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Page:Popular Science Monthly Volume 18.djvu/829

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THE PURIFICATION OF SEWER-WATERS.

forest of St. Germain would be as efficacious, if their action merely as an absorbing medium is contemplated, as 40,000 or 60,000 hectares would be in a scheme for irrigation. The demand for the water under the operations of irrigation would necessarily be very fluctuating. None would be wanted in the winter or in hot weather. While green crops might require much during the whole season of their growth, grain-crops must be carefully protected against it as the season of their ripening approaches. What is to be done with the constantly accumulating supplies of waste water during these seasons when it can not be used?

The whole difficulty arises from the fact that the fertilizing matter which we wish to make useful is drowned in an excess of water. It is this excess which renders sewage unfit for the fertilization of cereals, crops which are too often injured by the superabundance of rain-water. And it is the same excess which makes it impossible to find sufficient surfaces on which it can all be employed in the cultivation of kitchen-garden vegetables, with whose demands its organic constituents well agree. Were this matter extracted in a dry state, it would furnish a precious element of fertilization to a large agricultural interest; and we might preserve it without difficulty and without loss, and transport it at will to apply it to crops of all kinds. At the same time, the water in which it is held could be returned clear and wholesome to the rivers, whose salubrity it is now destroying. Clearly, there can be no discrepancy in the conditions required for the accomplishment of this double object; whatever favors one side must be equally favorable to the other. The two results are absolutely concordant, and may be produced at the same time by one and the same operation, an operation which we may call decantation. The practicability of this process is established by the fact that it has been adopted and is employed with complete success in a large factory in the neighborhood of Paris.

The paper-mill of Essonnes has to deal with 10,000 cubic metres of foul water a day. For two years it has returned to the river Essonnes these 10,000 cubic metres of water clarified, while it has at the same time extracted the mud which they held, and delivered it in a solid state as manure to the agriculturists of the neighborhood.

The apparatus employed at this establishment is composed of two parts, corresponding with two very distinct phases of the operations:

First, is a series of water-tight basins, or tanks, which are used in the decantation, properly so called, of the foul waters.

Second, is arranged a series of tanks having permeable bottoms, constructed parallel with the former tanks, but on a lower level; these are destined for the drainage of the mud which is deposited in the decanting-vats.

The process is as follows: The foul waters from the factory are drawn into a single conduit from about twenty inches to two feet wide, along and over which is disposed a series of circular bucking