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Page:Popular Science Monthly Volume 18.djvu/831

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811
THE PURIFICATION OF SEWER-WATERS

culty of drying the mud fast enough to enable it to be taken away at a profit, which has prevented the success of all previous efforts at purification by deposition. The mud which is spread on the permeable bottom does not soil the scoria, but leaves it perfectly clean, while the water flows clear from the end of the drains. After two or three days, according to the weather, the mud will appear cracked at some points, and finally over the whole surface. After a week it will have acquired consistency enough to be cut with a shovel. A cart is then brought into the basin, and after a few hours it is emptied and is ready to receive a new charge of mud. The scoria not having been soiled, requires no cleaning, and will be as ready for use even after the end of ten operations as at the beginning.

The drained mud is carried in the shape of large lumps to an open yard, where it is dried in the air without giving forth any odor. It contains about seventy-five, per cent, in weight of water at the time it leaves the basin, but the amount of water is reduced after two or three months of exposure to not more than fifteen or twenty per cent.

These operations are of the simplest character, and involve nothing cumbrous. The whole system, with its decanting and drainage tanks, its open yard and the necessary roads, occupies a surface of not more than two hectares, or five acres, for the effective purification of 10,000 cubic metres of water every twenty-four hours. Paris has to get rid of thirty times as much foul water as the Essonnes paper-mill, or 300,000 instead of 10,000 cubic metres a day. The system practiced at Essonnes would, therefore, have to be applied on a scale thirty times as large to be adapted to the needs of Paris. Can it be made to succeed on such a scale? What is there to prevent it?

No difficulty is offered by the composition of the sewer-waters. We have procured a quantity of water from the great collector of Asnières, and have subjected it to the same treatment that is given the waste water at Essonnes. On adding to it lime-water in the proportion of 250 grammes of lime—yes, even in the smaller proportions of 200 and 175 grammes—to a cubic metre of water, a complete precipitation was promptly produced. At the end of four or five hours the water became clear and limpid.

The extent of land required to conduct the operations of decantation, drainage, and drying, on the scale demanded by the city of Paris, seems formidable at the first sight, but it is not really so. As we have seen, the whole system at Essonnes occupies only two hectares, or five acres, of land. For the city of Paris thirty times as much land, sixty hectares, only 150 acres, would be needed to give room for all the apparatus and all the manipulations; that would be a small tract compared with the 1,500 hectares, or 3,750 acres, on which it is proposed to establish a nuisance in the forest of St. Germain.

The scheme will compare favorably with any other that has been proposed, in the cost of constructing and operating the works. The