The application of these laws, however, is limited in certain respects by the nature of sewage; one important factor being the tendency of the matters in suspension in the sewage to settle out if the velocity of flow becomes too low, thus placing a minimum limit upon the velocity of flow that is practicable. Another important factor is the offensiveness to sight and smell as well as the danger to health of the exposure of sewage in open conduits, as a result of which it seems to be necessary to carry sewage entirely in closed conduits.

There are several other essential or occasional characteristics of sewage which need to be considered in designing a sewerage system, and these and their effects upon the design form an important feature of the study of sewerage.

Aside from the nature of sewage, the principal factor in designing is the amount of sewage which the sewers will be called upon to carry and the fall in the hydraulic gradient of the sewers which creates the velocity with which it flows through them. The hydraulic gradient cannot for very long distances depart materially from approximate parallelism to the street surface without involving excessively deep cutting for constructing the sewer; and in general it may be said that the grades of a street under which a sewer is laid to a considerable degree determine the grade of the sewer.

The hydraulic gradient at the outlet of a sewer system can be no lower than that of the body of water into which it discharges. Generally speaking, also, it does not seem practicable to construct sewers above the surface of the ground, except for short stretches under unusual conditions. It is thus seen that the hydraulic gradient is to a large extent controlled by the topography of the city in question. The determination of this topography by surveying methods is therefore one of the important preliminaries to designing.

Where the topography does not permit the laying of the sewers at such grades as will produce the desired velocities and at the same time keeping the hydraulic gradient at the outlet at or above the level of the water into which the sewage is to be discharged, it is generally necessary to pump the sewage

at the outlet or at one or more other points, or use some other mechanical means for raising it from a lower to a higher level.

To a certain and sometimes a very considerable extent, the design of a sewerage system may be affected by the method to be employed in treating the sewage, or by the absence of any treatment. For instance, if treatment is to be provided it is generally facilitated by the use of the separate system rather than the combined. Also, some methods of treatment operate best on sewage that has been thoroughly agitated, other methods on sewage that is fresh and unagitated.

Unless a city which is to be sewered is practically level throughout, the topography more or less definitely and imperatively divides the total area into several districts or drainage areas, each of which will be sewered by a small secondary system of its own. The effluents from several such drainage areas may discharge independently, each through its own outlet, or all or several may discharge into a common main sewer and be carried to a common outlet. In this respect a sewerage system almost exactly duplicates, on a smaller scale, the system of rivulets, brooks, and river which go to make up the surface drainage system of a large water shed. ́

Every branch of a sewerage system, or at least of so much of it as has one common outlet, is to a greater or less extent dependent upon every other. The minor lines must be so located as to elevation that they can discharge into the larger ones, and, on the other hand, each of the latter must be given such depth and capacity that it will be possible for it to receive the flow of all of the smaller sewers for which it forms the natural outlet. Consequently, no one sewer line or group of sewers should be designed without a thorough study and understanding of its relation to all other portions of the entire sewerage system, both those now existing or to be constructed in the immediate program, and those which will be required in future extensions of this system.

All of the elements of a design are so interrelated that generally only tentative decision can be made of any one of them

until similar decision has been made of all and their effects on each other studied.

The more important matters to be investigated and data to be obtained in preparing a sewerage design are as follows:

Point of Discharge:

The bodies of water available.

Dilution furnished by each, involving a study of minimum flow (if a stream), current, tides, winds, etc.

Character of the water with respect to its effect upon sewage discharged into it. Possibility of the formation of deposits of sewage matters or the creation of other nuisances.

Whether water is or may be used as source of a potable supply.

Laws-federal, state, or local-bearing upon discharge of sewage into bodies

of water.

Method of Treatment, if any.

Chemical and physical analysis of sewage and of the water into which it is to be discharged.

Amount of dilution of sewage by ground or surface water; separate or combined system.

Amount of water available for diluting effluent, minimum flow

Climate-summer and winter temperature, humidity.

Amount of fall available at site of plant, or necessity of pumping.

Area of sites available and nature of soil.

Amount of sewage, mean and maximum.

Legal requirements.

Separate, Combined, or Mixei System.

Whether sewage will be treated, now or in the future.

Number, location, size, and condition of existing sewers that might be used in the new system.

Practicability of placing overflows at various points.

To what extent and over how large an area the removal of surface water is necessary.

Location, Depth, and Grades of Sewers.

Street layout, including widths of streets.

Existence and depth of rock, quicksand, ground water, and other difficult conditions under each street.

Depth of basements and cellars below street level along the line of sewer. Grades of the streets or other ground surface along the line of the proposed

sewers.

Limitations of velocity of low of sewage, to avoid deposits due to sluggishness on the one hand or abrasion due to high velocity on the other.

Size of sewer as affecting grade.

Sizes and Shapes of Sewers.

Amount of sewage, mean and maximum rates, for some years in future.

Grades of sewers.

Nature of ground (support offered for sewer).

Position of sewer relative to ground surface.

In addition to the above, there will be, in most cases, special considerations peculiar to the city in question that should be given more or less weight in solving the problems presented. Financial limitations imposed upon the engineer will generally restrict his choice of solutions and especially the extent of immediate construction.

CHAPTER II

AMOUNT OF HOUSE SEWAGE

ART. 5. COMPOSITION OF SEWAGE

SEWAGE may contain any combination of the innumerable materials that find their way into sewers, and varies considerably in the different cities, and from hour to hour in a given city.

Domestic sewage consists of the water used in the residences. of the city after pollution by such use, the pollution consisting chiefly of human excrement, both solid and liquid, dish water, (in which is found soap, grease, and particles of animal and vegetable matter), wash water (containing soap, bluing and cloth fibers), pieces of paper, matches, hair, bits of cloth, and occasionally discarded garments, bones and garbage and other matters that should not reach the sewers. Also more or less dirt enters many sewers through the covers of the manholes. The pollution carried by the water forms less than one-tenth of 1 per cent of the sewage in most cases, although this varies with the amount of water per capita entering the sewer, which amount may be increased by seepage of ground water into the sewer, and occasionally (but inadvisably) by rain water discharged into it from roofs. The appearance of domestic sewage is that of wash water in which float occasional particles of fæces, paper, matches, etc., and often with a film of grease on the surface. There is generally a slight but not particularly offensive odor. Both turbid appearance and odor increase with the age of the sewage, and most of the solid matter is rendered more comminuted by agitation of the sewage during its passage through the sewer. The temperature is generally about 45° to 55° in the coldest weather in northern cities and 65° to 75° in the hottest weather. As a general thing, the