Page:Popular Science Monthly Volume 18.djvu/830

810 tubs, containing lime-water, a substance which is known to be very efficient in securing the precipitation of organic matters. The tubs are provided with dashers, which keep the lime-water constantly in suspension, and with gauged faucets, which permit it to be introduced regularly in the proportion of two hundred to two hundred and fifty grammes of lime to the cubic metre of water into the current of foul water which is flowing at the foot of the tubs. A few eddies produced by means of artificial obstacles placed in the conduit secure immediately a complete mixture of the lime with the foul water. As soon as the lime-water is introduced, the waste waters almost wholly lose their offensive odor, and cease to offer the slightest danger from noxious exhalations.

The water having been thus prepared in its passage through the conduit, is distributed into ten basins of decantation, each about sixty-five feet long, twenty feet wide, and five and a half feet deep, which are arranged side by side. Each of the basins has a capacity for the decantation of a thousand cubic metres of liquid a day. The water is constantly entering at one end in each, and flowing out over the top at the other end. The rate of flow is almost imperceptible, being hardly a millimetre a second, and precipitation takes place as completely as if the water were quite still; consequently, the water goes out fully clarified. Thus a course of about sixty-five feet in length, which, at the rate of ·001 of a metre a second, represents a delay of nearly six hours in the basin, suffices to clear the water of all matters in suspension. A talus of mud is gradually formed in the bottom of the tank, which at the end of a week becomes flush with the surface of the water at the entrance-end, and just covers the bottom at the end of the outlet. The basin has now produced all the effect of which it is capable. If any more water is allowed to go through it, it will contain mud in suspension, for it is still in the act of precipitating it when it goes out. The operation must be stopped here. We close the feeding-gate of the tank, draw off the clear water that is left in it by means of a decanting-tube, and lay bare the talus of mud.

The bottom of the basin is slightly inclined in a contrary direction to the course of the water, and is provided with a large valve at the lower end. On opening this valve, the mud, which is still in a very liquid condition, is passed into a lower basin—the drainage-basin—of the same capacity as the former one, and so disposed that its upper surface is a little below the bottom of the same. The first tank may be put in operation again immediately after the mud has been drawn from it.

The side-walls of the drainage-basin are of tight masonry, but the bottom is made as permeable as possible. For this purpose a floor of scoria is prepared and provided with a series of pipes which lead the water out into a collector. The arrangement is admirably adapted to the purpose for which it is designed, and quite obviates the