The diameter of the vault of the _égouts collecteurs_ varies between four and six mètres; that of the _égouts secondaires_ from two mètres to three mètres, seventy centimètres; that of the _égouts ordinaires_, including ten varieties, from one mètre to one mètre, seventy-five. The size the most in use has a diameter of one mètre, forty. The problem of purifying and utilizing the contents of the sewers, which were provisionally discharged into the Seine at Saint-Denis and at Asnières, occupied the attention of the municipality from the period of the establishing of the collecteurs, but the vigorous local opposition which was encountered greatly delayed the carrying out of these projects. Consequently, the purification of the river is not yet complete. On the sandy and arid plain of Genevilliers, situated in the first loop of the Seine, beyond Clichy, the experiment of fertilizing with this drainage was commenced in 1869. At present, the ground thus under cultivation includes some seven hundred and ninety-five _hectares_,--about two and a half acres each,--of which six belong to the city of Paris and constitute the model garden. The remainder is held by private individuals, who pay a rental of from four to six hundred francs the hectare. The distribution of the sewage is effected by agents of the administration in regular rotation, in three zones. In 1896, each hectare absorbed thirty-seven thousand and sixty-seven cubic mètres. All varieties of vegetables are grown, and this land, on which were raised formerly only meagre crops of rye and potatoes, is now a flourishing garden.
A second agricultural establishment at Achères, farther on, on both sides of the Seine, was inaugurated in 1895, the larger portion of which is held by individuals, but as each hectare of land can absorb not more than forty thousand cubic mètres annually, it has been found necessary to seek additional _champs d'épuration_. These have been secured by the municipality at Méry and les Gresillons and in their neighborhood, still farther westward, and the completion of these is promised for the summer of 1899. In the model garden of Asnières, all varieties of culture are practised, the sewage is carried in trenches into the cultivated land in such a manner as to bathe only the roots of the plants. The extremely winding course taken by the Seine west of Paris renders it necessary for the conduits conveying this drainage to cross the river three times before reaching Achères, as may be seen by reference to the map. From the _usine elévatoire_ of Clichy it is carried under the Seine by a siphon, four hundred and sixty-three mètres in length; the aqueduct crosses the river again near the usine de Colombes, opposite Argenteuil, on a steel bridge, and again near Herblay, by another siphon. In 1897, on the total surface, a thousand hectares, under cultivation, there were spread seventy million cubic mètres of sewage.