the screenings makes it difficult to dispose of them without offense. Also it should not be possible for the screen to become so clogged as to prevent free flow of the sewage through it.

In a general way, any kind of plant used for pumping water may be employed for pumping sewage. But pieces of paper, rags, match sticks and the like are likely to interfere with the operation of valves and other moving pump parts, and for this reason reciprocating pumps are seldom employed, although they offer the advantage of permitting considerable variation. in speed. Largely for this reason, they are sometimes used for large plants, where attendants are always present, fine screening being employed and special provision made for frequent removal and immediate disposal of the screenings. Centrifugal pumps, however, are most commonly used, since there is little danger that small floating matters will interfere with their continuous action. Another contrivance used for sewage pumping is the pneumatic ejector, in which the sewage is raised intermittently by compressed air acting expansively on the surface of sewage in a pot or tank.

For operating pumps, any of the common steam, gas, gasoline, or kerosene engines, or electric motors may be used. Steam is probably most common for large plants, electric motors for the small ones. The chief advantage of electrical operation for small plants is that they can be made to operate automatically, receiving no attention except a daily visit for inspection and oiling.

A type of plant that has almost become standard consists of a vertical, submerged, centrifugal pump, driven by an electric motor direct connected to the upper end of the vertical pump shaft. The motor is started and stopped by a switch that is operated by a float in the suction well. When the sewage rises in the well above a certain level the pump is started, and continues to operate until the sewage is drawn below a certain lower level, when it is stopped. If the sewage continues to rise during pumping, a second float starts a second pump operating. A third pump started at a still higher level should be provided against a stoppage of either of the other two.

While the pumps are below the low sewage level in the suction well, they are preferably in a dry well, between which and the suction well is a tight wall, through which the suction pipe passes. There should always be two pumps and preferably three or more, one for reserve and as a third-level emergency pump, one or more for peak loads, and one or more others with capacity for all but the peak loads. This throwing in and out of service of supplementary pumps is necessary, since the centrifugal pumps so operated cannot vary their velocity, while the rate of flow of sewage into the suction well varies considerably. To insure that a pump will be "primed" when its motor starts, It is necessary that the sewage flow into it by gravity.

The motors should be of sufficient capacity to operate their pumps continuously at maximum flow and head and be provided with overload stopping devices to shut off the power in case of a clogged impeller. If an operator is always present at the plant, a tell-tale device should be used to show what pumps are operating. If it is not under constant inspection, a float should, by closing an electric circuit, operate a bell or whistle where some one is always present, if the sewage in the suction well should rise above a given level, indicating the failure of a pump to function.

The amount of storage and number and capacity of pumps required for reliable and economical operation depend upon the maximum rate of flow and frequency and amount of variations in rate. It should not be necessary for a pump to stop and start more frequently than two or three times an hour. The greater the storage capacity the less frequent the stops; but on the other hand, the less the storage the less the danger of sedimentation in the well and resulting putrefaction, and the less the cost.

A pneumatic ejector is operated by compressed air. Where there is pumping at more than one point, the air may be piped to all points from a central plant or be furnished by an electricallydriven compressor located near each ejector. The cost of one large compressor plant and the necessary air pipes, together with the cost of operation and loss of energy by transmission,

is to be balanced against the cost of several individual plants and their cost of operation. The efficiency of the central plant system is greatest when the lift by all the ejectors is practically the same. The ejector is especially adapted to small quantities of sewage or great variations in quantity. Sewage flows from the sewer directly into the pot or chamber of the ejector, and when it has almost filled this, it opens a valve in the air-pipe, admitting compressed air from above that forces it into the rising or pressure pipe. No storage tank is required, and the ejector, if large enough, adapts itself to

any rate by its frequency of operation. The compressor is generally operated by electricity and started and stopped automatically, so as to keep the air in the air reservoir at the proper pressure. A duplicate ejector and compressor should be provided to permit repairs or cleaning of either.

From no pumping station need there be any odor, with good management. The suction well and screen are the only parts of the plant where odors are at all probable. The former should be so designed and operated as to prevent sedimentation or stagnation and consequent putrefying of any part of the sewage. Pumping plants can therefore be placed at any convenient point. The small pumping plants and pneumatic

ejectors are usually placed in vaults beneath the surface, the larger plants above ground. The sewage-pumping stations of London, Berlin, New Orleans, and other cities are within the city limits, no odor whatever being perceptible near them.

Eighty plants are described in Municipal Journal for March 18, 1915, of which 52 were motor-driven centrifugal pumps, 10 were centrifugal pumps driven by steam engines, 3 centrifugal pumps driven by gasoline or gas engines, 5 were reciprocating pumps, and 10 were pneumatic lifts.

CHAPTER VII

COLLECTING THE DATA

ART. 28. DATA REQUIRED

ANY plans made before the full and complete data are at hand may be shown by further information to be inadvisable, while their very existence may create a prejudice against the substitution of more efficacious ones. Therefore, although the development of the plans may suggest the desirability of further data the necessity for which was unforeseen, all such data should be collected and surveying done preliminary to any designing as would seem to be necessary for completing it.

The first necessity will be for a map of the district under consideration. This will usually include the city or town and all land over which it may spread in the future; also all adjacent areas which shed their water into or across the surface of this territory. This map should show all streets, lanes, etc.; all parks or other areas permanently devoted to vegetation; all rivers, creeks, ponds, or other bodies of waterin fact, all natural and artificial divisions of the area embraced by the corporate limits. It usually happens that this much can be found already mapped for other purposes; but unless it is known that the measurements from which such map was prepared were accurately taken, a sufficient number of check measurements should be made to establish its accuracy or the reverse. On the point of accuracy a question may arise as to how exactly the measurements should be taken. If these should involve an error of no more than .2 per cent they would be sufficiently accurate for the work in hand. For, as sewer grades are ordinarily run from manhole to manhole, and these are about 300 feet apart, an error of .2 per cent would mean