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.

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.

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.

Fig. 14 shows a form made in England and largely used as a cheap valve for the purpose of emptying a tank rapidly. The peculiarity of the valve is that a sidewise motion of the long handle locks the valve into position so that the moving part of the valve may be readily set at any height. The one shown in the figure is taken from the catalogue of the Adams Hydraulic Company, Westminster, London, and is listed in their catalogue at $6.50 for a six-inch pipe.

Fig. 15 shows another type of valve which is supplied by some firms making sewer pipe and consists, as may be seen, of a light moving valve which is attached to a projection cast on the top of the vitrified tile pipe in such a way that the valve comes to an even seat on the bevelled end of the pipe. It is found that with pressure acting against the valve the thin metal of which it is composed is pressed against the pipe so that little, if any, water or sewage will escape. The valve can easily be opened by attaching a cord or chain to the ring at the lower edge of the valve, and when released the valve shuts automatically. This is a very cheap and convenient design, and answers every purpose for emptying tanks by hand.

More elaborate structures of the same general type have been made, using cast iron as the metal, the stationary collar with the bevelled end being built into the masonry wall of the tank. This type of flap valve is faced with bronze, and the bearings or joints have bronze bushings. A satisfactory valve of this sort can be made at a local foundry and machine shop, but there is danger that the valve will not be water-tight. Fig. 16 shows such a valve with the metal seat which is intended to be bolted into the masonry of the tank wall.

Fig. 17 shows another form of this same sort of valve, taken from the catalogue of the Adams Hydraulic Company, and noteworthy because of the loose-link connection at the upper part of the valve, the object of this being to prevent the valve closing at the upper part without, at the same time, closing at the bottom.

If the dosing tank is to work automatically and independently of human agency, an arrangement which is always preferable, there must be installed some mechanism which takes the place of the valve operated by hand. This mechanism is in almost every case a siphon which is put into action when the water level reaches a certain height, and which discharges rapidly until the water falls to a point when air is admitted to the inside of the siphon pipe, thereby interrupting the flow.

There is on the market a dosing apparatus which does not involve a siphon, and which is shown in Fig. 18. This is made by the Ansonia Manufacturing Company, 30 Church Street, New York City, and its operation may be described as follows: It consists of two floats connected by means of a chain which passes over a wheel supported in the upper part of the chamber. As the water in the chamber rises, the left-hand float shown in the drawing rises and the right-hand float falls, thereby communicating a rotary motion to the wheel. A projection on this wheel at a certain point when the left-hand valve has reached the desired height communicates with an inside portion of the wheel, to which a chain connected with the valve is attached. Thus the valve is opened at the right height, and remains open until the water has fallen to the bottom of the chamber. Then the left-hand float falls, and the apparatus is ready to repeat the operation. This apparatus, for a small installation, will probably cost, set up in place, about $15.

Fig. 19 shows the simplest form of siphon arranged to discharge water from a tank. It will be noticed that it consists of an inverted bent pipe, one leg being longer than the other, and extending into a pool of water formed in the end of the discharge pipe. When the water level in the tank reaches the bent portion of the siphon pipe, the water begins to flow out, and will continue to flow until air is drawn in at the lower end of the short leg. This stops the flow and the tank begins to fill again.

Fig. 20 shows another method of working the siphon and insuring its rapid initial action. This is known as the Van Vranken flush tank, and the feature of this arrangement is the movable bucket, which in one position seals the lower end of the longer leg. Then, however, the siphon begins to act, and the bucket, which is hung on trunnions, is disturbed and its contained water is dumped out. This allows the escape of the water in the longer leg and insures a vigorous starting up of the siphon into action.

A more simple type, however, is the inverted siphon arrangement, developed perhaps most completely by the Pacific Flush Tank Company under the Miller patents. Fig. 21 shows their ordinary design, the upper part of the siphon being replaced by a bell and the discharge starting when the level of the water in the long leg of the siphon has been depressed sufficiently to reach the curved part of the pipe. The principle on which this siphon works is as follows:

When the water rises in the tank above the lower edge of the bell, the air which remained between the water in the siphon pipe and in the bottom of the tank is confined, and, as the water rises, is gradually compressed. The effect of this compression is to force down the water in the long leg of the siphon and to hold down the level of the water inside the bell lower than the level outside. When sufficient head of water in the tank is secured, the water inside the pipe will be forced down to the curved part of the pipe, and, the siphon being so designed, the water level inside the bell will be just at the top of the same pipe, but on the outside. Any slight additional height then allows the contained and compressed air to escape around the bend in the pipe, suddenly relieving the pressure and allowing the water to enter the pipe from under the bell readily. Thus the siphon starts and continues to flow until the water level falls so that the air is drawn in under the bell. That stops the action of the siphon and the tank fills again. These siphons are generally sold in two pieces, the cast iron bell and the curved pipe being the factory product. At the plant they have to be set in place, generally bedded in concrete and properly connected with the outlet pipe. For a small installation a three-inch or four-inch siphon is ample, and will cost, delivered, from $10 to $15, depending on the freight.

Fig. 22 shows two siphons with auxiliary air-pressure chambers installed in the same chamber for the purpose of automatically diverting the flow from one bed to another. This may be done more simply by installing two ordinary siphons of the Miller or similar type. If one of these siphons is filled half full when the tank is empty, that siphon will discharge first because of the amount of water already present in the U-shaped tube. During the filling of the tank previous to its discharge, the other siphon will partially fill, so that when the tank begins to fill for the second time the second siphon is half full and the first nearly empty. In this way alternate action is secured and the discharge takes place as often as the tanks fill.

Fig. 23 shows three similar siphons installed with some auxiliary piping attached for the purpose of making the periodic discharge more positive. These small auxiliary pipes are so put together that there is an auxiliary siphon passing under the edge of the bells. When one siphon discharges, the auxiliary siphon of the corresponding large siphon is filled with water, and at the same time part of the water in the auxiliary siphon of the other is discharged, so that it will be the first to operate at the next filling. When the water is forced to the bottom of the small siphon, it is blown out through the vent pipe, and, the air following, the large siphon is started.

Fig. 24 shows an automatic discharging siphon made by the Merritt Company, of Camden, N. J., and embodying a different principle. The main discharge pipe is built in the form of a “U” tube, the longer leg containing an auxiliary small air pipe, with a return bend at its lower end. When the chamber starts to fill, this small pipe bend or seal is filled with water, so that the rising water confines and compresses air in both the large and small “U” pipes. In time, and at any desired height, determined by changing the relative lengths of the parts of the small pipe siphon, the seal is broken and the air escaping draws air enough from the large pipe to start it in action. The method has an advantage in that it requires no deep excavation, and the mechanism can be set after the siphon chamber is built.

Fig. 25 shows a method of securing the alternate discharge of sewage by siphons whose action depends upon an air trap, each siphon being of the type shown in Fig. 24. The installation of the figure is further complicated by the fact that it is arranged to discharge sewage from the four contact beds as well as to discharge sewage onto the beds. The compartments, and the piping connected therewith, at the four corners operate to admit the sewage from the central channel onto the four beds in rotation. The four square wells between the corner wells operate to empty those beds in turn into the pipe shown at the centre of the drawing, the pipe leading to the nearest stream. The operation may be described as follows: Sewage enters at the top of the drawing, and from the inlet channel flows into the siphon channels marked A. A_{1} is ready to discharge if bed No. 4 was the last one to fill, since, when that bed filled, the small bell D_{4} forced the siphon A_{1} open. Sewage therefore flows through siphon A_{1} into bed No. 1. As the sewage level rises in bed No. 1, the outlet siphon from bed No. 2, G_{2}, is locked by the air pipe from B_{1} so that bed No. 2 will be ready for the next dose. Also the air pipe from the bell H_{1} opens the siphon G_{4}, and allows bed No. 4 to drain into the outlet drain. Also bell D_{1}, when bed No. 1 is full, opens the siphon A_{2} through the connecting air pipe so that bed No. 2 begins to fill as soon as bed No. 1 is full. And finally bell C_{1} locks the siphon A_{1}, and stops further flow into bed No. 1. The other beds operate in the same way in turn.

The manufacturers of siphons are always glad to advise prospective buyers of the proper arrangement of siphons and the details of placing, with dimension sketches.

As a summary, it may be pointed out that in any installation, one of the three methods above described may be adopted.

1. A simple valve worked by hand may be adopted and the alternate distribution of the sewage regulated by choice of the several valves placed at the head of the several discharge pipes.

2. An automatic discharge mechanism may be installed which will operate regularly and intermittently, but lacking any automatic selection of the bed onto which the discharge is to be made. These siphons will discharge as often as the tank fills, but the particular valve must be opened in order that the discharge may take place onto any one of the several beds.

3. An apparatus may be installed which will both discharge intermittently and will also automatically select different beds in turn onto which the discharge shall take place. It may even discharge onto contact beds and also empty those beds, entirely automatically.

Which of these mechanisms shall be selected depends upon the amount of money available and on the value to be placed on the freedom from constant care which an automatic installation gives. Not that a sewage-disposal plant may be ignored because an automatic mechanism has been installed. No machine is infallible, and sewerage machinery may give out or stop working just as that for any other purpose. But instead of a daily routine of duties which may not be interrupted, by means of automatic apparatus one may avoid everything except casual inspections and periodic cleaning.