likely to be observed. intercept all matter in the catch-basins which would deposit in the drains after they reach the flat grades in the lower part of your city. The cleaning of the drains would, therefore, be necessary in any event, and the additional amount of silt that would be intercepted by the catch-basins will not cost much more to remove. In the city of Paris, even though a combined system of sewers is used, it is not found objectionable to allow all the street-dirt to enter the sewers and therefore the catch-basins at the inlets are omitted." (Report of Rudolph Hering and Samuel M. Gray on Sewerage of Baltimore.) (See also Appendix No. 1.)

Also it is found impracticable to

As a matter of fact catch-basins are not infrequently left uncleaned after light storms, or even heavy ones, for weeks together, and the odors from them are usually attributed to the sewers, which in most cases are far less foul. Moreover, catch-basins are usually cleaned with shovels only and sufficient filth left upon the sides and bottom to become noticeable by its odors. When cleaned so infrequently the catch-basin often stands full of material and is until cleaned practically non-existent so far as any useful effect is concerned.

sewer.

For these reasons the universal use of catch-basins is, in the author's opinion, not to be advised, but rather the inlet should be so designed that all material shall at once reach the The inlet-connection he would also make without a trap, that it may assist in the ventilation of the sewer; and if the sewer and its appurtenances are properly designed, constructed, and maintained there will be very few instances where any odor can be detected at the inlet.

There may well be cases where catch-basins are desirable, as where the wash from a steep hillside is caught, or for other reason a large amount of coarse soil or "clean dirt" finds its way to the inlet; and there the catch-basin will need to be large, that but a small proportion of this may reach the sewer,

and should be cleaned after every heavy shower. A small catch-basin is in most locations worse than useless.

Catch-basins are also desirable where the sewer grades are very flat and the velocity is less than 3 feet per second; also on combined sewers where the streets are unpaved.

ART. 42. PUMPING OF SEWAGE.

There will frequently occur instances where, even if the sewers be laid at the flattest permissible grades, either the outlet will come too low, or the upper ends or some intermediate point will be too high for proper service. This is especially likely to occur where the outlet is at a considerable distance from the city; also where treatment of the sewage is necessary. Under such circumstances there is but one solution of the difficulty-the sewage must be raised at some one or more points from a low to a higher level. (Where a street has not yet been graded or built upon it may often be practicable to lay the sewer above the ground-surface in crossing a valley or basin, and so grade the street finally as to give it a proper covering, thus avoiding the necessity of pumping.)

Where the sewage is discharged into tidal waters and the outlet is below high tide the lower stretch of the sewer will be filled twice a day, and the velocity therein cannot then exceed the quotient obtained by dividing the volume of sewage by the area of the sewer. It would therefore be well to make this sufficiently large for present needs only and duplicate it when greater capacity becomes necessary. In some instances tidal basins are constructed, which are closed -automatically in most cases-against the rising tide, and receive and hold the sewage flow during high tide, their contents being discharged on the falling of the tide. In some cases the sewers themselves are made sufficiently large near the outlet to serve as reservoirs in the same way. But these

reservoirs are seldom satisfactory, owing largely to the difficulty of cleansing them from the deposits made while they are filled with stagnant sewage. It would be better, though of course more expensive, to pump the sewage during high tide; or better still to raise the streets and sewers generally, where this is possible, and discharge above high tide. (The city of Chicago some years ago raised the streets over its entire area to permit of better drainage.)

In certain places the conditions are such that the water rises above the sewer-outlet, which is ordinarily free, for periods of days or even weeks; as on a lee shore during a storm or on rivers subject to extended floods. In such a case pumping is necessary; but the first cost of the plant should be kept at a minimum, since the interest on this will far exceed any saving that could be made in running-expenses for a few days. If possible it is well to locate the plant where power can be obtained from an outside source-as steam from the boilers of a water-works pumping-plant, electricity from a power or traction company, etc.-by which means both first cost and running-expenses may be reduced.

Where house-sewage only is to be raised the apparatus should be of a capacity sufficient for the maximum flow. Storm-sewage, or at least the entire run-off from heavy storms, is not often pumped, owing to the enormous capacity required in the machinery. It will in most cases be found more economical to build special outlets for the storm-sewage to the nearest watercourse, where this is practicable. In the case of a combined sewer the house-sewage should all be pumped, as should even the run-off from light storms, which carries street-washings. But it will usually be permissible to allow the run-off from heavy rains with the admixture of house-sewage to escape by overflows and special storm-sewers to nearer outlets. If this would give rise to danger or a nuisance, owing to even the small proportion of house-sewage

contained, it is probable that the separate system should be employed, all house-sewage being pumped and each stormsewer seeking the nearest outlet.

In a very flat country it may be desirable to raise the sewage at a great number of points to prevent deep and expensive excavation. A sewer under a level surface, beginning at a depth of 8 feet and falling 1 foot in 300, would in 2100 feet have a depth of 15 feet. Beyond this the cost of construction would rapidly increase unless the sewage could be lifted and started again at a depth of 8 feet.

Whether the lifting of the sewage shall be done at one station or at several is usually a question of cost only. It can be exactly settled only by a comparison of the sum of the interest on first cost and the operating-expenses of one method as compared with another. (It is assumed that the depth of every sewer is made sufficient to meet all requirements.) The fewer the lifting-stations and the further apart they are the greater will be each lift; also the greater will be the average depth of sewer. Hence, while the greater the distance between lifts the less will be the total cost of lifting machinery or apparatus, and also of maintenance of the same; on the other hand the greater will be the cost of the construction of the sewer and also of its maintenance. The proper decision as to the number and location of the lifting-stations is frequently a problem requiring much careful study. While in one locality, where excavation is expensive, 5 feet may be the maximum lift which will be economical, in another this limit may reach 30 feet or more. 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.

The methods and apparatus to be employed may be: pumping by steam, gas, gasoline, or hot-air engines or electric motors, lifting by a Shone Ejector, an Adams Sewage-lift, or other appliance which seems adapted to the circumstances.

If steam, gas, gasoline, or hot air be employed a complete plant must be placed at each lifting-station. Where electricity is the motive power a motor and pump only are required at each station. This renders possible a saving by using electricity, under certain conditions, such as many liftstations with a small horse-power required at each, or even when the horse-power is considerable. For five stations at New Orleans, of very great pumping capacity, B. M. Harrod estimates the annual cost as follows:

If the difference in cost of real estate for the two systems be allowed for, the annual cost would probably balance very closely.*

The pumps usually employed are the piston- or plungerpump and the centrifugal pump. Other devices have been employed, such as screw and oscillating pumps, but few with any success. The centrifugal pump requires a quite constant volume of sewage for its proper working; hence, usually, a storage-basin, which is objectionable. For ordinary lifts, however, it is frequently more economical than a piston-pump; also the wear due to grit in the sewage is neither so great nor so injurious to the pump, and hence the necessity for screening the sewage is not so great as with the piston-pump. With the latter particularly care should be taken to remove all large solids and gritty matter. For this purpose gratings, wire screens, and settling-tanks are employed, the last being of such cross-section that the velocity through them is less than one foot per second. These should be near or in the pump

* Since the above was written electric pumping has been adopted for this work.