CHAPTER III
VALVES, SIPHONS, AND SIPHON CHAMBERS
It was explained in Chapter I that one of the essentials of successful sewage purification is an intermittent application of the sewage to the beds in which bacteria are to act. This intermittent action is secured by providing a small additional tank or by setting aside a part of the settling tank and by installing therein some kind of mechanism for the purpose of changing the more or less regular flow into an intermittent or periodical flow. The proper capacity of this tank will be considered later in the chapters dealing with the several methods of final purification. Now it may be said only that the size depends on the amount of sewage to be cared for per day and on the size of the dose demanded by the purification method. The size of dose depends directly upon the method of treatment and on the size of the particles in the beds intended to receive the sewage. On sand beds, for example, it is customary to discharge the sewage from the dosing tank three times a day, although many plants operate with a daily discharge. The size of the dosing tank, however, in the latter case has to be three times as large as in the former, and it is usually worth while to take the additional trouble of having more frequent operation in order to save the cost of the larger tank.
The simplest method of construction of the dosing tank is to make it a part of the sedimentation tank by means of a cross wall, which latter must be strong enough to withstand the pressure of the water on one side when the dosing tank is empty. (See Figs. [2] and [3].) There is no objection to this tank being separate and some distance away from the sedimentation tank, and sometimes, for convenience in distributing the sewage from the dosing tank onto several beds in turn, the dosing tank is placed at the centre of a group of beds with the settling tank outside. If the dosing tank is a part of the main tank, the sewage flows into it over a dividing wall between the two tanks or through a pipe laid through this wall, while if the tank is separate from the other, then a longer pipe connection is required.
Fig. 10.—Sludge Valve for Floor of Tank.
It is economical to arrange that the level of the sewage in the dosing tank, at the time when that tank discharges, shall be at the level of the sewage in the settling tank, since then no head is lost. It is better still to arrange the mechanism in the dosing tank so that the level of the sewage there at the time of its discharge will be from four to eight inches higher than the normal level in the settling tank. The effect of this is to back up the sewage and raise the general level in the settling tank, and when the dosing tank discharges there is drawn off not only the sewage in that tank, but also an amount in the large settling tank equivalent to that which is above the normal level of the sewage there. The advantage of this is plain in that it reduces the necessary volume of the dosing tank by that of the back water in the settling tank, and, while it was thought at one time that such a frequent variation in the level of the main tank might affect injuriously the scum which forms there, and perhaps also the bacterial action going on in the tank, there seems to be no real reason why this method may not be used with considerable advantage in economy of construction.
The bottom of the dosing tank, which is preferably made of concrete, should have a slope toward the point from which the outlet pipe leads, thus enabling the outward rush of sewage to carry off any material which would otherwise settle in the bottom and perhaps decompose there.
Fig. 11.—Sludge Valve for Side Wall of Tank.
The simplest method of operating the dosing tank is by means of a hand valve fastened either to the floor of the chamber or to the bottom of the outside wall. Fig. [10] shows a simple form of a valve suitable for the floor and intended to be operated by a rod extending up through the sewage to the outside air. Such a valve can be made at any local foundry, the bearing surfaces turned up in any machine shop, and a piece of leather for packing purchased at any hardware store. Such a design, however, is not suitable for a large valve or for a great depth of water, since the pressure on the valve is dependent on the weight of the column of water acting on its area. If the outlet pipe is six inches in diameter, the diameter of the upper surface of this valve would be about ten inches, and the area of the top of the valve would be about half a square foot, so that, with six feet of water above, weighing 62½ pounds per square foot, the weight on the valve to be lifted would be 186 pounds, rather more than could be lifted by one man. Under such conditions it would be necessary, using such a valve, to rig a lever, the fulcrum being fastened to the edge of the tank, the short end of the lever to the rod, and the long end so arranged as to reduce the load in the ratio of about one to four. Fig. [11] shows another type of valve intended to be set into the side of the tank with the floor sloping rapidly toward the low point at which this valve is set. These valves require better workmanship and are preferably purchased from one of the dealers in valves who make this type as one of their regular stock forms. Fig. [12] shows the design made by the Coffin Valve Company, of Troy, N. Y., and a similar form of valve is made by the Caldwell-Wilcox Company, Newburg, N. Y. For a six-inch pipe, these valves are so made that the danger of the moving parts rusting together is avoided by having one surface bronze or some similar noncorrosive metal. Fig. [13] shows an ordinary gate valve generally used for water works, but applicable to sewerage works. Such a valve is shown in Fig. [5].