SECTION VII

SEWAGE DISPOSAL

By GEORGE C. WHIPPLE

Professor of Sanitary Engineering in Harvard University

Importance of Speedy Removal of Fecal Matter.-The basic principle that underlies all methods of sewage disposal is to get rid of the sewage as speedily as possible, with the least nuisance to the smallest number of people, with the least damage to health or property, and at the smallest cost. Experience has shown that failure to remove human excrementitious matter from a community promptly or properly is a menace to the public health. Privies and cesspools should not be tolerated in a closely built up area. Unless more than ordinary care is exercised their existence may give opportunity for the spread of disease by insects and animals and by the pollution of local wells. Statistics show that the abandonment of privies and the substitution of sewerage systems have reduced the general death rate in many a city. Thus Dr. Boobyer has reported that at Nottingham, England, in a period covering ten years typhoid fever cases occurred in 2.7 per cent. of the houses that were provided with privies, in 0.83 per cent. of the houses where pail closets were used, and in only 0.18 per cent. of the houses that had water-closets connected with the sewers. Similarly, Dr. Porter has stated that in Stockport, England, during the years 1893-7 typhoid fever occurred in 3.4 per cent. of the houses where there were privies, but in only 1.2 per cent. of the houses that had sewer connections, these figures being based on a study of over 18,000 houses. In Munich, when sewers were constructed in 1856-9 the typhoid fever death rate fell from 242 to 166 per 100,000; later, after an improved water supply and other sanitary reforms had been brought about, the typhoid fever death rate fell to a much lower figure.

By taking special precautions against the spread of infection through the agency of flies, either by preventing their breeding or preventing them from obtaining access to fecal matter, and by closing polluted wells in crowded districts, the dangers from privies and cesspools may be greatly reduced.1 Sometimes it is wiser to do this in villages and 'For the dangers of polluting the soil with feces see chapter on "Soil."

small towns than to go to the expense of introducing sewerage systems, with perhaps the attendant difficulty and expense of purifying the sewage after collection.

Ordinarily in this country sewerage systems and public water supplies are introduced in towns where the population exceeds about 3,000, and in smaller places if the population is concentrated. This is so generally true that towns that have less than 2,500 or 3,000 population are classed as "rural," the larger towns being called "urban.”

Dry Earth System. The dry earth system, much in vogue before the general introduction of the water carriage system, is now but little used; yet under some conditions it has advantages. With this method the water-closets are replaced by removable water-tight receptacles, or pails, in which the fecal matter is kept covered with dry earth, ashes, or some similar material. The pails are collected at frequent intervals, preferably daily, and a clean, empty pail substituted. The material is usually buried in the ground. For isolated houses, for temporary camps of laborers, for small scattered summer colonies, and for houses situated near streams or lakes used for public water supplies this method is satisfactory, and is often the best possible method, provided that proper care is taken by the user and the collector. Cleanliness in handling, the protection of the material against flies, regular and frequent collection, occasional disinfection of the pails, and prompt burial in proper soil are essential to success.

Water Carriage System.-So accustomed are we to present methods of sewerage that it is hard to realize that the system of water carriage of fecal matter is less than a century old. Up to 1815 the public drains of London were not permitted to receive excreta; in Boston fecal matters were rigidly excluded from the sewers until 1833; and in Paris this was the case even up to 1880.

Following the report of the Health of Towns Commission in England in 1844, water-closets were rapidly introduced, and in 1847 their connection with the sewers was required by law. The modern sewerage system, therefore, dates from about the middle of the last century. Chesbrough designed a general sewerage system for Chicago in 1855. Boston's first sewerage commission was appointed in 1875. Baltimore was without a sewerage system until within a few years, and even now, 1912, the system has not been fully completed, nor have many houses been connected, with it.

The introduction of the water carriage system accomplished its purpose and effectually did away with the offensive accumulations of filth around city dwellings, but it gave rise to a series of other problems that sanitarians are now endeavoring to solve. The sewers were naturally built to discharge their contents into the nearest available body of water -into river, lake, or harbor, according to the situation of the city.

Where the streams were relatively large, no nuisance was caused by doing this, but where the streams were relatively small foul conditions soon arose, and it became necessary to reduce the amount of organic matter discharged from the sewers into them. Water supplies also became infected and in some instances great epidemics followed, while infection was spread in other minor ways. Thus the problem of the removal of fecal matter was sometimes solved at one place only to reappear elsewhere. Litigation also arose between riparian owners along the water courses, involving damages caused by the pollution of the water.

The problem has thus broadened from a local one to one in which different cities and even different states have become involved. It is to the solution of these problems of maintaining our streams and lakes. and harbors in a satisfactory condition that sanitarians are now earnestly devoting themselves.

Separate and Combined Systems.-The sewers and drains of a city are used for various purposes, the two most important ones being the removal of domestic house sewage, and the rain water that falls on roofs, yards, sidewalks, and streets. Sometimes the same system of sewers is used to carry both domestic sewage and storm water. Such is called a combined system. Sometimes the storm water is carried in relatively large drains, or allowed to flow along in the street gutters, while the domestic sewage is carried in a separate system of sewers of smaller size. The choice of the two systems depends upon the local situation, but in general the following conditions control.

The combined system is the older and the one more commonly used in large cities and crowded communities, for it is cheaper than a dual system, where both separate sewers for the house sewage and drains for the storm water are required. Where the storm water can be allowed to flow off in the gutters without serious inconvenience from flooding the separate system is cheaper, as the pipes are smaller. Where the sewage must be pumped or carried long distances in pipes or purified by expensive methods the advantages lie with the separate system, as the quantity of sewage is less and its flow more constant. From the sanitary standpoint either method is satisfactory. The choice of the two systems depends upon various engineering questions involving cost, so that the matter is one that should be submitted to an engineer.

Sewerage systems consist of house sewers or house drains that convey the sewage to the street sewers or lateral sewers. These unite in what are termed district sewers, and the latter sometimes unite in one or more trunk sewers of large size. Relief sewers are sometimes built parallel to old sewers of inadequate capacity, and storm sewers are sometimes built to carry away surface water, while underdrains may be used in connection with the separate system to remove some of the

ground water. Intercepting sewers are sometimes built parallel to a stream for collecting the sewage from a number of district sewers and conveying it to a safer point of discharge. When intercepting sewers are used with the combined system they are not designed to carry all of the flow at times of storm, but are provided with overflows, so that the excess of storm water discharges into the river at various points of overflow. This is a matter of importance and one to be remembered in connection with the purification of sewage, for the quantity of sewage that passes these overflows at times of heavy rain may amount to 25 per cent. or 50 per cent. or more of the sewage, and during the course of the year may amount to from 2 per cent. to 5 per cent., or even more, of the entire sewage of the city. Such overflow water is almost never purified. At Birmingham, England, Watson has estimated that, in spite of the elaborate provisions for purification, a large part of the city's sewage is at times discharged untreated, and at Milwaukee the Sewerage Commission estimated that nearly 2 per cent. of the sewage would fail to be collected by a very liberally designed system of intercepting sewers.

Quantity of Sewage. The volume of sewage flowing in a separate system, or in a combined system during dry weather, does not differ materially from the water consumption of the city. In small towns this may be as low as 40 or 50 gallons per capita daily, although ordinarily it is rather more than this. In large cities it may amount to from 100 to 200 gallons per capita, and more than this in extreme

cases.

Intercepting sewers are commonly designed to provide for a flow of 300 to 400 gallons per capita daily. The amount of storm water depends upon climatic conditions, and for this subject engineering books should be consulted. The flow of sewage fluctuates hourly, and the maximum may be from 50 to 100 per cent. of the daily average, while greater fluctuations may be found, especially in cities where large quantities of water are used in manufacturing.

Composition of Sewage.-A city's sewage consists of the public water supply soiled with the waste products of human life and refuse from household and factory, increased by a certain amount of ground water which leaks into the sewers, and, in the combined system, by varying quantities of rain water and street wash. Disintegrating and decomposing as it flows, the sewage gradually becomes a more or less homogeneous suspension of fine particles in water, with organic and mineral matter in solution. The longer the sewage flows or stands, the more its constituents become disintegrated; fecal matter and paper become unrecognizable as such; bacteria increase enormously, and assist in the breaking down of the complex organic compounds. The oxygen originally present in the water becomes reduced and finally disappears,

so that from a fresh condition the sewage becomes first stale and then "septic." Mixed with the putrefying organic matter and the swarming hosts of bacteria harmlessly engaged in their beneficent work of destroying the organic matter, there may be also bacteria which have come from persons sick with typhoid fever, dysentery, tuberculosis, and other diseases.

Sewage is obnoxious to the senses because of its decomposing organic matter, but it is dangerous to health because of the possible presence of these pathogenic bacteria.

Among the important constituents of sewage from the standpoint of purification are urea, various proteid substances such as albumin, fibrin, casein, starch, sugar, and other carbohydrates, fats, soaps, and other organic substances. Important among the elements present in the easily decomposable matter are nitrogen and sulphur. The concentration of these substances, that is, the amount present in a given volume of sewage, depends upon the per capita volume of the sewage, and varies widely in different places. Somewhat more constant, however, are these constituents when compared with the number of persons dwelling in houses connected with the sewers.

The following figures show the approximate constituents of sewage expressed in terms of grams per capita daily and in parts per million when the volume of sewage amounts to 100 gallons per capita daily.

ESTIMATED CONSTITUENTS OF AVERAGE SEWAGE

Bacteria, 322 billion per capita daily.

These figures also indicate parts per million if the per capita volume of

sewage is 264 gallons per day.

Assuming a per capita volume of 100 gallons per day.