An abundant supply of pure water is a necessity in every town and city; and such a supply having been secured brings up the question of its disposal after being used. This is plainly the reverse of its introduction. As it was distributed through a network of conduits, diminishing in size, with its numerous branches, so it may be collected again by similar conduits, increasing in size, as one after another they unite in a common outlet.

This fouled water is called sewage, and the conduits which collect it constitute a sewerage system. In general, sewage is disposed of in two ways; either it must be turned into a body of water so large as to dilute it beyond all possibility of offence, and where it cannot endanger human life by polluting a public water-supply, or it must be purified in some manner.

The conduits which carry water collected from street surfaces during and after rains, or ground water collected from beneath the surface, are called drains. When one set of conduits removes sewage and another carries surface and ground water, it is said that the separate system of sewerage is in use. Where one system conveys both sewage and drainage water it is called the combined system. Various modifications of these two systems are possible, both for whole cities and for limited areas within the same town or city.

A sanitary sewerage system cannot be installed until a public water-supply has been provided. It is needed as soon as that is accomplished, for while the wells can then be abandoned the volume of waste water is greatly increased by the water-works system. Its foulness is also much increased through the introduction of waterclosets. Without sewers and with a public water-supply cesspools must be used, and with these begins a continuous pollution of the soil much more serious than that which commonly results from closets and the surface disposal of slops.

Among the data which should first be obtained in laying out a sewerage system are:


The area to be served, with its topography and the general character of the soil. - A contour map of the whole town or city, showing the location of the various streets, streams, ponds or lakes, and contour lines for each 5 feet or so of change in elevation, is necessary for the best results. The general character of the soil can usually be obtained by observation and inquiry among residents or builders who have dug wells or cellars, or have observed work of this kind which was being done. The kind of soil is important as affecting the cost of trenching and its wetness or dryness, and this, together with a determination of the groundwater level, will be useful in showing the extent of underdraining necessary.


Whether the separate or combined system of sewerage, or a compromise between the two is to be adopted. - These points will depend almost wholly upon local conditions. The size and cost of combined sewers is much greater than the separate system, since the surface drainage in times of heavy rainfall is many times as great as the flow of sanitary sewage. In older towns and cities it sometimes happens that drains for removing the surface water are already provided, and in this case it is only necessary to put in the sanitary sewers; or again, the latter may be provided, leaving the matter of surface drainage for future consideration.

If the sewage must be purified, the combined system is out of the question, for the expense of treating the full flow in times of maximum rainfall would be enormous. Sometimes more or less limited areas of a town may require the combined system, while the separate system is best adapted to the remainder; and again it may be necessary to take only the roof water into the sewers. As already stated, local conditions and relative costs are the principal factors in deciding between the separate and combined systems.


Whether subsoil drainage shall be provided. - In most cases this also will depend upon local conditions. It is always an advantage to lower the ground-water level in places where it is sufficiently high to make the ground wet at or near the surface during a large part of the year. In addition to rendering the soil dry around and beneath cellars, the laying of underdrains is of such aid in sewer construction as to warrant their introduction for this purpose alone. This is the case where the trenches are so wet as to render the making and setting of cement joints difficult. The aim in all good sewer work is to reduce the infiltration of ground water into the pipes to the smallest amount; but in very wet soil, tight joints can be made only with difficulty, and never with absolute certainty. Cases have been known where fully one-half the total volume of sewage discharged consisted of ground water which had worked in through the joints.


The best means for the final disposal of the sewage. - Until recently it was turned into the nearest river or lake where it could be discharged with the least expense. The principal point to be observed in the disposal of sewage is that no public water-supply shall be endangered. At the present time no definite knowledge is at hand regarding the exact length of time that disease germs from the human system will live in water. The Massachusetts legislature at one time said that no sewer should discharge into a stream within 20 miles of any point where it is used for public water-supply, but it is now left largely in the hands of the State Board of Health. There may be cases where sewage disposal seems to claim preference to water supply in the use of a stream, but each case must be decided on its own merits. Knowing the amount of water and the probable quantity and character of the sewage, it is generally easy to determine whether all of the crude sewage of a city can safely be discharged into the body of water in question. Averages in this case should never be used; the water available during a hot and dry summer, when the stream or lake is at its lowest, and the banks and beds are exposed to the sun, is what must be considered. Where sewage is discharged into large bodies of water, either lakes or the ocean, it is generally necessary to make a careful study of the prevailing currents in order to determine the most available point of discharge, in order to prevent the sewage becoming stagnant in bays, or the washing ashore of the lighter portions. Such studies are commonly made with floats, which indicate the direction of the existing currents.


Population, water consumption and volume of sewage for which provision should be made, together with the rainfall data, if surface drainage is to be installed. - The basis for population studies is best taken from the census reports, extending back many years. By means of these the probable growth may be estimated for a period of from 30 to 50 years. In small and rapidly growing towns it must be remembered that the rate of increase is generally less as the population becomes greater.

It is desirable to design a sewerage system large enough to serve for a number of years, 20 or 30 perhaps, although some parts of the work, such as pumping or purification works, may be made smaller and increased in size as needed.

The pipe system should be large enough at the start to serve each street and district for a long period, as the advantages to be derived from the use of city sewers are so great that all houses are almost certain to be connected with them sooner or later. It is often necessary to divide a city into districts in making estimates of the probable growth in population. Thus the residential sections occupied by the wealthiest classes will be comprised of a comparatively small population per acre, due to the large size of the lots. The population will grow more dense in the sections occupied by the less wealthy, the well-to-do and finally the tenement sections. In manufacturing districts the amount of sewage will vary somewhat, depending upon the lines of industry carried on.

The total water consumption depends mainly upon the population, but no fixed rule can be laid down for determining it beforehand. It is never safe to allow less than 60 gallons per day per capita as the average water consumption of a town if most of the people patronize the public water-supply. In general it is safer to allow 100 gallons. The total daily flow of sewage is not evenly distributed through the 24 hours. The actual amount varies widely during different hours of the day. In most towns there should be little if any sewage, if the pipes are tight enough to prevent inward leakage, between about 10 o'clock in the evening and 4 in the morning. From 2/3 to 3/4 of the daily flow usually occurs in from 9 to 12 hours, the particular hours varying in different communities. This is not of importance in designing the pipe system, but only affects the disposal.

Rainfall data is usually hard to obtain except in the cities and larger towns. In cases of this kind the data of neighboring town or cities may be used if available. Monthly or weekly totals are of little value, as it is necessary to provide for the heaviest rains, as a severe shower of 15 minutes may cause more inconvenience and damage, if the sewers are not sufficiently large, than a steady rain extending over a day or two. A maximum rate of 1-inch per hour will usually cover all ordinary conditions. The proportion which will reach the sewers during a given time will depend upon local conditions, such as the slope of land, whether its surface is covered with houses and paved streets, cultivated fields or forests, etc.


Extent and cost of the proposed system. - This is a matter largely dependent upon the local treasury, or the willingness of the people to pay general taxes or a special assessment for the benefits to be derived.