1. Sewerage and the Sanitary Engineer.—Present day conceptions of sanitation are based on the scientific discoveries which have resulted so much in the increased comfort and safety of human life during the past century, in the increase of our material possessions, and the extent of our knowledge. The danger to health in the accumulation of filth, the spreading of disease by various agents, the germ theory of disease, and other important principles of sanitation can be counted among the more recent scientific discoveries and pronouncements. Experience has shown, and continues to show, that the increase of population may be inhibited by accumulations of human waste in populous districts. The removal of these wastes is therefore essential to the existence of our modern cities.

The greatest need of a modern city is its water supply. Without it city life would be impossible. The next most important need is the removal of waste matters, particularly wastes containing human excreta or the germs of disease. To exist without street lights, pavements, street cars, telephones, and the many other attributes of modern city life might be possible, although uncomfortable. To exist in a large city without either water or sewerage would be impossible. The service rendered by the sanitary engineer to the large municipality is indispensable. In addition to the service necessary to the maintenance of life in large cities, the sanitary engineer serves the smaller city, the rural community, the isolated institution, and the private estate with sanitary conveniences which make possible comfortable existence in them, and which are frequently considered as of paramount necessity. Training for service in municipal sanitation is training for a service which has a more direct beneficial effect on humanity than any other engineering work, or any other profession. W. P. Gerhard states:

A Sanitary Engineer is an engineer who carries out those works of civil engineering which have for their object:

(a) The promotion of the public and individual health;

(b) The remedying of insanitary conditions;

(c) The prevention of epidemic diseases.

A well-educated sanitary engineer should have a thorough knowledge of general civil engineering, of architecture, and of sanitary science. The practice of the sanitary engineer embraces water supply, sewerage, and sewage and garbage disposal for cities and for single buildings; the prevention of river pollution, the improvement of polluted water supplies; street paving and street cleaning, municipal sanitation, city improvement plans, the laying out of cities, the preparation of sanitary surveys, the regulation of noxious trades, disinfection, cremation, and the sanitation of buildings.

The need of the work of the sanitary engineer in the provision of sewers and drains is thrust upon us in our daily experience by the clogging of sewers, the flooding of streets by heavy rains, filthy conditions in unsewered districts, increased values of property and improved conditions of living in sewered districts, and in many other ways. The increasing demand for sewerage and the amount of money expended on sewer construction is indicated by the information given in Table I.

2. Historical.—An ordinance passed by the Roman Senate in the name of the Emperor about A.D. 80, states:

I desire that nobody shall conduct away any excess water without having received my permission or that of my representatives; for it is necessary that a part of the supply flowing from the delivery tanks shall be utilized not only for cleaning our city, but also for flushing the sewers.^[1]

Neither the sewers mentioned nor the distributing pipes of the public water supply were connected to individual residences. The contributions to the sewers came from the ground and the street surface. The streets were the receptacles of liquid and solid wastes and were often little more than open sewers. A promenade after dark in an ancient, medieval, or early modern city was accompanied not only by the underfoot dangers of an uneven pavement or an encounter with a footpad, but with the overhead danger from the emptying of slops into the streets from the upper windows. Sewers were used for the collection of surface water; the discharge of fecal matter into them was prohibited. The problem of the collection of sewage remained unsolved until the Nineteenth Century.

The development of the London sewers was commenced early in the Nineteenth Century. The sewerage system of Hamburg, Germany, was laid out in 1842 by Lindley, an English engineer who with other English engineers performed similar work in other German cities because of their earlier experience in English communities. Berlin’s present system dates from 1860. The construction of storm-water drains in Paris dates from 1663.^[5] They were intended only as street drains but are now included in the comprehensive system of the city. The first comprehensive sewerage system in the United States was designed by E. S. Chesbrough for the City of Chicago in 1855. Previous to this time sewers had been installed in an indifferent manner and without definite plan. The installation of a comprehensive sewerage system in Baltimore in 1915 marks the completion of installation of sewerage systems in all large American cities.

In the early days of sewerage design it was considered unsafe to discharge domestic wastes into the sewers as the concentration of so much sewage was expected to create great nuisances and dangers to health. That the fear that the concentration of large quantities of sewage would create a nuisance was not ill founded is proven by the conditions on the Thames at London in 1858–59. Dr. Budd states:^[6]

For the first time in the history of man, the sewage of nearly three millions of people had been brought to seethe and ferment under a burning sun in one vast open cloaca lying in their midst.

The result we all know. Stench so foul we may well believe had never before ascended to pollute this lower air. Never before at least had a stink risen to the height of an historic event.... For months together the topic almost monopolized the public prints.... ‘India is in revolt and the Thames stinks’ were the two great facts coupled together by a distinguished foreign writer, to mark the climax of a national humiliation.^[7]

The problem of sewage disposal followed the more or less successful solutions of the problem of sewage collection. In England the British Royal Commission on Sewage Disposal was appointed in 1857 and issued its first report in 1865. The first studies in the United States were started in 1887 by the establishment of an experiment station at Lawrence, Massachusetts, where valuable work has been done. The station is under the State Board of Health, which issued its first report containing the results of the work at the station, in 1890.

Various methods of sewage treatment preparatory to disposal have been devised from time to time. Some have fallen into disuse, such as the A. B. C. (alum, blood and clay) process, and others have taken a permanent place, such as the septic tank. The unsolved problems of sewage collection, and the number of persons still unserved by sewerage and sewage disposal opens a wide field to the study and construction of sewerage works.

3. Methods of Collection.—The method of collection which involves the removal of night soil from a privy vault, the pail system which involves the collection of buckets of human excreta from closets and homes, indoor chemical closets, and other makeshift methods of collection are of extreme importance where no sewers exist, but they are not properly considered as sewerage systems or sewerage works. These methods of collection are generally confined to rural districts and to outlying parts of urban communities. They require constant attention for their proper conduct and little skill for their installation, the principal requirements being to make the receptacles fly-proof.

The pneumatic system was introduced by Liernur, a Dutch engineer.^[8] It is used in parts of a few cities in Europe, but it is not capable of use on a large scale. It consists of a system of air-tight pipes, connecting water closets, kitchen sinks, etc., with a central pumping station at which an air-tight tank is provided from which the air is partly exhausted. As little water as possible is allowed to mix with the fecal matter and other wastes in order not to overtax the system. Solid and liquid wastes are drawn to the central station when the waste valve on the plumbing fixture is opened.

The collection of sewage in a system of pipes through which it is conducted by the buoyant effect and scouring velocity of water is known as the water-carriage system. This is the only method of sewage collection in general use in urban communities. In this system solid and liquid wastes are so highly diluted with water as either to float or to be suspended therein. The mixture resulting from this high dilution follows the laws of hydraulics as applied to pure water, or water containing suspended matter. It will flow freely through properly designed conduits and will concentrate the sewage wastes at the point of ultimate disposal.

4. Methods of Disposal.—Sewage is disposed of by dilution in water, by treatment on land, or occasionally by discharging it into channels that contain no diluting water. Some form of treatment to prepare sewage for ultimate disposal is frequently necessary and will undoubtedly be required in a comparatively short time for all sewage discharged into watercourses. The solid matters removed by treatment may be buried, burned, dumped into water, or used as a fertilizer.

If the volume of diluting water, or the area and character of land used for disposal are not as they should be, a nuisance will be created. The aim of all methods of sewage treatment has so far been to produce an effluent which could be disposed of without nuisance and in certain exceptional cases to protect public water supplies from pollution. Financial returns have been sought only as a secondary consideration. A few sewage farms and irrigation projects might be considered as exceptions to this as the value of the water in the sewage as an irrigant has been the primary incentive to the promotion of the farm.

It is to be remembered that since the aim of all sewage treatment is to produce an effluent that can be disposed of without causing a nuisance, the simplest process by which this result can be attained under the conditions presented is the process to be adopted. No attempt is made to purify sewage completely, or on a practical scale to make drinking water.

5. Methods of Treatment.—Screening and sedimentation are the primary methods for the treatment of sewage. By these methods a portion of the floating and settleable solids are removed, preventing the formation of unsightly scum and putrefying sludge banks. Chemicals are sometimes added to the sewage to form a heavy flocculent precipitate which hastens sedimentation of the solid matters in the sewage. The process in these methods is mechanical and the solid matters removed from the sewage must be disposed of by other methods than dilution with the sewage effluent. More complete methods of treatment are dependent on biologic action. Under these methods of treatment complete stabilization of the effluent is approached, and in the most complete treatment an effluent is produced which is clear, sparkling, non-odorous, non-putrescible, and sterile. Sterilization of sewage, usually with chlorine or some of its compounds, has been used, not to reduce the amount of diluting water necessary, but to reduce the number of pathogenic germs and to minimize the danger of the transmission of disease.

6. Definitions.—Sewage and sewerage are not synonymous terms although frequently confused. Sewage is the spent water supply of a community containing the waste from domestic, industrial or commercial use, and such surface and ground water as may enter the sewer.^[9] Sewerage is the name of the system of conduits and appurtenances designed to carry off the sewage. It is also used to indicate anything pertaining to sewers.

A difference is made between sanitary sewage, storm sewage, and industrial wastes. Sanitary sewage, sometimes called domestic sewage, is the liquid wastes discharged from residences or institutions, and contains water closet, laundry and kitchen wastes. Storm sewage is the surface run-off which reaches the sewers during and immediately after a storm. Industrial wastes are the liquid waste products discharged from industrial plants.

A sewer is a conduit used for conveying sewage.

The names of the conduits through which sewage may flow are:

Soil Stack.—A vertical pipe in a building through which waste water containing fecal matter or urine is allowed to flow.

Waste Pipe.—A vertical pipe in a building through which waste water containing no fecal matter is allowed to flow.

House Drain.—The approximately horizontal portion of a house drainage system which conveys the drainage from the soil stack or waste pipe to the point of discharge from the building.

House Sewer.—The pipe which leads from the outside wall of the building to the sewer in the street.

Lateral Sewer.—The smallest branch in a sewerage system, exclusive of the house sewers.

Sub-main or Branch Sewer.—A sewer from which the sewage from two or more laterals is discharged.^[10]

Main or Trunk Sewer.—A sewer into which the sewage from two or more sub-main or branch sewers is discharged.^[11]

Intercepting Sewer.—A sewer generally laid transversely to a sewerage system to intercept some portion or all of the sewage collected by the system.

Relief Sewer.—A sewer intended to carry a portion of the flow from a district already provided with sewers of insufficient capacity and thus preventing overtaxing the latter.^[12]

Outfall Sewer.—That portion of a main or trunk sewer below all branches.

Flushing Sewer.—A conduit through which water is conveyed for flushing portions of a sewerage system.

Force Main.—A conduit through which sewage is pumped under pressure.