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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 12.—Sluice-Gate Valve made by Coffin Valve Co.]
[Illustration: FIG. 13.—Ordinary Gate Valve.]
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.
[Illustration: FIG. 14.—English Slide Valve with Wedge-lock Handle.]
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.
[Illustration: FIG. 15.—Flap Valve Attached to Length of Sewer Pipe.]
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.
[Illustration: FIG. 16.—Flap Valve with Metallic Seat Attached.]
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.
[Illustration: FIG. 17.—Flap Valve with Loose-link Hinges.]
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.
[Illustration: FIG. 18.—Intermittent Dosing Apparatus made by Ansonia
M’f’g Co.]
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:
[Illustration: FIG. 19.—Simplest form of Automatic Siphon.]
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.
[Illustration: FIG. 20.—Van Vranken Automatic Siphon.]
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.
[Illustration: FIG. 21.—Miller Automatic Siphon.]
[Illustration: FIG. 22.—Double Alternating Siphons of the Miller Type.]
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.
[Illustration: FIG. 23.—Triple Alternating Siphons of the Miller Type.]
[Illustration: FIG. 24.—Single “Merritt” Automatic Siphon.]
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.
[Illustration: FIG. 25.—Air-lock Siphon for Admitting and Releasing
Sewage from each one of Four Beds in Regular Order.]
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.