In the larger cities there are many instances where plumbing fixtures are located below the level of the street sewer, in which case it is obviously impossible to discharge the waste coming from them, into the sewer by gravity. Such conditions must be dealt with in the sub-basement floors of numerous tall city buildings, underground toilet rooms or public-comfort stations, and in underground or subway passenger stations.
Briefly stated, the method of handling such sewage is to convey it by gravity through the ordinary soil and waste lines into a receiving tank, from which it is pumped or ejected by other means, into the house sewer of the gravity system.
Plate XLVI. The Disposal Of Sewage Of Fixtures Located Below Sewer Level - Automatic Sewage Lifts - Automatic Sump Tanks
Automatic Sewage Lift
Automatic Sump Tank.
In addition to fixture drainage, the matter of subsoil drainage, which is often a very considerable matter in underground work, must be taken care of.
There are several methods of raising the low-level sewage into the gravity house drain.
A sectional view of such a sewage lift or ejector is to be seen in Plate 46.
When pumps are to be used, the low-level sewage is discharged into a receiving tank located below the level of the lowest fixtures, each soil or waste inlet to the tank being trapped, and the trap supplied with a vent, which may be connected into any main vent of the gravity system.
A tank of this kind should be large enough to hold the sewage collecting during several hours, if the discharge from it is automatic, and if not, it should be large enough to hold the sewage entering it during twenty-four hours.
As nearly above the tank as possible, a centrifugal pump is set, which is operated by an electric motor. A float inside the tank is arranged to rise with the sewage in the tank, and when it has filled to a certain point, the rising of the float locks an electric switch which controls the motor. The motor is thus set in action, operating the pump, and the latter quickly draws out the contents of the tank and forces them into the house sewer of the gravity system. The suction of the pump should reach down to the bottom of the tank in order to draw out all the heavy matter. To the tank a fresh-air inlet should be connected, not only to serve the ordinary purpose of the fresh-air inlet, but to relieve the tank while it is filling and to aid the pump by admitting air when the latter is in action. The pump may also be set on the same level as the tank, and, in fact, works to better advantage when so set, as no primer is necessary, and the apparatus is thereby considerably simplified. Piston pumps are also used in raising sewage from low levels.
The centrifugal form of pump is best adapted to large volumes of sewage which are not to be raised very high, while piston pumps will raise smaller amounts through much greater distances.
In the use of piston pumps, however, it is necessary to prevent anything but clear sewage from entering, as the coarser and gritty matter works destructively on the working parts of the pump.
The great objection to the use of pumps in disposing of low-level sewage is the cost of operating.
The use of automatic sewage ejectors, however, is accompanied with small running expenses, and they have many advantages over the use of pumps, chief among which is the fact that there are almost no working parts to get out of order, and very few auxiliary devices, which are expensive to operate, as in the case of electric motors used on pumps.
In Plate 46 is shown such an apparatus, operating automatically, and designed especially for this kind of work.
There are several other makes that may be obtained, all working on more or less similar principles. Compressed air has proved the most satisfactory motive power, but very often these machines are provided with appliances by means of which steam or water may be used to operate them in the event of an interruption in the compressed-air apparatus.
The action of the automatic sewage lift is the following: Sewage from the levels below the crown of the sewer is conducted, through various lines of soil and waste pipe, into a sewage tank or receiver.
Inside the receiver an open bucket rests upon the surface of the sewage, rising as the latter rises. When it has risen to a certain point, the rod to which it is connected, and which passes through a stuffing box at the top of the tank, by means of a lever attachment trips a valve on the compressed-air supply pipe, the same action closing a valve on the vent pipe of the apparatus. Compressed air is at once admitted upon the surface of the sewage in the receiver, and is sufficient in pressure to raise this sewage through the outlet and into the house sewer of the gravity system.
A pressure of 2 pounds should be provided for each foot in height through which the sewage is to be raised.
When the pressure of the compressed air is exerted on the sewage, it closes the check valve on the inlet to the receiver, and opens the check valve on the outlet, and as the closing of the vent pipe closes the only other path for the sewage, it must pass out through the proper outlet.
As the water in the receiver falls, the bucket, which is weighted with the water which it holds, follows with it, and when it reaches a point near the bottom, the lever attachment shuts the valve which controls the compressed-air supply, and opens the vent valve, thus venting the air confined in the receiver. The ejector is now ready for another operation. It will be seen that the ejector acts as a trap, and therefore the use of a main trap is unnecessary in connection with it.