipe network.