Preliminary Work
14. Preparing for Design.—Methods for the design of sewerage systems are given in Chapter V. Before the design is made certain information is essential. A survey must be made from which the preliminary map can be prepared as described in Art. 42. Other necessary information which is the basis of subsequent estimates of the quantity of sewage to be cared for must be obtained by a study of rates of water consumption and the density and growth of population, the measurement of the discharge from existing sewers, and the compilation of rainfall and run-off data. If no rainfall data are available estimates must be made from the nearest available data. Observations of rainfall or run-off for periods of less than 10 to 20 years are likely to be misleading. Methods for gathering and using this information are explained in subsequent chapters.
Underground surveys are desirable along the lines of the proposed sewers to learn of obstructions, difficult excavation and other conditions which may be met. All such data are seldom gathered except for sewerage systems involving the expenditure of a large amount of money. For construction in small towns or small extensions to an existing system the funds are usually insufficient for extensive preliminary investigation. The saving in this respect is paid unknowingly to the contractor as compensation for the risk in bidding without complete information.
15. Underground Surveys.—These may be more or less extensive dependent on the character of the district in which construction is to take place. In built-up districts the survey should be more thorough than in sparsely settled districts where only the character of the excavated material is of interest and no obstructions are to be met.
Underground surveys furnish to the engineer and to prospective bidders on contract work information on which the design and estimate of cost and the contractor’s bid may be based and without which no intelligent work can be done. By removing much of the uncertainty of the conditions to be met in the construction of the sewer, the design can be made more economical and the contractor’s bid should be markedly lower, sufficiently so to repay more than the expense of the survey. The information to be obtained consists of the location of the ground-water level, and the location and sizes of water, gas, and sewer pipes, telephone and electric conduits, street-car tracks, steam pipes, and all other structures which may in any way interfere with subsurface construction. These structures should be located by reference to some permanent point on the surface. The elevation of the top of the pipes, except sewers, rather than the depth of cover should be recorded, as the depth of cover is subject to change. The elevation of sewers should be given to the invert rather than to the top of the pipe.
A portion of the map of the subsurface conditions at Washington, D. C., is shown in Fig. 3. Many of the dimensions and notations are not shown to avoid confusion on this small reproduction.[[15]] Colors are generally used instead of different forms of cross hatching to show the different classes of pipe and structures. In addition to a record of the underground structures the character of the ground and the pavement should be recorded. A comprehensive underground survey is seldom available nor does time usually permit its being made preliminary to the design of a sewerage system. The character of the material through which the sewer is to pass should be determined in all cases.
Fig. 3.—Record Map of Underground Structures, Washington, D. C.
Eng. Record, Vol. 74, p. 263.
The various subsurface lines are differentiated by colors as follows: A—Sewers, vermilion. B—Water mains, blue. C—Potomac Electric Power Co., carmine. D—Washington Railway and Electric Co., carmine. E—Capital Traction Co., violet. F—Chesapeake and Potomac Telephone Co., green. G—Washington Gas Light Co., green. H—Western Union Telegraph Co., orange. I—Postal Telegraph Co., orange. K—Private vaults, black. L—City Electric Co., yellow.
Fig. 4.
Punch Drill.
Underground pipes and structures are located by excavations, which may be quite extensive in some cases. Their position is fixed by measurements referred to manholes and other underground structures which are somewhat permanent in position. A city engineer should grasp every opportunity to record underground structures when excavations are made in the streets. The character of the material through which the sewer is to pass is determined by borings.
16. Borings.—Methods used for the investigation of subsurface conditions preliminary to sewer construction are: punch drilling, boring with earth auger, jet boring, wash boring, percussion drilling, abrasive drilling, and hydraulic drilling. The last three methods named are used only for unusually deep borings or in rock.
Punch drills are of two sorts. The simplest punch drill consists of an iron rod ⅞ of an inch to 1 inch in diameter, in sections about 4 feet long. One section is sharpened at one end and threaded at the other so that the next section can be screwed into it without increasing the diameter of the rod, as shown in Fig. 4. The drill is driven by a sledge striking upon a piece of wood held at the top of the drill to prevent injury to the threads. The drill should be turned as it is driven to prevent sticking. It is pulled out by a hook and lever as shown in Fig. 5. It is useful in soft ground for soundings up to 8 to 12 feet in depth. Another form of punch drill described by A. C. Veatch[[16]] consists of a cylinder of steel or iron, one to two feet long split along one side and slightly spread. The lower portion is very slightly expanded and tempered into a cutting edge. In use it is attached to a rope or wooden poles and lifted and dropped in the hole by means of a rope given a few turns about a windlass or drum. By this process the material is forced up into the bit, slightly springs it, and so is held. When the bit is filled it is raised to the surface and emptied. Much deeper holes can be made with this than with the sharpened solid rod.
Fig. 5.—Lever for Pulling Punch Drill.
Fig. 6.—Earth Augers.
Types of earth augers about 1½-inches in diameter are shown in Fig. 6. They are screwed on to the end of a section of the pipe or rod and as the hole is deepened successive lengths of pipe or rod are added. The device is operated by two men. It is pulled by straight lifting or with the assistance of a link and lever similar to that shown in Fig. 5. The device is suitable for soft earth or sand free from stones, and can be used for holes 15 to 25 feet in depth. For deeper holes a block and tackle should be used for lifting the auger from the hole. It is not suitable for holes deeper than about 35 feet.
In the jetting method water is led into the hole through a ¾-inch or 1–inch pipe, and forced downward through the drill bit or nozzle against the bottom of the hole. The complete equipment is shown in Fig. 7.[[17]] It is not always necessary to case the hole as shown in the figure as the muddy water and the vibration of the pipe puddle the sides so that they will stand alone. The jet pipe may be churned in the hole by a rope passing over a block and a revolving drum. In suitable soft materials such as clay, sand, or gravel, holes can be bored to a depth of 100 feet and samples collected of the material removed. An objection to the method is the difficulty of obtaining sufficient water.
Fig. 7.—Jetting Outfit.
U. S. Geological Survey, Water Supply Paper, No. 257
1. Simple Jetting Outfit. 2. Jetting Process. 3. Common Jetting Drill. 4a and 4b. Expansion Bit or Paddy. 5. Drive Shoe.
Methods of drilling in rock up to depths of 20 feet are described in Chapter XI under Rock Drilling. For deeper holes percussion, abrasive, or hydraulic methods as used for deep well drilling must be employed.
CHAPTER III
QUANTITY OF SEWAGE
17. Dry weather Flow.—Estimates of the quantity of sewage flow to be expected are ordinarily based on the population, the character of the district, the rate of water consumption, and the probable ground-water flow. Future conditions are estimated and provided for, as the sewers should have sufficient capacity to care for the sewage delivered to them during their period of usefulness.
18. Methods for Predicting Population.—Methods for the prediction of future population are given in the following paragraphs.
The method of graphical extension. This is the quickest and most simple of all. In this method a curve is plotted on rectangular coordinates to any convenient scale, with population as ordinates and years as abscissas. The curve is extended into the future by judgment of its general tendency. An example is given of the determination of the population of Urbana, Illinois, in 1950. Table 4 contains the population statistics which have been plotted on line A in Fig. 8 and extended to 1950. The probable population in 1950 is shown by this line to be about 21,000.
The method of geometrical progression. In this method the rate of increase during the past few years or decades is assumed to be constant and this rate is applied to the present population to forecast the population in the future. For example the rate of increase of population in Urbana for the past 7 decades has varied widely, but indications are that for the next few decades it will be about 20 per cent. Applying this rate from 1920 to 1950 the population in 1950 is shown to be about 17,800. It is evident that this method may lead to serious error as insufficient information is given in the table to make possible the selection of the proper rate of increase.
| TABLE 4 | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Population Studies | ||||||||||
| Year | Urbana, Illinois | Population of | ||||||||
| Population | Absolute Increase for Each Decade | Per Cent Increase for Each Decade | Decatur | Danville | Champaign | Kankakee | Peoria | Bloomington | Ann, Arbor Michigan | |
| 1850 | 210 | 736 | 5,095 | 1,594 | ||||||
| 1860 | 2,038 | 1828 | 85.6 | 3,839 | 1,632 | 1,727 | 2,984 | 14,045 | 7,075 | 5,097 |
| 1870 | 2,277 | 239 | 10.5 | 7,161 | 4,751 | 4,625 | 5,189 | 22,849 | 14,590 | 7,368 |
| 1880 | 2,942 | 665 | 22.6 | 9,547 | 7,733 | 5,103 | 5,651 | 29,259 | 17,180 | 8,061 |
| 1890 | 3,511 | 569 | 16.2 | 16,841 | 11,491 | 5,839 | 9,025 | 41,024 | 20,484 | 9,431 |
| 1900 | 5,728 | 2217 | 38.7 | 20,754 | 16,354 | 9,098 | 13,595 | 56,100 | 23,286 | 14,509 |
| 1910 | 8,245 | 2517 | 30.5 | 31,140 | 27,871 | 12,421 | 13,986 | 66,950 | 25,786 | 14,817 |
| 1920 | 10,230 | 1985 | 19.4 | 43,818 | 33,750 | 15,873 | 16,721 | 76,121 | 28,638 | 19,516 |
Fig. 8.—Diagram Showing Methods for Estimating Future Population.
The method of utilizing a decreasing rate of increase. This method attempts to correct the error in the assumption of a constant rate of increase. After a certain period of growth, as the age of a city increases its rate of increase diminishes. In applying this knowledge to a prediction of the future population of a city the population curve is plotted, as in the graphical method and a straight line representing a constant rate or increase is drawn tangent to the curve at its end. The curve is then extended at a flatter rate in accordance with the rate of change of a similar nearby larger city. This method has not been applied to any of the cities included in Table 4, as none has reached that limiting period where the rate of increase has begun to diminish.
The method of utilizing an arithmetical rate of increase. This method allows for the error of the geometrical progression which tends to give too large results for old and slow-growing cities. This method generally gives results that are too low. The absolute increase in the population during the past decade or other period is assumed to continue throughout the period of prediction. Applying this method to the same case, the increase in the population during the past decade was 2,000. Adding three times this amount to the population in 1920, the population of Urbana in 1950 will be about 16,000.
The method involving the graphical comparison with other cities with similar characteristics. In this method population curves of a number of cities larger than Urbana but having similar characteristics, are plotted with years as abscissas and population as ordinates, with the present population of Urbana as the origin of coordinates. The population curve for Urbana is first plotted. It will lie entirely in the third quadrant as shown by the heavy full line in Fig. 8. The population curves of some larger cities are then plotted in such a manner that each curve passes through the origin at the time their population was the same as that of the present population of Urbana. These curves lie in the first and third quadrants. The population curve of the city in question is then extended to conform with the curves of older cities in the most probable manner as dictated by judgment. Such a series of plots has been made in Fig. 8. The results indicate that the population of Urbana in 1950 will be about 25,500.
The last method described will give the most probable result as it is the most rational. For quick approximations the geometrical progression is used. The arithmetical progression is useful only as an approximate estimate for old cities.
19. Extent of Prediction.—The period for which a sewerage system should be designed is such that each generation bears its share of the cost of the system. It is unfair to the present generation to build and pay for an extensive system that will not be utilized for 25 years. It is likewise unfair to the next generation to construct a system sufficient to comply with present needs only, and to postpone the payment for it by a long term bond issue. An ideal solution would be to plan a system which would satisfy present and future needs and to construct only those portions which would be useful during the period of the bond issue. Unfortunately this solution is not practical, because, 1st, it is less expensive to construct portions of the system such as the outfall, the treatment plant, etc., to care for conditions in advance of present needs, and 2nd, the life of practically all portions of a sewerage system is greater than the legal or customary time limit on bond issues.
A compromise between the practical and the ideal is reached by the design of a complete system to fulfill all probable demands, and the construction of such portions as are needed now in accordance with this plan. The payment should be made by bond issues with as long life as is financially or legally practical, but which should not exceed the life of the improvement.
The prediction of the population should therefore be made such that a comprehensive system can be designed with intelligence. Practice has seldom called for predictions more than 50 years in the future.
20. Sources of Information on Population.—The United States decennial census furnishes the most complete information on population. Unfortunately it becomes somewhat old towards the end of a decade. More recent information can be obtained from local sources. Practically every community takes an annual school census the accuracy of which is fairly reliable. The general tendencies of the population to change can be learned by a study of the post office records showing the amount of mail matter handled at various periods. Local chambers of commerce and newspapers attempt to keep records of population, but they are often inaccurate. Another source of information is the gross receipts of public service companies, such as street railways, water, gas, electricity, telephone, etc. The population can be assumed to have increased almost directly as their receipts, with proper allowance for change in rates, character of management, and other factors.
21. Density of Population.—So far the study of population has been confined to the entire city. It is frequently necessary to predict the population of a district or small section of a city. A direct census may be taken, or more frequently its population is determined by estimating its density based on a comparison with similar districts of known density, and multiplying this density by the area of the district. In determining the density, statistics of the population of the entire city will be helpful but are insufficient for such a problem. A special census of the area involved would be conclusive but is generally considered too expensive. A count of the number of buildings in the district can be made quickly, and the density determined by approximating the number of persons per building. Statistics of the population of various districts together with a description of the character of the district are given in Table 5.
Fig. 9.—Density, Area, and Population, Cincinnati, Ohio. 1850 to 1950.
| TABLE 5 | |||
|---|---|---|---|
| Densities of Population | |||
| City | Character of District | Area, Acres | Density per Acre |
| Philadelphia | Thomas Run. Residential. Mostly pairs of two and three-story houses. 1204 acres settled. | 1,840 | 59 |
| Pine Street. Residential. Mostly solid four to six-story houses. 156 acres settled. | 160 | 97 | |
| Shunk Street. Residential. Mostly pairs of two and three-story houses. 539 acres settled. | 539 | 119 | |
| Lombard Street. Tenements and hotels, 145 acres settled. | 147 | 113 | |
| York Street. Residential and manufacturing. 354 acres settled. | 358 | 94 | |
| New York City | Residential. Three-story dwellings with 18–foot frontage, and four-story flats with 20–foot frontage. | 100 | |
| Residential. Five-story flats. | 520–670 | ||
| Residential. Six-story flats. | 800–1000 | ||
| Residential. Six-story apartments. High class. | 300 | ||
| Chicago | 1st Ward. Retail and commercial. The “Loop”. | 1,440 | 20.5 |
| 2d Ward. Commercial and low-class residential solidly built up. | 800 | 53.5 | |
| 3d Ward. Low-class residential. | 960 | 48.1 | |
| 5th Ward. Industrial. Some low-class residences. Not solidly built up. | 2,240 | 25.51 | |
| 6th Ward. Residential. Four and five-story apartments. A few detached residences. | 1,600 | 47.0 | |
| 7th Ward. Same as Ward 6. Not solidly built up. Contains a large park. | 4,160 | 21.7 | |
| 8th Ward. Industrial. Sparsely settled. | 13,624 | 4.8 | |
| 9th Ward. Industrial and low-class residential. Solidly built up. | 640 | 70.0 | |
| 10th Ward. Same as Ward 9. | 640 | 80.8 | |
| 13th Ward. Low-class residential. Solidly built with three and four-story flats. | 6,100 | 36.7 | |
| 16th Ward. Middle-class residential. Some industries. Well built up. | 800 | 81.5 | |
| 19th Ward. Industrial and commercial. Some low-class residences. | 640 | 90.7 | |
| 20th Ward. Low-class residential. Some industries. Entirely built up. | 800 | 77.1 | |
| 21st Ward. Industrial. Entirely built up. | 960 | 49.9 | |
| 23d Ward. Industrial and residential. | 800 | 55.4 | |
| 24th Ward. Residential apartment houses and middle-class residences. | 1,120 | 46.8 | |
| 25th Ward. Residential. High-class apartments. Wealthy homes. Contains a large park. | 4,160 | 24.0 | |
| 26th Ward. Residential. Middle-class homes and apartments. Fairly well built up. | 4,640 | 16.1 | |
| 27th Ward. Residential. Sparsely settled. | 20,480 | 5.5 | |
| 29th Ward. Low-class residential. Two-story frame houses. “Back of the Yards”. | 6,400 | 12.8 | |
| 30th Ward. The Stock Yards. | 1,280 | 40.1 | |
| 32d Ward. Scattered residences. | 8,480 | 8.3 | |
| 33d Ward. Scattered residences. | 12,944 | 5.5 | |
| 35th Ward. Scattered residences. | 4,960 | 12.0 | |
| General average | The most crowded conditions with five-story and higher, contiguous buildings in poor class districts. | 750–1000 | |
| Five and six-story contiguous flat buildings. | 500–750 | ||
| Six-story high-class apartments. | 300–500 | ||
| Three and four-story dwellings, business blocks and industrial establishments. Closely built up. | 100–300 | ||
| Separate residences, 50 to 75–foot fronts, commercial districts, moderately well built up. | 50–100 | ||
| Sparsely settled districts and scattered frame dwellings for individual families. | 0–50 | ||
The density of population in Cincinnati from 1850 to 1913 with predictions to 1950 is given in Fig. 9.[[18]] This shows the densities for the entire city and is illustrative of the manner in which future conditions were predicted for the design of an intercepting sewer. The data given in Table 5 are of value in estimating the densities of population in various districts. The Committee on City Plan of the Board of Estimate and Apportionment of New York City obtained some valuable information on this point, especially in Manhattan. Three-story dwellings with 18–foot frontage, or four-story flats with 20–foot frontage, presumably contiguous, were found to hold 100 persons to the acre. Five-story flats held 520 to 670 persons per acre. Six-story flats held 800 to 1,000 persons per acre, and high-class six-story apartments held less than 300 per acre.
22. Changes in Area.—In order to determine the probable extent of a proposed sewerage system it is important to estimate the changes in the area of a city as well as the changes in the population. With the same population and an increased area the quantity of sewage will be increased because of the larger amount of ground water which will enter the sewers. Predictions of the area of a city are less accurate than predictions of population because the factors affecting changes cannot be so easily predicted. An area curve plotted against time would be helpful in guiding the judgment, but its extension into the future based on past occurrences would be futile. A knowledge of the city, its political tendencies, possibilities of extension, and other factors must be weighed and judged. The engineer, if he is ignorant of the city for which he is making provision, is dependent upon the testimony of real estate men, business men and others acquainted with the local situation.
23. Relation between Population and Sewage Flow.—The amount of sewage discharged into a sewerage system is generally equal to the amount of water supplied to a community, exclusive of ground water. The entire public water supply does not reach the sewers, but the losses due to leakage, lawn sprinkling, manufacturing processes, etc., are made up by additions from private water supplies, surface drainage, etc. The estimated quantity of water used but which did not reach the sewers in Cincinnati is shown in Table 6. The amount shown represents 38 per cent of the total consumption. Unless direct observations have been made on existing sewers or other factors are known which will affect the relation between water supply and sewage, the average sewage flow exclusive of ground water, should be taken as the average rate of water consumption. Experience has shown that water consumption increases after the installation of sewers.
| TABLE 6 | |
|---|---|
| Estimated Quantity of Water Used but not Discharged into the Sewers in Cincinnati | |
| Expressed in gallons per capita per day, and based on a total consumption of 125 to 150 gallons per capita per day. | |
| Steam railroads. | 6 to 7 |
| Street sprinklers. | 6 to 7 |
| Consumers not sewered. | 9 to 10½ |
| Manufacturing and mechanical. | 6 to 7 |
| Lawn sprinklers. | 3 to 3½ |
| Leakage. | 18 to 21 |
The public water supply is generally installed before the sewerage system. By collecting statistics on the rate of supply of water a fair prediction can be made of the quantity of sewage which must be cared for. The rate of water supply varies widely in different cities. It is controlled by many factors such as meters, cost and availability of water, quality of water, climate, population, etc. In American cities a rough average of consumption is 100 gallons per capita per day. Other factors being equal the rate of consumption after meters have been installed will be about one-half the rate before the meters were installed. Low cost, good quantity and good quality will increase the rate of consumption, and the rate will increase slowly with increasing population. Statistics of rates of water consumption are given in Table 7.
24. Character of District.—The various sections of a city are classified as commercial, industrial, or residential. The residential districts can be subdivided into sparsely populated, moderately populated, crowded, wealthy, poor, etc. Commercial districts may be either retail stores, office buildings, or wholesale houses. Industrial districts may be either large factories, foundries, etc., or they may be made up of small industries housed in loft buildings.
In cities of less than 30,000 population the refinement of such subdivisions is generally unnecessary in the study of sewage flow, all districts being considered the same. The data given in Tables 8 and 9 indicate the difference to be found in different districts of large cities. The Milwaukee data are presented in a form available for estimates on different bases. These data are shown in Table 10.
| TABLE 7 | |||
|---|---|---|---|
| Rates of Water Consumption | |||
| From Journals of American and New England Water Works Associations | |||
| City | Population in Thousands | Per Cent Metered | Consumption, Gal. per Capita per Day |
| Tacoma, Wash. | 100 | 11.6 | 460 |
| Buffalo, N. Y. | 450 | 4.9 | 310 |
| Cheyenne, Wyo. | 13 | 270 | |
| Erie, Pa. | 72 | 3.0 | 198 |
| Philadelphia, Pa. | 1611 | 4.6 | 180 |
| St. Catherines, Ont. | 17 | 3.2 | 160 |
| Port Arthur, Ont. | 18 | 14.7 | 145 |
| Ogdensburg, N. Y. | 18 | 0.2 | 140 |
| Los Angeles, Cal. | 516 | 77.9 | 140 |
| Wilmington, Del. | 92 | 43.7 | 125 |
| Lancaster Pa. | 60 | 34.6 | 120 |
| Richmond, Va. | 120 | 75.2 | 115 |
| St. Louis, Mo. | 730 | 6.7 | 110 |
| Springfield, Mass. | 100 | 94.4 | 110 |
| Keokuk, Ia. | 14 | 64.5 | 105 |
| Jefferson City, Mo. | 13.5 | 34.4 | 100 |
| Muncie, Ind. | 30 | 23.8 | 95 |
| Burlington, Ia. | 24 | 4.5 | 90 |
| Council Bluffs, Ia. | 32 | 75.5 | 80 |
| San Diego, Cal. | 85 | 100 | 80 |
| Monroe, Wis. | 3 | 100 | 80 |
| Yazoo City, Miss. | 7 | 84.1 | 75 |
| Oak Park, Illinois. | 26 | 100 | 70 |
| Portsmouth, Va. | 75 | 8.1 | 65 |
| New Orleans, La. | 360 | 99.7 | 60 |
| Rockford, Ill. | 53 | 93.0 | 55 |
| Fort Dodge, Ia. | 20 | 96.0 | 50 |
| Manchester, Vt. | 1.5 | 69.0 | 45 |
| Woonsocket, R. I. | 47.5 | 95.6 | 35 |
Attempts have been made to express the rate of sewage flow in different units other than in gallons per capita per day. A unit in terms of gallons per square foot of floor area tributary has been suggested for commercial and industrial districts. It has not been generally adopted. The rates of flow in New York City as reported in this unit by W. S. McGrane are given in Table 11.
The most successful way to predict the flow from commercial or industrial districts is to study the character of the district’s activities and to base the prediction on the quantity of water demanded by the commerce and industry of the district affected.
25. Fluctuations in Rate of Sewage Flow.—The rate of flow of sewage from any district varies with the season of the year, the day of the week, and the hour of the day. The maximum and minimum rates of sewage flow are the controlling factors in the design of sewers. The sewers must be of sufficient capacity to carry the maximum load which may be put upon them, and they must be on such a grade that deposits will not occur during periods of minimum flow. The maximum and minimum rates of flow are usually expressed as percentages of the average rate of flow.
| TABLE 8 | |||
|---|---|---|---|
| Sewage Flow from Different Classes of Districts | |||
| Arranged from data by Kenneth Allen in Municipal Engineer’s Journal, Feb., 1918. | |||
| District | Gallons per Capita per Day | Gallons per Acre per Day | |
| Buffalo, N. Y. From Report of International Joint Commission on the Pollution of Boundary Waters: | |||
| Industrial: Metal and automobile plants. Maximum. | 13,000 | ||
| Industrial: Meat packing, chemical and soap. | 16,000 | ||
| Commercial: Hotels, stores and office buildings. | 60,000 | ||
| Domestic: Average. | 80 | ||
| Domestic: Apartment houses. | 147 | ||
| Domestic: First-class dwellings. | 129 | ||
| Domestic: Middle-class dwellings. | 81 | ||
| Domestic: Lowest-class dwellings. | 35.5 | ||
| Cincinnati, Ohio. 1913 Report on Sewerage Plan: | |||
| Industrial, in addition to residential and ground water. | 9,000 | ||
| Commercial, in addition to residential and ground water. | 40,000 | ||
| Domestic. | 135 | ||
| Detroit, Mich.: | |||
| Domestic. | 228 | ||
| Industrial, in addition to residential and ground water. | 12,000 | ||
| Commercial, in addition to residential and ground water. | 50,000 | ||
| Milwaukee, Wis. 1915 Report of Sewerage Commission: | |||
| Industrial, maximum. | 81 | 16,600 | |
| Industrial, average. | 31 | 8,300 | |
| Commercial, maximum. | 60,500 | ||
| Commercial, average. | 37,400 | ||
| Wholesale commercial, maximum. | 20,000 | ||
| Wholesale commercial, average. | 9,650 | ||
| TABLE 9 | ||||||||
|---|---|---|---|---|---|---|---|---|
| Observed Water Consumption in Different Classes of Districts in New York City | ||||||||
| From data by Kenneth Allen in Municipal Engineers Journal, for 1918 | ||||||||
| Hotels | Daily Cons. Gals. per 1000 Sq. Ft. Floor Area | Tenements | Daily Cons. Gals. per 1000 Sq. Ft. Floor Area | Office and Loft Buildings | Daily Cons. Gals. per 1000 Sq. Ft. Floor Area | |||
| Building | Max.[[19]] | Avg. | Location | Max.[[19]] | Avg. | Building | Max.[[19]] | Avg. |
| Hotel Biltmore. | 470 | 368 | 78th–79th St. and B’way. | 256 | 192 | McGraw Bldg. | 309 | 206 |
| Hotel McAlpin. | 753 | 694 | 410 E. 65th St. | 350 | 295 | N. Y. Telephone Bldg. | 194 | |
| Hotel Plaza. | 630 | 578 | 30th St. and Madison Ave | 306 | 188 | Met. Life Bldg. | 256 | |
| Hotel Waldorf Astoria. | 618 | 482 | 27 Lewis St. | 307 | 250 | 42d St. Bldg | 271 | |
| Hotel Astor. | 732 | 492 | 258 Delancey St. | 267 | 226 | Municipal Bldg. | 118 | |
| Hotel Vanderbilt. | 604 | 545 | Equitable Bldg. | 366 | 268 | |||
| Average | 634 | 526 | Average | 297 | 230 | Average | 338 | 219 |
| TABLE 10 | |||
|---|---|---|---|
| Sewage Flow from Different Classes of Districts Based on 1915 Report of Milwaukee Sewerage Commission | |||
| Ratio of maximum to average rate for department store district. | 1.755 | ||
| Ratio of maximum to average rate for hotel district. | 1.65 | ||
| Ratio of maximum to average rate for office building district. | 1.51 | ||
| Ratio of maximum to average rate for wholesale commercial district. | 2.1 | ||
| Average and maximum gallons per thousand square feet of floor area: | Avg. | Max. | |
| For department store district. | 232 | 407 | |
| For office building district. | 541 | 891 | |
| For wholesale commercial district. | 164 | 344 | |
| For all districts except wholesale commercial. | 381 | 618 | |
| Average and maximum gallons per day: | |||
| For all districts except wholesale commercial. | 17,700 | 29,800 | |
| For wholesale commercial district. | 9,650 | 20,000 | |
| TABLE 11 | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Rates of Consumption Predicted for Different Districts in New York City | ||||||||||
| District | Net Bldg. Area in Sq. Ft. per Acre for Ultimate Consumption | Avg. Number of Floors | Observed Cons. in g.p.d. per 1000 Sq. Ft. Max. | Observed Cons. in g.p.d. per 1000 Sq. Ft. Avg. | Predicted Mean Cons. | Predicted Mean in Million Gals. per Acre per Day | Predicted Dry Weather Flow, c.f.s. per Acre | Predicted Max. Dry Weather Flow, c.f.s. per Acre | Measured Avg. Dry Weather Flow, c.f.s. per Acre | Measured Max. Dry Weather Flow, c.f.s. per Acre |
| Hotel and midtown. | 24,800 | 15 | 634 | 526 | 500 | .20 | .29 | .34 | 1.04 | .146 |
| Midtown and financial. | 24,800 | 15 | 338 | 219 | 300 | .12 | .18 | .23 | .078 | .110 |
| East and West of midtown. | 24,800 | 10 | 297 | 230 | 300 | .074 | .12 | .15 | .057 | .097 |
| Apartment, 59th to 155th Sts. | 20,400 | 7 | 230 | 300 | .043 | .06 | .09 | |||
| Manhattan north of 155th St. | 20,400 | 5 | 230 | 300 | .031 | .05 | .08 | |||
| Midtown district consists of department stores, large railroad terminals, industrial and loft buildings, and sky-scraper office building. | ||||||||||
It is difficult to set any definite figure for the percentage which the maximum rate of flow is of the average. Fluctuations above and below the average are greater the smaller the tributary population. This relation can be expressed empirically as
M = 500
P⅕,
in which M represents the per cent which the maximum flow is of the average, and P represents the tributary population in thousands. The expression should not be used for populations below 1,000 nor above 1,000,000. Having determined the expected average flow of sewage by a study of the population, water consumption, etc., the maximum quantity of sewage is determined by multiplying the average flow by the per cent which the maximum is of the average. In this connection W. G. Harmon[[20]] offers the relation
M = 1 + 14
4 + √P,
which was used in the design of the Ten Mile Creek intercepting sewer at Toledo, Ohio. For rough estimates and for comparative purposes the ratio of the average to the minimum flow can be taken the same as the ratio of the maximum to the average flow, unless direct gaugings or other information show it to be otherwise.
Fig. 10.—Daily and Hourly Variations of Sewage Flow.
1. Toledo, O.; Manufacturing average. 2. Toledo, O.; Manufacturing, Monday. 3. Toledo, O.; Manufacturing, Sunday. 4. Toledo, O.; Residential, average. 5. Toledo, O.; Residential, Monday. 6. Toledo, O.; Residential, Sunday. 7. Cincinnati, O., Industrial, average. 8. Cincinnati, O.; Residential, average. 9. Cincinnati, O.; Commercial, average. 10. Average of 7 cities.
The fluctuations of flow in commercial and industrial districts are so different from those in residential districts that the formulas given should not be used in the design of sewers other than those draining residential areas. It is reasonable to suppose that fluctuations in rates of flow from industrial districts are dependent upon the character of the tributary industries. A study of these industries will give valuable light on the maximum and minimum rates at which sewage will be delivered to the sewers.
Hourly, daily, and seasonal fluctuations in rates of sewage flow are of interest in the design of pumping stations to give knowledge of the rates at which the pumps must operate at various periods. The fluctuations in rates of sewage flow during various hours and days in different cities and districts are shown in Fig. 10. Fluctuations in rate of flow of sewage lag behind fluctuations in rate of water consumption, the time being dependent on the distance through which the wave of change must travel in the sewer.
26. Effect of Ground Water.—Sewers are seldom laid with water-tight joints. Since they usually lie below the ground water level it is inevitable that a certain amount of ground water will enter. Various units have been suggested for the expression of the inflow of ground water in an attempt to include all of the many factors. Some of these units are: gallons per acre drained by the sewer per day, gallons per mile of pipe per day, gallons per inch diameter per mile of pipe per day, etc. Since the ground water enters pipe sewers at the joints, the longer the joints the greater the probability of the entrance of ground water. The last unit is therefore the most logical but the accuracy of the result is scarcely worthy of such refinement and the unit usually adopted is gallons per mile of pipe per day.
No definite figure can be given for the amount of ground water to be expected in sewers since the character of the soil and the ground water pressure must be considered. Relatively normal infiltration may be found from 5,000 to 80,000 gallons per mile of pipe per day. The minimum is seldom reached in wet ground and the maximum is frequently exceeded. Table 12 shows the amount of ground water measured in various sewers as given by Brooks.[[21]]
27. Résumé of Method for Determination of Quantity of Dry weather Sewage.—The steps in the determination of the quantity of sewage are: determine the period in the future for which the sewers are to be designed; estimate the population and tributary area at the end of this period; estimate the rate of water consumption and assume the sewage flow to equal the water consumption; determine the maximum and minimum rates of sewage flow; and finally, estimate the maximum rate of ground water seepage and add it to the maximum rate of sewage flow to give the total quantity of sewage to be carried by the proposed sewers.
| TABLE 12 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Data on the Infiltration of Ground Water into Sewers | |||||||||
| Abstracted from paper by J. N. Brooks in Transactions Am. Society of Civil Engineers, Vol. 76, p. 1909. | |||||||||
| Place | Shape | Diameter or Dimensions in Inches | Material | Wet Trench, Per Cent of Total Length | Avg. Head of Ground Water, Fee | Character of Subgrade | Gallons per 24 Hours | ||
| Per Foot of Joint | Per Inch Diameter Per Mile of Pipe | Per Mile of Pipe | |||||||
| Boston, Mass. | Circ. | 8 to 36 | V.P. | 2.6 | 1,818 | 40,000 | |||
| East Orange, N. J. | 10 | Q. | 22,400 | ||||||
| East Orange, N. J. | 8 to 24 | V.P. | 0.8 | 540 | 8,650 | ||||
| Joint trunk sewer, New Jersey | G. & Q. | 25,000 | |||||||
| Rogers Park, Ill. | 6 | 0.3 | 207 | 1,240 | |||||
| Altoona, Pa. | 30 | 5.0 | 2,890 | 86,592 | |||||
| Concord, Mass. | 18 | 8 | 43,000 | ||||||
| Malden, Mass. | Circ. | V.P. | 60 | 50,000 | |||||
| Westboro, Mass. | 15 | V.P. | 100 | 88,100 | 1,320,300 | ||||
| Fond du Lac, Wis. | Circ. | 24 | V.P. | 100 | 5 | C. | 1.5 | 1,010 | 24,370 |
| East Orange, N. J. | Circ. | 10 to 24 | V.P. | 100 | 4.7 | 2,540 | 43,250 | ||
| Ocean Grove, N. J. | Circ. | 4 to 12 | V.P. | 100 | 3 | S.C. | 2.7 | 1,890 | 15,126 |
| Ocean Grove, N. J. | Circ. | 4 to 12 | V.P. | 100 | 4 | S.C. | 7.9 | 5,480 | 43,764 |
| East Orange, N. J. | Rect. | 24 × 36 | Brick | 100 | 570,000 | ||||
| Westboro, Mass. | Brick | 415,850 | |||||||
| Altoona, Pa. | Rect. | 33 × 44 | B. & C. | 5,390 | 264,000 | ||||
| Columbus, Ohio. | H.S. | 42 × 42 | Concrete | 120 | 6,340 | ||||
| Bronx Valley, N. Y. | Circ. | 44 to 72 | Concrete | G. | 123 | 7,266 | |||
| Cincinnati, Ohio. | Estimated in design. Data not from Brooks | 67,500 | |||||||
| Milwaukee, Wis. | Residential districts, gals. per acre per day. Not taken from Brooks | 1460 to 2200 | |||||||
| Abbreviations: H.S. = horseshoe shaped; B. & C = Brick and concrete; V.P. = vitrified pipe; G. = gravel; Q. = quicksand; S. C. = sand clay; C. = clay. | |||||||||