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To prevent these deposits the only practicable way known is to keep all sewers constantly flowing with a depth at least the radius of the invert, water being introduced for this purpose if necessary, and also to maintain a velocity of at least feet per second. To remove them the methods. employed are either to occasionally turn through the sewer streams of water of sufficient quantity and velocity to dislodge and remove the deposits, or to employ shovels, hoes, "pills," scrapers, or similar appliances to be described in Chapter XV.

The method of prevention, if applied near a dead end, where the sewage flow is minimum in quantity, even in the case of a sewer laid at minimum grade, would require about 47,000 gallons per day for each line of 6-inch pipe and 83,000 gallons for each 8-inch line. These quantities it will usually be impracticable to supply; and were it practicable the addition to the sewage of this amount in each of several branches would compel a large increase in the size of the sewer-mains, and greatly increase the cost of treatment in case this method of disposal was employed. There will occasionally be instances, however, where a convenient stream of water can be utilized to advantage in this way.

It sometimes happens that an old sewer-main or other large drainage-channel is at so flat a grade as to be, in part at least, a sewer of deposit. Flushing can be used to advantage in such a case to stir up and remove the matter deposited. A notable instance of this may be found at Milwaukee, Wis., where 40,000 gallons of lake-water per minute are pumped into the Milwaukee River (the flow of which is largely sewage) for flushing it.

In general a sewer in which there is a continuous flow with a depth of at least the radius of the invert and a velocity exceeding 2 feet will need but infrequent cleaning if legitimate sewage only be admitted. If for any reason or at

any time these conditions be not fulfilled artificial cleaning will probably need to be resorted to.

ART. 26. METHODS OF FLUSHING.

As stated, there are two general methods of cleaning sewers: flushing, and by the use of some kind of scraper or similar tool. The latter usually calls for no special provisions in the construction and will be treated of in Part III. Flushing, however, is frequently accomplished by appliances built into the system, and the principles involved are other than those controlling hand labor; it is therefore necessary to consider it in designing. Flushing may be done by hand, by automatic appliances, or by use of rain-water.

By the first the sewer can be flushed from any manhole, as well as from flush-tanks; by the second from fixed points only, usually the heads of laterals; by the third the flushingwater enters from roofs through all or many house-connections, or in some instances the inlets are so constructed as to store the rain-water from the street-surfaces or from watercourses and flush with periodic discharges of the same.

The secret of successful flushing lies in compelling a large mass of water to move at considerable speed down the sewer. If the sewer be less than 24 inches or 30 inches in diameter water should as far as possible completely fill it, that deposits may be removed from its entire circumference and also that the effect of the flush may be felt far down the sewer. With the sewer flowing full bore at the upper end the depth of the water will decrease as the flushing-wave progresses down the sewer, until at some point below, at a distance varying with the size and grade of the sewer, with the head of water at the upper end and the volume of sewage flowing, the depth and velocity of the sewage will be but little affected by the flush.

The initial velocity will depend upon the head and upon

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the facility offered the water for entering the sewer. should be a free and open orifice at the entrance end, and if possible the angle between the inside of the sewer and that of the manhole or flush-tank should be rounded. Speed is of as much value in flushing as quantity, and with a given. amount of flushing-water the more quickly it can be made to pass through the sewer the better. In most cases little if any benefit would result should a faucet be left continuously running in each house in a city, but of the same amount of water used in a proper way would be of great benefit to the system.

Although for creating velocity the head in the flush-tank should generally be as great as possible, it must be limited by the amount of internal pressure which the sewer can stand without rupture. A few years ago a brick sewer in Washington, D. C., was, on account of insufficient size, put under such a head of water by the run-off from a cloudburst that its upper half was completely severed from the lower and the sewer destroyed, and a similar result might follow from too great pressure of flushing-water. With a pipe sewer this danger is not so great. A head of 6 or 8 feet at the manhole or flush-tank-which is more than can usually be obtainedshould not endanger a pipe sewer. Brick sewers as ordinarily constructed should not be filled to a point more than 5 feet above the invert or, for those more than 5 feet in diameter, higher than the crown. In no case should the water be backed up a sewer-line to such a height as would flood any connected cellars.

The flushing-water should move down and not up the sewer, since the effect of the latter would probably be to sweep the intermediate deposits nearly to the upper limit of the wave and leave them there to dam the flow. The interval which should elapse between flushings will vary under different conditions. In sewers where there is a constant

ample flow of water, where stoppages are few and due solely to accident or design of ignorant or malicious persons, flushing need be resorted to only when such stoppages occur. If it is found from experience that stoppages are frequent or that there is a constant depositing of material in the sewers, or if it is foreseen that this will occur from causes mentioned in the previous article, frequent flushings should be provided for. In the case of a dead end of a house or combined sewer, or one which has but few house-connections made with it, the flushing should be done once in each 24 or at least 48 hours.

Both separate and combined systems have been built and satisfactorily maintained without flushing at any point oftener than two or three times a year. It is probable that this is possible only where there is considerable ground-water entering the sewers at their upper ends, or where the dead ends occur only in thickly populated districts and on grades a little greater than the minimum herein advocated. There is too little definite information on this subject to justify a positive statement as to when, if ever, flushing at dead ends may be profitably omitted. It is advisable so to arrange every

house or combined sewer, where the conditions will be those given as favoring deposits, that it can be satisfactorily flushed.

A few experiments have been made on the actual effect of flushing-water in a sewer, chiefly with reference to the velocity and depth of the flushing-water at different distances from the point of entering. Andrew Rosewater found by experiment with a 400-gallon tank at the head of an 8-inch line of sewer discharging 11 gallons per second that at the first manhole, 200 feet below the flush-tank, the water was 6 inches deep and had a velocity of 5.6 feet per second; 200 feet further the depth was 5 inches, velocity 2.8 feet; and 400 feet further the depth was 4 inches, velocity 2 feet— showing the flushing effect to be practically exhausted in 800 feet. Mr. Ogden, in experiments made in Ithaca, N. Y.,

in 1897, found that with discharges from flush-tanks through 8-inch pipes of from .89 to 1.1 cubic feet per second the flow was reduced to 2 inches at 1123 feet from the flush-tank in two cases where the grades varied from .52% to 1.31%, at about 1000 feet in another where the grades varied from 1.02% to 3.14%, and in another where the grades varied from .80% to .89% the depth was 4 inches at 895 feet from the flush-tank. In the first two the sewer was scoured clean for 529 feet and some effect felt at 819 feet; in the third the sewer was cleaned for 556 feet and the effect slight at 970 feet; in the last the pipe was "disturbed, but not cleaned," at 636 feet, until 600 gallons were discharged, when it was cleaned for more than 636 feet, but less than 900 feet. The other discharges referred to were of 300 gallons each. An interesting series of experiments were conducted and their results plotted by S. H. Adams in England. These appeared to show, as do the above, that 300 gallons is in some cases. insufficient to properly flush an 8-inch pipe; also that the effect of such a quantity is felt for about 800 to 1000 feet.*

In flushing by hand the sewer is usually stopped at the down-grade side of a manhole or flush-tank, this is filled to a desired height with water or by allowing the sewage to accumulate in and above it, the gate, plug, or other stopper is removed and the water allowed to enter the sewer under the head due to its height. Where outside water is used for flushing and is limited in quantity another stopper should be placed at the upper orifice in manholes, to prevent a flow up the sewer, and left in until the flushing is over. The stoppers are made of various forms and to act in various ways, and to close the whole or only the lower half or two thirds of the sewer. The water is obtained from different sources and introduced by different methods, a further discussion of which will be given in Part III.

In England the separate system, when first constructed,

* See also Transactions Am. Soc. C. E., vol. xL, pp. 1-30.

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