through a force main (generally of cast iron) into a main leading by gravity to the outlet.

(2) Where all the sewage of the city is collected at one point near the body of water that is to receive it, but below the level of such water, and is there raised to such level.

(3) Where a ridge separates a part of the tity from the outlet. In such case there may be the alternative either to pump the sewage over the ridge, or to carry it through the ridge by tunnel or deep cut construction.

(4) Where, owing to flat land and a long outfall, or other conditions, a continuous grade to the outlet not only would bring this much below the surface of the water that is to receive the sewage, but also would require that a considerable part of the mains or outfall sewer be laid at a too expensive depth below the surface. In such case a number of pumping plants sometimes are located at intervals along the mains, which then consist of a series of lifts (generally vertical) and long, flat inclines, alternating.

(5) Where a treatment plant is required and must be placed higher than the outfall sewer. In such case the pumping plant is commonly placed close to the treatment plant for convenience and economy of operation.

Pumping house sewage is employed intermittently where

(6) Occasional high water in a stream receiving the sewage would back up the sewer and perhaps flood connected properties. A gate (to exclude the high water from the sewer) and pumping plant are then placed at each outlet and the sewage pumped into the stream during such periods, but not at other times. This plan is generally practicable for separate systems only.

(7) Unusually high tides at ocean outlets would have the same effect. Where each high tide backs up the sewer, but the outlet is free at low tide, pumping during the former period is desirable, although the expense may not be considered warranted.

Storm-water is almost never pumped, because of the enormous amount to be handled. (New Orleans is a notable excep tion to this.)

For Case 1, the only alternative to pumping is omission of the low area from the sewerage program, unless it can all be raised by filling in. It may be omitted from immediate construction, but should always be provided for in the plans; and the main into which the sewage from this area is to be pumped should generally be given capacity for removing the sewage from it whenever in the future pumping may be adopted. The force main should be made as short as possible, since it is much more expensive than a gravity main and house-connections cannot discharge into it.

Case may occur because the ground generally is flat, because there is little fall from the head of the outfall to the outlet, or because of a low area at some point along the route of the outfall or of one of the mains. The last case may sometimes be solved without pumping by using an inverted siphon, or better still by carrying the main through on the gradient, letting it come above the surface where necessary. Where so

elevated, the grade of the street should ultimately be raised sufficiently to cover it if possible. If not, the main may be carried on a trestle or built into a masonry wall along the side of the street, where there are no intersecting streets or other local conditions to make this impossible.

Case 3 can in some cases be solved by carrying the sewage by gravity to the natural drainage outlet of the land in question; by arranging with an adjoining city to remove it if the outfall enters the borders of such city, or by treating it if necessary to discharge it into a stream so small that the untreated sewage would create a nuisance. But pumping may be cheaper than building and operating a treatment plant or than building a long outfall to another outlet.

Case 4 may be considered as Case 2 with the added feature of considerable and expensive depth of sewer throughout a long distance. The outfall, beginning a few feet (possibly 5 or 6) below the surface at each pumping station, continually falls and reaches a maximum depth that is generally greater the greater the distance to the next pumping station. The less the distance between pumping stations, the less the average depth of the outfall and resulting cost and the less the lift at each station. The number and location of pumping plants are generally decided upon the basis of which will cause the least annual charges (interest and sinking fund, maintenance and repairs). If all the lifting can be done at one or two points, it is usually most economical to so arrange it, even at great expense for excavation. It would be possible in many cases to use only one pumping station, all of the outfall sewer beyond it being laid at a practically uniform depth, regardless of grade, and acting as a force main; but the cost of such a main, since it must withstand pressure, would be several times that of a gravity main, and deposits would be apt to form near the bottom of any rising grade. It is evident that no house, inlet or other gravity connection can be made with a pumping main, inverted siphon, or other sewer under internal pressure.

Case 5 is more or less common because the outlet of a treatment plant should be above the highest water, while there is

more or less head lost in the plant itself (10 to 15 feet fall is required for tank treatment followed by sprinkling or contact filter). Even where the topography would permit bringing the sewage to the plant by gravity, it may be that the outfall was built before treatment was considered, and when the treatment plant is built either the sewage must be pumped or the outfall and a part of the system be rebuilt. If the combined system is used and can be laid at such an elevation as to discharge into a stream by gravity, the outlet can be so arranged that the storm sewage will so discharge but the dry-weather flow and light storm run-off will be pumped, the separation being effected by interceptor, regulator, overflows, or other method.

Case 6 also may result from change in conditions, which cause higher water in a stream than was anticipated when the plans were made. Or it may be practically impossible to lay the outfall high enough to discharge above the level of the highest freshets, or more expensive to do so than to install and occasionally use a pumping plant.

thus reducing the velocity to

In case 7, if pumping is not employed, the high tide backing up the outfall sewer compels it to flow full at the lower end, cubic feet per second which area of sewer in square feet' diminished velocity will probably cause deposits. The length of sewer so flooded may then be made only large enough to carry the sewage for three to five years to come, thus increasing the velocity when flowing full; adding another outfall sewer when the increased amount of sewage demands it.

In case 6, and in case 7 when only occasional tides flood the sewer, efficiency and low cost of operation of pumps are of much less importance than cost of installation, since operation will be so infrequent. It is desirable to locate the plant where power can be obtained from an outside source-electricity from a power or traction company, steam from the boilers of a factory or water-works pumping plant, etc.—which will cost less and be more reliable in an emergency than creating energy in a plant operated at very infrequent intervals.

In all of the above cases an effort should be made to reduce

the amount of sewage to be pumped by carrying to the outlet or treatment plant by gravity, generally through an intercepting sewer laid at a minimum grade, sewage from so much of the city as lies at a sufficient elevation to permit this.

In any case where low land causes the necessity for pumping, it is decidedly preferable, for other reasons as well as for sewering (although, of course, more expensive) to raise the streets and sewers generally throughout the low area a sufficient amount to permit gravity discharge. (The city of Chicago, when rebuilding after the fire, raised the streets over its entire area. chiefly to secure better drainage.)

PUMPING PLANTS

Sewage pumps must handle water containing more or less suspended matter, and at constantly varying rates. A pump cannot ordinarily vary its rate to meet instantly every variation in sewage flow, and therefore more or less storage is required at a pumping station to provide for the continuous variations from minute to minute, and in some cases the greater variations from hour to hour. The amount of storage should be the least that is safe, for deposits are liable to form in the storage tank (which is generally an enlarged suction well) unless the sewage therein be kept constantly in motion, and the cost of underground storage is considerable. For this reason, also, pumping is generally continued night as well as day.

Even with storage, there must be a considerable variation in the rate of pumping in most cases; as a general thing, the smaller the amount pumped the greater the variation. This variation may be obtained by varying the speed of the pumps, by varying the number of pumps in action, or by intermittent pumping. In most cases the two latter are combined.

It is desirable to screen sewage before pumping it, to remove sticks and other large substances that might clog or injure the pumps. But the screens should not intercept small matters like pieces of paper and fæces (unless located at and serving as part of a treatment plant), since the presence of these in