No one type of moving part device has been used extensively in different sewage treatment plants. Designing engineers have exercised their ingenuity at different plants, resulting in the production of different types.[[207]] Among the best known forms is the apparatus designed by J. W. Alvord for the intermittent sand filters at Lake Forest, Illinois.[[208]] In its operation....
A float in the dosing chamber lifts an iron ball in one of a series of wooden columns, and at a certain height the ball rolls through a trough from one column to the next, in its passage striking a catch, which opens an air valve attached to one of ten bell-siphons in the dosing chamber. Each of the siphons discharges on one of the ten sand beds, which are thus dosed in rotation.
Since air-locked dosing devices are in more general use their operation will be explained in greater detail.
284. Operation.—The simplest form of these devices is the automatic siphon used for flush-tanks, the operation of which is described in Art. 61.
In the operation of sand filters, sprinkling filters, or other forms of treatment where there are two or more units to be dosed it is desirable that the dosing of the beds be done alternately. A simple arrangement for two siphons operating alternately is shown in Fig. 182. They operate as follows: with the dosing tank empty at the start water will stand at bb′ in siphon No. 2 and at aa′ in siphon No. 1. As the water enters through the inlet on the left the tank fills. When the water rises sufficiently, air is trapped in the bells, and as the water continues to rise in the tank, surfaces a and b are depressed an equal amount. When b has been depressed to d, a has been depressed to c. Air is released from siphon No. 2 through the short leg, and siphon No. 2 goes into operation. Surface c rises in siphon No. 1 as the tank empties and when the action of Siphon No. 2 is broken by the admission of air when the bottom of the bell is uncovered the water in siphon No. 1 has assumed the position of bb′ and that in No. 2 is at aa′. The conditions of the two siphons are now reversed from that at the beginning of the operation and as the tank refills siphon No. 1 will go into operation. It is to be noted that these siphons are made to alternate by weakening the seal of the next one to discharge and by strengthening the seal of the one which has just discharged.
Fig. 182.—Diagram Showing the Operation of Two Alternating Siphons.
Fig. 183.—Diagram Showing the Operation of Three Alternating Siphons.
285. Three Alternating Siphons.—This principle can be extended to the operation of three alternating siphons as shown in Fig. No. 183. These operate as follows: with the dosing tank empty at the start and water at aa′ in siphons 1 and 2, and at bb′ in siphon No. 3, the dosing tank will be allowed to fill. As the water rises in the tank air is trapped in all the bells and surfaces a and b are depressed. When surface b has been depressed to d, a has been depressed to c. Air is released from siphon No. 3 and this siphon goes into action. Surface c rises in siphons 1 and 2 to the position b, as the dosing tank is emptied. At the same time a small amount of water is passed from siphon No. 3 to the short leg of siphon No. 1, through the small pipes shown, thus filling this leg so that when siphon No. 3 ceases to operate the water in siphons 1 and 3 stands at aa′ and that in No. 2 stands at bb′. Siphon No. 2, having the weaker seal, will be the next to operate. During its operation it will fill siphon No. 3, leaving No. 1 weak. When No. 1 operates it will refill No. 2, leaving No. 3 weak, thus completing a cycle for the three siphons. This principle has not been applied to the operation of more than three alternating siphons and is seldom used on recent installations.