Sewerage : the designing, construction, and maintenance of sewerage systems
o M M A ,^-N ^—s Sk « o 9 A O 9 A A 1 ^ ar 9* f » p P o 9 »ihB *\ O •-•» ^ >J M M o» ca o» ca M M M % b *♦ % 00 OO Ov Ov O^ Ov cn cn cn pr • 9P « 00 9P • 9P • « 00 OO  00 QO o 0 0 *N r» *^ •(^ •»^ oa oa c»> o» M M N D* '^ M M OO  o» M 4^ Ov QO ■ • • B V ■ • • • • 9 g 8 8 8 8 2 o O o O cn H P ^ en ^ cn *vi (>) C4 • O 6 o^ Ov 00 9P QO 00 o^ Ov Ok 1 ■8 o 8 8 t -8 cn cn cn < M A •»k. m Oi  Ok kO 00 kO ^ Oa Oa C^J Oa oj c^j oj Oj  Ln >o >o M >o o ^* ^* ^* r < • • O • 00 t • • cn Ok • Ok • Ov * On * O •^ •o p • • t • • OK) A H 1 1 8 cn 1 §. Co >s •H M M M a» Ok cn 4k. OJ v^ t t • • • •s ^^ 4k. M M C»J  *»» >o M M • • t * • •Q 5S - ^ Ok o 4k. O A < 4k 4k 4k. 4k. Size of Sewer. w 09 % 8 ■8^ 4k. 10 4k. 8 Velocity of Flow. Feet per Minute. P* 9 a OO cn C»> (M O o > < A • « _o_ ? tN oa a > P e» A • • > • • > A • < A • < A • A CO 1^ D o 0 w 0 • 0 0 • 5P > • > A • > < A o • • r • CA r ! 114 SEWERAGE. The quantity q{=alr) as well as Q should be calculated for each sub-area, and if the Q for any stretch of sewer is at any place less than the q immediately tributary to the same the latter should determine the size, Plate IV will be found convenient for determining a, and also R when the rates of rainfall of the place in question can be represented by any of the curves there given, these being for rains of the second class. To find a in acres from the diagram, use one dimension (in feet) of the area (or of an equivalent rectangle if it is not rectangular) as an ordinate and find the corresponding abscissa of the acre-curve in the diagram; divide this into the other dimensions of the area and the quotient will be a in acres. By the table the run-off from the undeveloped territory is placed at 60.0 cubic feet per second, which is carried by a 42-inch sewer on a .6% grade for 360 feet, where it receives still more sewage; the maximum amount to be received there, both over the surface and through the sewer, being 62.4 cubic feet, although q for the block No. i alone is 7.6 cubic feet. But Q is not equal to 60.0 -h 7.6, because the latter quantity was due to a rainfall of 3.9 minutes' diu-ation, or rather to the maximum rate for that time, during which only such water would have arrived from the upper end of the drainage- area as was due to a lower rate of rainfall; but the time of 14.3 minutes is that for which the run-off is calculated from both the undeveloped territory and block No. i. Blocks No. 2 and No. 3 both reach the sewer at the same point, and, taking the rate of rainfall for 15.1 minutes, we have a total run-off from all the territory above of 72.0 cubic feet pet second and, the grade being .5%, a 44-inch sewer is found to be necessary. Blocks No. 4 and No. 7 discharge first into a branch sewer, which it is found should be 24 inches in diameter. Where this joins the main the run-off from blocks No. 5 and THE DESIGN. II 5 No. 8 and from half of No. 6 and No. g also reaches it, and it must consequently be increased in size. The time T at this point is 13.4+0.9 (in the Ave. B sewer) +0.8 (in the Second Street sewer) +1.3 (in the Ave. C sewer), or 16.4 minutes, and the rate of rainfall for thie time is used for the run-off from the entire area. It will be seen that the method here employed is but a practical application of the principles stated in Art. 13. More, and more accurate, data for determining t and /, as weU as R, are needed before this or any method can be relied upon to give more than general approximations to the run-off^ Fortunately with the method here given the approximation becomes more close as the area becomes more urbani and is most so in the most densely populated districtSi where the danger from gorged sewers would be greatest. Aet. 32, Grade, Size, and Depth of Sewers. For both determining and recording the grades of the proposed sewers use is usually made of the profiles of the streets, plotted from the level notes. Upon these a vertical longitudinal section of the proposed sewer through its centre line is placed, thus showing the size, grade, and depth of the sewer. While designing, however, it will be found convenient to pencil in the line of the invert only, since then changes in its vertical location can more readily be made. A short experience in sewer-designing will demonstrate how mutually involved are Qj 5, the diameter and the depth of the sewer. In many cases it will be necessary to alter and realter the' grade and diameter before obtaining for each reach of sewer the best obtainable depth and velocity. 0 is a fixed quantity for any given case, 5 may vary between fixed limits, the size also has its limits in some cases, but the depth of the Il6 SEWERAGE. sewer may vary from any distance below to any distance above ground. A depth of 25 or 30 feet is obtained in many sewerage systems, and even 50 feet or more has been reached in open cut, while sewers have been laid in tunnel at still greater depths. Where possible deep sewers should run through wide streets, that the danger to building-foundations may be kept as small as possible; and they should avoid the busiest thoroughfares unless these are also the widest streets and the soil is treacherous. The sewer may in some cases be carried on bridges or trestles, as in crossing a stream or ground lower than the hydraulic gradient. In many such locations, however, this position will be impossible, owing to traffic on the river, to danger from floods, to blocking of streets, or to prohibitive cost of construction. In such cases the pipe may be placed under the surface of the ground or in the bed of the stream, being thus below the hydraulic gradient. Such a downward loop is called an inverted siphon. Many instances of these are in use, and if care be taken in their design and construction they need give no trouble. It will not be possi- ble, or at least advisable, to connect any buildings to an inverted siphon, since the sewage will continually stand in the connections up to the level of the hydraulic gradient of the siphon. The depth of storm-sewers is usually fixed by grade requirements only; the covering over them, however, should be not less than two feet and would better be three or four feet. The minimum depth to which house or combined se.wers should be laid will usually be decided by local circum* stances or customs. It is generally desirable to lay them somewhat deeper than the gas- or water-pipes, that these may not interfere with them. The city of Brooklyn some years ago fixed 12 feet as the depth to which all (combined) sewers are to be laid, unless the maintenance of proper velocity requires a less or greater one. In Philadelphia 14 feet is the THE DESIGN. 117 standard depth, in Washington, D. C, 10 fedt. In residence districts in the smaller cities 7 to 10 feet is usually sufficient, although in a street running along a hillside a much greater depth may be called for by the depth of basements upon the lower side of the street. In streets which are already built up the sewer should be deep enough to drain all basements and cellars, with the exception, perhaps, of an occasional one of unusual depth. To insure this the cellar depths taken during the survey should be indicated in their proper positions upon the profiles of their respective streets. In many Southern cities where there are no cellars under the dwellings and there is little danger of frost the sewers may be given a depth of covering of only 3 or 4 feet. In the North 6 feet is probably the least depth which should be given to the flow- line save under exceptional circumstances. The maximum depth should be kept at 14 to 16 feet if possible, since below this the cost rapidly increases. When the depth is consider- able the expense of making house-connections may become excessive. It may in such cases be found cheaper to lay a small sewer about 7 or 8 feet below the surface and following the surface grade, which may be with or against the grade of the deep sewer, to a manhole in the deep sewer into which this shallow onexan discharge. Before fixing the grade it is well to prepare tables similar to those given in Articles 35 and 36, and also to calculate as closely as possible the total amount of sewage reaching each outlet. Very often the main sewer for a long distance from the outlet must be laid at a minimum grade if pumping is to be avoided or the lift kept as small as possible. In such a case the grade of this main will be the first to be located upon the profile, the outlet being placed as low as is permissible. This should never be below ordinary high water unless absolutely necessary, and under no consideration should it be below or Ii8 SEWERAGE, even as low as ordinary low water; or rather this should be true for the hydraulic gradient, although the last few feet of the sewer may be given a steeper grade to bring the outlet below the water-surface or into the channel. There may be other lines also where the surface elevations demand the flattest possible grades; that is, the grades which will give the minimum permissible velocity. This grade will depend upon the size of the sewer, and this again upon the quantity of sewage. To ascertain this size, reduce the maximum sewage flow to cubic feet per second, divide by the desired velocity of flow in feet per second, multiply the quotient by 1.5 for mains or 2 for laterals, and find the diam- eter of a circle having this product as its area, which will be the sewer diameter required. Or, divide the gallons of sewage per day times 1.5 or 2, as the case may be, by the required velocity in feet per second, and take the size corre- sponding to the next highest quantity in the following table: Table No. 16. Siieofsewer . 8" 10" la" 15" Gallons of sewage per day 225,500 352,500 507,600 792,800 V Size of sewer 18" 20" 24" Gallons of sewage per day 1,141,700 1,410,300 2,030,50a V Where possible the grades of house-sewers should be such as to give a velocity of from 3 to 4 feet per second, and those of storm-sewers from 4 to 5 feet per second. The demands of economical construction and the necessity for sufficient fall in house-connections should not, however, be sacrificed to reduce velocities to less than 10 or 12 feet, which, however^ should be the maximum allowed. If it is possible the grade of the various sewers should be so proportioned that the velocity of the sewage shall increase as the outlet is approached, or at least it should not decrease^ THE DESIGN. 119 since a decrease in velocity may cause a deposit of suspended matter. Frequently, however, it is impossible to attain this in the design, since the flattest surface slopes are usually nearest to the outlet and the sewer grades are largely con- trolled by these. From the formulas Q'=. aV and F= ^ VRSy considering the sewer as flowing full, and giving c a constant value of 85, which will in no case vary more than 10$^ from Kutter's c for pipes ranging from 8 to 18 inches diameter, we have the formula V = -^ = .0796c'S'Q, or F being in feet and Q in cubic feet per second. This formula should not be used where any considerable accuracy is demanded, but will be found convenient for use in fixing the first approximate grades. If V is to be a constant 5 must vary inversely as VQ. Sy however, equals 7 if / equals the fall of the grade for a length /. If /, /', / , etc., be taken as the lengths between successive manholes, /, /', /'\ etc., as the corresponding falls, and Q, Q\ Q'\ etc., as the quantities of sewage flowing through these lengths, then (if V is constant) {VG =-^ ^' = -p ^'' etc. ; also/+/' +/'' + etc. = F, the total fall from the head to the outlet of the system. Knowing /% and / and Q for each length between manholes, we can obtain the values of/, /', y', etc. As just stated,. it is seldom that an entire system can be designed to give a constant velocity to the sewage, I20 SEWERAGE. but this is sometimes possible in separate drainage-areas, a constant velocity being obtained in each area. Still more important than obtaining a constant or con- stantly increasing velocity is the keeping of the velocity within the limits given in Art. 17. If the ground-surface is too flat to permit of obtaining this velocity by gravity, pumping must be resorted to (see Art. 37). If the surface is steeper than is permissible for the sewer the sewer grades can be broken and a drop made at each manhole (see Art. 36). A slight drop in the grade should be made at each manhole on flat grades to compensate for the obstruction ofiFered by curves, etc., at this point, and for slight errors in measurement; 0,02 or 0.03 foot is usually sufficient. This is generally unnecessary in the case of masonry sewers or others with continuous inverts with straight alignment. Of the above principles the most important is that the velocity of the sewage shall be within the proper limits; then that all basements and cellars to a reasonable depth shall be drained by house-sewers; also the depth of excavation should be kept as- light as possible, and the principles outlined in Art. 29 should be regarded. The obtaining of the nearest possible approach to an ideal design will usually require many changes in, and rearrangement of, both lines and grades, since a change in those of one lateral may in some way affect the entire system. The preliminary grades having been thus fixed according to the desirable depths and velocities of flow, the size of the sewer for each reach should be calculated or taken from the diagram and the velocities checked by accurate calculation. Additional changes, usually slight, will probably be required to obtain the best values for each interdependent velocity, size, and depth. The junctions and crossings of the sewer-lines must be carefully examined and adapted to each other. It is a good plan to make a list of all the manholes, showing for THE DESIGN, 12 1 each the elevation at which each sewer enters and leaves it. Two sewer-lines should never intersect each other, each having a continuous grade; either one should discharge from both directions into the other or they should cross, the one above the other. At junctions the surface of the sewage in the contributing sewer should never be designed to be lower than that in the other; that is, if they are both branch sewers the centre of the tributary should not be below the centre of the intercept- ing sewer; if the larger is a main the centre of the smaller should not be lower than a point two thirds the diameter of the larger above its invert. It would be still better to place the invert of the tributary above the sewage-surface in the interceptor, particularly when the former drains but a small district; but where the total fall possible is slight none of it need be utilized for this purpose. Difficulty will sometimes be found in so arranging the comparative depths of storm- and house-sewers that the house-connections can pass under or over the former. In some cases this may be impossible, and it may be necessary to place a house-sewer on each side of the storm-sewer. Reference to the data of locations and depths of gas- and water-pipes and other existing sub-surface systems should be constantly made and the sewers so designed as to interfere with them as little as possible. On the profile of each sewer-line the elevation of all trans- verse sewers should be indicated and a cross-section of the sewer shown. On the finished profile it is well to indicate the thickness and material of the sewer-walls and of all man- holes, lamp-holes, and other appurtenances. The materials may be indicated by colors, as red for brick, brown for sewer- pipe, etc. The grade, length, and size of the sewer between each two manholes should be given in figures, as well as the exact elevation of the invert at each change of grade. 122 SEWERAGE, Art. 33. Inverted Siphons. Since the ordinary sewer is designed to flow only i to f full, while an inverted siphon, being under a head, will flow full bore, the velocity in the latter will be only i to f that in the sewer laid to the hydraulic gradient, if they are of the same size. On account of the difliculty of access and repairs it is especially necessary that the velocity of flow in the siphon should be at least as great as that in the ordinary sewer, that deposits may be prevented. This can be attained only by reducing the size of the siphon-pipe. Moreover, this velocity should be had from the beginning of the use of the system; and therefore this size should be designed to give sufficient velocity to the sewage from the first. This first sewage flow may be doubled or trebled as time passes, and the increase may then be provided for either by giving sufficient fall to the siphon originally to produce the greater velocity necessary or by additional siphon- pipes. Usually at least two siphon- pipes are laid at the first, that while one is being emptied and cleaned the other may be used. The friction-head in the inverted siphon will be greater than if the sewer were laid to the hydraulic gradient, and consequently the gradient must be steeper. The difference in elevation of the two ends of the siphon should be equal to the fall required by a sewer of the same size flowing full and of the length of the entire siphon (which is not the horizontal distance between its ends) to pass the given amount of sewage. The velocity of flow in an inverted siphon is entirely inde- pendent of the fall therein, but depends upon the quantity of sewage, since all of this must, but no more can, pass through it. If the fall in the inverted siphori is not sufficient the sewage will back up the sewer until sufficient head is obtained to produce the required velocity. Hence to prevent THE DESIGN, 123 this the fall in the siphon itself should be made great enough to create the velocity which will be required by the largest quantity to be passed at any time. An inverted siphon may at times be necessary for passing under some obstruction in the street — as a large conduit of one kind or .another, but this should be avoided where possible. For details of inverted-siphon construction see Articles 44 and 72. Art. 34. Sub-drains. Very frequently storm-sewers are placed at such a short distance from the surface that they cannot be utilized fot draining damp cellars, particularly since a cellar should b< connected with no sewer whose crown is above its level, from danger of back-water when the storm-sewer flows full. Ordinarily the house-sewer is below the cellar-level; but this should not be utilized as a drain, both because the amount of sewage may thus be too largely increased ; and still more on account of the danger from sewer-air, which would have free access through the drain should the trap-seal evaporate during a drought, which it is very apt to do, and from the cellar this air might permeate the entire house. From a sanitary point of view the drainage of wet soils is almost, if not quite, as important as the sewerage and should not be neglected. The mere opening of sewer-trenches tends to drain the soil, even after they arc refilled. But in many cases it is extremely desirable to provide other and more positive drainage. It is almost impossible to make a perfectly tight sewer without great expense, and when laid in wet ground sewer- joints may admit in the aggregate large quantities of water. This could be prevented and the land adjacent drained, to its 124 SEWEHAGE. great improvement and the health of residents thereon, if this ground-water could be lowered along the trench by some means. During construction in wet ground much trouble will be experienced, even when the pumping facilities are ample, by water rising and flowing over newly laid inverts, to their per- manent injury (see Arts. 70 and 71). These difiiculties can each and all be met in most cases by the use of sub-drains — that is, drains laid a little below the sewers. These are ordinarily laid in a narrow trench in the bottom of, and at one side or in the centre of, the sewer- trench. Their use for construction drainage will be consid- ered in Part II. When properly designed for this purpose their size will in most cases be sufficient for the continuous drainage of the land and also for cellar-drainage. The in- stances will be very few, however, in which any approach to an accurate estimate can be made of the amount of sub- drainage which will be required in a system. But provision should always be made for sub-drainage wherever the soil is wet, for permanent drainage if for no other purpose. The water flowing into such drains must have some outlet, and the most natural course would be, when the sewage is disposed of by dilution, to place the outlets of sewers and sub-drains at the same point. It may happen, however, that the necessity for sub-drains is not foreseen when the sewer- outlet is being built; or the place where they will be neces- sary may be so far from this outlet that a great length of otherwise useless drain-pipe must be laid to reach it; also the amount of ground-water may be so much greater than was anticipated, in spite of all investigations, that the drain-pipe near the outlet will not carry it all. In any of these cases another outlet may be desirable or necessary. This can fre- quently be found by leading the sub-drain in a special trench to a near storm-sewer or natural watercourse. In some THE DESIGN. 125 casesy however, special means must be resorted to, such as one of the methods of pumping (see Art. 37). If the sub-drain is necessary for construction purposes only it may be led to a sump-well where a pump is stationed, and broken and sealed at several points after construction is com- pleted. (This last will be necessary, as otherwise the drain would continue to lead the ground-water to this point, which might become permanently and dangerously water-soaked.) Although the sub-drain is in most cases smaller than the sewer, it must be laid at practically the same grade. The objection to flat grades in house-sewers does not apply to these so urgently, however, since the watier flowing through them, after construction is completed at least, is usually free from suspended matter likely to cause deposits. The size and position, then, are the only elements of the general design to be decided upon. The size it will not be advisable to make less than 6 inches at the outlet or for long stretches, but for stretches of a few hundred feet only and through ground but moderately wet 4- or even 2-inch pipe may be used. Pipe larger than 10 or 12 inches is seldom used in any but excep- tional cases. If a larger would be required (and instances can be named where the sub-drainage from a small town would more than fill a 36-inch pipe) special methods may be em- ployed ; such as dividing the sub-drainage system into small sub-systems, each having its own outlet, which may, when constructed under a storm-sewer, discharge into the sewer immediately above it or which may be at a near water- course. Art. 35. House- and Inlet-connections. The connections between the sewers and opposite houses and storm-water inlets are of an importance second only to the sewer-mains. Any defect in one of the connections, while 126 SE WEE AGE. limited in the range of its effect, is fully as detrimental within that range to the proper working of the system as a defect in the main itself. Since the house-connections are subject to extreme fluctuations of discharge and hence to stoppages, as also to the formation of grea^ deposits, it is desirable that they be equally as accessible as sewer-mains for both inspec- tion and cleaning, and also that their grade and alignment be given equal care in both the design and the construction. They should, if possible, be given a uniform grade of not less than 2j$^. Where the house sits back from the street an observation-hole (see Art. 42) should be placed at the fence- line, and one should be placed wherever there is a change in the line or grade. There should also be a hand-hole in the pipe just after it enters the cellar. The junction with the sewer should be made by means of branches, either Y or T. It should never be made in pipe sewers by breaking a hole into the shell and inserting a pipe. If the sewer be larger than 20- to 24-inch a T is advisable, both because this offers easier inspection of the house-connection from its lower end, which inspection can be made by a person entering the sewer, and because the branch can be placed entirely above the ordinary level of the sewage, which position it should occupy when possible so as to cause no interference with the sewage flow. When the sewer is too small to admit a man, which size will also not admit of raising the branch entirely above the ordinary sewage flow without giving it too steep a pitch, a Y branch is preferable, because this will retard the flow less than a T, and because the house-sewage will enter the sewer at a less angle with its flow. The vertical angle which the branch makes with the horizontal should not ordinarily ex- ceed 45° in small sewers, because of the interference with the flow and of the splashing caused by a vertical drop of sewage into their relatively small stream, and because of the danger THE DESIGN. 127 that the weight of the house-connection may break in the crown of the sewer. It is well to so place the branch in masonry sewers that a trickling discharge from it will flow over the surface for the least possible distance, that deposits from such discharge may be avoided. In the case of combined sewers this would call for placing the branch but a short distance above the invert, but it should be given such a grade as to bring it higher than the crown of the sewer when it reaches the cellar. Some engineers always use T brandies, more always use Y branches, for house-connections; but the practice here recommended seems to best utilize the advantages and avoid the disadvantages of each. The connections with inlets should never enter the sewer at an .angle with its axis greater than 45^, on account of the great disturbance to the flow which would be occasioned. Where possible, and particularly in small sewer-mains, a manhole should be placed where each connection enters the *ewer and the connection continued by a curved invert in the bottom of said manhole (see Plate VIII, Fig. 5). It is difiicult to calculate the proper size for a storm-water connection, but, since there is little disadvantage in having it larger than is actually required, while the effect of too small a pipe may be disastrous, it is advisable to make the size fully ample to discharge all the run-off from the heaviest storms. A 12-inch pipe is probably the smallest which should ever be used ; while a 24-inch may be required if the sewer lies near the surface (thus giving little fall to the connection) and if the tributary area is large. Where considerable undeveloped territory drains into the head of a sewer-main, or a small stream is there received, it may be necessary to continue the sewer to the inlet, not only not diminished in size but even enlarged into a bell mouth. It would be advi^ble to use an 128 SE^'ERAGK increaser at the upper end of every inlet-connection, since, owing to the churning of the water in the inlet, a "standard orifice" will not pass more than two-thirds the water which can be carried by a pipe of the same size. Art. 36. Manholes, Inlets, Flush-tanks, etc. The necessity for frequent connections between the air of the sewer and the outer air has been shown (Articles 23 and 24). As one means for this, and one which can always be adopted, manholes should be adapted to serve this end by having perforated covers. For this purpose, also, the more numerous they are the better. The other and greater neces- sity Tor their use, that of providing access to the sewers, should, however, have greater weight in fixing the distances which should separate them. It has been found in practice that a 6- or 8-inch sewer can be easily inspected and cleaned if this distance be not greater than 300 feet; a 12- or 18-inch sewer, when not more than 400 feet separates successive man- holes. A sewer which can be entered may, for this purpose, have its manholes even 600 or 1000 feet apart; but the cost and difficulty of cleaning are thereby increased, owing to the distance the material removed in cleaning must be carried through the sewer. Ventilation also is not so well served by so great intervals. It is better to fix 500 or 600 feet as the maximum distance between manholes on lines of the largest sewers. Economy would suggest placing a manhole at each sewer intersection, where it would serve both lines. This is also desirable as permitting a curved junction between the sewer- channels. Where a curved bend is made in the entire sewer a manhole should be placed at each end of the curve unless the sewer is sufficiently large to be entered. A manhole should be placed, in general, at each change THE DESIGN. 129 of line or grade, in order that every part of the sewer may be easily inspected. Economy will set a limit to the number of manholes which may be introduced; the number of the breaks in the street- paving caused by their covers it is also desirable to keep at a minimum. Principally for the first reason a manhole is some- times omitted in small sewers when it would come less than 200 feet distant each way from another manhole, and a lamp- hole substituted. While the sewer cannot be inspected from this, a light can here be lowered into it to light up the sewer for inspection from the next manhole either way. Lamp- holes have objectionable features, however, and are seldom used. The use of flush-tanks has ahready been discussed (Articles 20-22). The grades of the laterals and the conditions of their use should be carefully examined to determine where fre- quent flushing will probably be needed. In some cases, such as where a flat grade on a long line of small sewer is unavoid- able, it may be desirable to place automatic flush-tanks at intervals of 800 to icxx) feet along its length, the tanks being placed at one side of the sewer and discharging into it through a short connecting-pipe. If automatic appliances are not employed no special tanks need be built in such a case, but manholes at intervals along the line can be used for flushing. All the local conditions should be examined that advan- tage may be taken of any opportunities for flushing offered by springs, streams, or any available sources of water, and in general decision made as to the places and methods of flush- ing. As a general rule every dead end of a house- or com- bined sewer should be flushed frequently and some arrange- ment for this placed at each such point. Inlets should be provided at frequent intervals throughout the area drained to receive the surface-water. In districts where the street traffic is considerable and where any great 130 SEWEPAGE, depth of water in the gutters would inconvenience a large proportion of the population the inlets should be not more than 200 or 300 feet apart, while in residence districts they may be so situated as to require the run-off to flow for 600 or 700 feet over the surface. They should generally be so placed that all the run-off can reach them by flowing along the gutters only, and need not flow across the streets. The plan Plate V shows how this can be accomplished in most cases. Where this is impossible a culvert should be placed under the street-pavement in line with the gutter. Where street grades are continuous from one intersecting street to another inlets should be placed on street-corners. They are frequently placed at the gutter intersection ; but a better plan in many cases, particularly on steep grades, is to place two openings, one just above each cross-walk, as this avoids the vehicle-trap caused by the ordinary corner inlet. Also an inlet should be placed at every point where two falling grades meet, and if this be between street intersections an inlet must be placed there on each side of the street. In the majority of cities a large proportion of the inlets are provided with catch-basins — more than the best practice would warrant, in the author's opinion. The object of using a catch-basin is to retain there the silt and other heavy matter and not permit it to be carried into and deposited in the sewer. Catch-basins should be cleaned after every storm. The objection to catch-basins is that several days some- times must elapse — and several weeks usually do — between the beginning of a storm and the cleaning of the catch-basin; and during this time the organic matter which has been washed or thrown into the inlet, including horse-droppings, fruit and vegetable refuse, etc., is putrefying and frequently emitting objectionable odors. ** Such foulness is less offensive in the drains [storm-sewers] than in the catch-basins, which are situated at the sidewalks and where it is much more THE DESIGN. 131 likely to be observed. Also it is found impracticable to 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 objection- able to allow all the street-dirt to enter the sewers and there- fore the catch-basins at the inlets are omitted." (Report of Rudolph Hering and Samuel M. Gray on Sewerage of Balti- more.) 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 suffi- cient 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. 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 sewer. 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, con- structed, 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. 132 SEWERAGE. 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. 37. 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 inter- mediate 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 solu- tion 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 prac- ticable 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 con- tents 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 THE DESIGN. 133 reservoirs are seldom satisfactory, owing largely to the diffi- culty of cleansing them from the deposits made while they are filled with stagnant sewage. It would be better, though of course more expensive, to pimip 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, when rebuilding after the fire, raised the streets over its entire area to pernyt 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 piunping is necessary during this period; but the first cost of the plant should be kept at a minimum, since the interest on the saving in cost will far exceed any saving that could be made in nmning-expenses for a few days. 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 trac- tion company, etc. — ^by which means both first cojt 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 134 SEWERAGE. contained, it is probable that the separate system should be employed, all house-sewage being pumped and each storm- sewer 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, begin* ning at a depth of 8 feet and falling i 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 require- ments.) 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 construc- tion 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 some pattern of automatic lift or other appliance which seems adapted to the circumstances. THE DESIGN. 13S 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 lift-stations with a small horse-power required at each, or even when the horse-power is considerable. Improvements in electricity and low rates for cur- rent, combined with the conveniences of automatic action, have led to the adoption of electric plants for most recent installations of smaQ and some of large capacity. The pumps usually employed are the piston- or -plunger-pump and the centrifugal pump. Other devices have been employed, such as screw and oscillating pumps, but few with any success. 