The siphons in each instance should be so placed that the lower edge of the bell of the siphon will be at a distance below the roof of the tank equal to twelve inches plus the drawing depth or discharging depth of a siphon of the diameter indicated. There should be three inches of space between the siphon bell and the floor of the chamber. The discharging depths of siphons as used in forming Tables 2, 3, 4, and 5 are as follows:
| Diameter of Siphon. | Discharging Depth. |
|---|---|
| 3 inches | 13 inches |
| 5 inches | 23 inches |
| 6 inches | 30 inches |
| 8 inches | 35 inches |
| 10 inches | 60 inches |
| 12 inches | 72 inches |
If the siphons installed are larger or smaller than those shown in these tables, or if the particular make of siphon purchased has the same diameter but a different discharging depth, proper allowance must be made in proportioning the size of the dosing chamber.
In order to quickly convey the dose from the siphon chamber to the filter beds at the rate at which the siphon discharges, the sewer from the siphon chamber should be of proper size and should have a sufficient gradient. For instance, with a 3–inch siphon the sewer should be 6 inches in diameter, with a gradient or fall of at least 12 inches per 100 feet; with a 5–inch siphon, the sewer should be 8 inches in diameter, with a gradient of at least 6 inches in 100 feet; with a 6–inch siphon, the diameter of the sewer should be 8 inches, and should have a gradient of at least 12 inches per 100 feet, or 10 inches with a gradient of at least 3 inches per 100 feet; with an 8–inch siphon, 12 inches, with a gradient of at least 12 inches per 100 feet.
Sewage is sometimes applied directly to the beds without treatment in settling tanks, generally, in such cases, after having been screened to remove the larger suspended matters, but it is decidedly preferable in the case of the smaller plants under discussion to pass the sewage first through settling tanks, as in the method of sub-surface irrigation. Therefore, the areas of beds given in the table are for sewage which has been passed through settling tanks. It is even necessary, in the case of sand filters for institutions where considerable grease and soaps are contained in the sewage, to provide grease traps through which the sewage must pass before it reaches the settling tank. The effluent from the tank should be discharged intermittently by means of a dosing chamber and siphon and should be distributed quickly over the surface of the bed as uniformly as possible. This is generally accomplished in the case of the larger beds by laying on the surface of the bed, wooden troughs, with short branches, as shown in Figs. [38] and [39]. A detail of a portion of these distributing troughs is given in Fig. [40]. This view shows the hinged gates which are used to effect a proportionate division of the flow of the various branches of the main trough. The view also shows the slots in the sides of the troughs which allow the sewage to flow out onto the bed.
Fig. 38.—Layout for Intermittent Sand Filtration.
If the ground-water level is within three feet of the surface at any time, or if the sand is very fine and contains a slight proportion of clay, underdrains should be laid at depths of four feet to prevent the beds from becoming waterlogged.
Fig. 39.—Intermittent Sand-filtration Beds.