[28]. Includes depreciation and repairs, but not general supervision.
SEWAGE DISPOSAL
EFFICIENCY OF PROCESSES USED BY AMERICAN CITIES—OPINIONS OF AUTHORITIES—EXPERIMENTS WITH NEW METHODS.
Recognition of the necessity for the proper disposal of sewage is now quite prevalent in most American communities, whether large or small. In many sections the problem has become vital, and as the population increases, it is only a matter of time when all will be compelled to solve the problem, for its importance grows in direct proportion to the rapid increase in inhabitants. The continued concentration of population makes it increasingly difficult and expensive for a municipality to secure and maintain a pure water supply and forces community activity for protection against disease germs. It also causes the demand for the improvement of the esthetic condition of bodies of water within or near a city’s boundaries. Many states have already recognized the conditions due to these nuisances and have enacted strict legislation with a view to preventing the pollution of streams and other bodies of water, for the protection of water supplies, surface and underground, and for the elimination of disease germs accompanying sewage. States and even nations have realized that sewage disposal is more than a local problem. In every case it is an inter-community problem, in some it is inter-state and in a few the question must be settled by national governments.
Even those communities which have not already provided a proper method of disposal of their sewage know that it must be done sooner or later, and many are preparing for it either by making a preliminary study, by preparing tentative plans, by reconstructing their sewerage systems or planning new extensions with that end in view, or by shaping their financial programs so that the community will be prepared to assume the financial burden when the necessity becomes imperative.
The quantity of harmful waste produced by a community is surprisingly small in comparison with the disastrous effects it may produce. All authorities agree that in cities provided with an abundant water supply sewage contains less than one-tenth of one per cent. of foreign substances. This organic matter and the products of its decomposition the Massachusetts State Board of Health has found rarely exceed one-half of one per cent. of the sewage. George W. Fuller, consulting sanitary engineer, says that 99.9 per cent. of sewage is ordinarily pure water and that even much of the remainder is harmless matter of a mineral nature. The experience of George S. Webster, Chief Engineer of the Bureau of Surveys and of the Philadelphia Sewage Testing Station, with sewage works, indicates that on an average 1,000 persons produce per annum forty-five tons of dry sludge matter, or the solid part of the sewage after treatment; and the United States Census Bureau reports that the volume of sewage discharged daily during the year per person is 164 gallons. Yet the small amount of decomposing matter must be properly treated for it is that which gives sewage its offensive character and power to cause disease.
The proper solution of the sewage disposal problem involves first, the construction of a sewerage system that will remove the sewage from the community completely and as rapidly as possible, and secondly, the construction of a disposal plant at which the sewage can be treated in such a way that when it is discharged into the body of water it will not cause a nuisance and disease.
The Sewerage System
There are two types of sewerage systems in use, the separate and the combined. In the former the storm water is removed in one set of pipes and the domestic sewage in another. The combined system removes both in the same set of pipes. In deciding which system to adopt three factors must be first considered, the cost, the topography of the city and the method of disposal. The general conclusions of sanitary engineers at present regarding the relative merits of the two systems are that either is satisfactory from a sanitary point of view when properly constructed, that the separate system is usually best for suburban districts not closely built up and for all communities where the sanitary sewage requires treatment, and that often a combination of the two systems can be used to advantage. Most engineers point to the advantage of combined sewers in narrow streets and congested districts where only one pipe and one house connection are required.
The belief has been expressed by John H. Gregory, consulting engineer, that as a general proposition the cost of building a combined system is less than that of constructing a separate system, especially where the territory to be served is more or less closely built up and streets paved. In suburban territory, not closely built up and where storm water is easily and quickly diverted into natural water courses, he believes the separate system will in general cost less, for then only sanitary sewers need to be built first, the storm water sewers being deferred for years or only such drains constructed as are immediately required. When there are steep grades and relatively high velocity all authorities agree with Gregory that it is advisable to build combined sewers, even though the development of the territory may hardly be such as to require the removal of the storm water.
Discussing the merits of the two systems so far as they affect the cost of disposal Clark P. Collins, sanitary engineer, concludes that generally speaking “it is unwise to dilute sewage with storm water and to befoul storm water with sewage in the attempt to remove both by the same underground channel.” Gregory has expressed the opinion that if sewage is to be discharged into a body without treatment the combined system will offer the simplest and cheapest solution of the problem.