Page:The New International Encyclopædia 1st ed. v. 18.djvu/36

* SEWERAGE. 22 SEWERAGE. Goth, dniyan, to draw, OHG. iraf/an, Ger. tragcn, to carry). The removal and disposal of liquid and water-borne solid household wastes, the free- ing of towns and citie's from surface water, and the lowering and removal of subsoil water. The two fundamental principles in the design of sewerage systems are (1) the removal of sew- age before offensive decomposition sets in, which may be effected by providing sewers of ample capacity, uniform and sullicieiit slope, and smooth interiors ; and (2) the disposal of sewage in such a manner that neither water, soil, nor air will be polluted thereby. Sewerage systems are gen- erally divided into two portions: the collecting sewei's and appurtenances and the outfall sewer or sewers. In addition there may be disposal works, including either a pumping or a purifica- tion plant, or both. The aim always is so to design the collecting and outfall sewers that the discharge may be by gravity, thus avoiding the expense of a pumping plant. Sewerage systems, as now understood, date chiefly from the middle of the nineteenth cen- tury. A few ancient cities had sewers for the removal of fouled liquids, as well as for drain- age. The most notable instance of this was Rome. (See Cloaca.) But the Roman sewerage system did not serve the whole population, by any means. The drainage of London was the subject of legislation as early as 1225. but down to 1815 it was a penal offense to discharge ex- crement or other offensive matter into the drains of that city. In 1847 the first act was passed making it compulsory to drain London houses into the sewers, and in 1859 work was begun on a system of intercepting sewers and storage tanks to cut off the discharge of sewage into the Thames within the city. Paris had drains prior to 1536, but in 1663 their total length is said to have been only about six miles, of which one and one-half miles were closed and the remainder open channels. In 1820 Paris made the use of cesspools obligatory, but permitted the liquid overflow to be discharged into the sewers. In 1880 a move was made to permit the di.scharge of all house sewage into the sewers, but up to the close of 1893. or just before the full adoption of the sewerage plan, of 266.044 houses in the city, only 10,934 were di- rectly connected with the sewerage system. In the United States. Boston had drains as early as 1701. After the adoption of a city charter in 1823 Boston assumed the ownership and control of all the drains and sewers which had been built by private parties. The date on which the sewers were opened for the reception of water-closet matter generally is not available; but presumably it followed shortly after the introduction of an ample public water supply, in 1848. It may be said of all cities that a sanitary sewerage system, as now conceived, is out of the question until a copious water supply has been provided. In most of the larger cities provisions for surface drainage preceded the introduction of sanitary sewers. Convenience graduallj' led to the use of these surface or storm sewers for the disposal of liquid, and then of solid house wastes, the connections for the latter purpose often being surreptitious at first. As public water supplies were introduced and the per capita water con- sumption greatly increased, the disposal of the water thus brought into the houses often became even more serious a matter than the removal of surface and ground drainage. This led to the construction of sewers on the combined plan. The expense involved in building sewers large enough to carry off the rainfall was almost or quite prohibitive for all but the larger, closely , built cities, so as the need for house sewerage systems increased sewers were built more and more frequently for this purpose alone. About 1850 the separate system was intro- duced in several English towns. In 1875-76 a separate system of sewerage was built at Lenox, JIass., and in 1880 a more extensive one was constructed at Jlemphis, Tenn. Both these were designed by the late Col. Geo. E. Waring, Jr. The Memphis system attracted great attention, owing largely to the }-ellow fever epidemic which preceded and led up to its adoption. Neverthe- less, the separate system, often but not always slightly modified to avoid controversy, has been widely adopted in the United States. Designing a sewerage system necessitates first of all an accurate and complete topographical map of the city or town. The next step is to di- vide the city into its natural drainage areas, par- ticularly if storm-water sewers are to be built. This done, the location of the main sewer for each district is determined and the tributary population estimated. The grades, or rate of fall per 1000 feet, should be so adjusted as to give self-cleansing velocities. At the same time, economy in construction will keep the sewers as near the surface as is consistent with proper grades and serving the lowest plumbing fixtures in the houses. The relative advantages of the combined and separate systems of sewerage will depend largely upon the size of the city and whether either pumping or purification is necessarj'. If either of the latter, and particularly if both, are re- quired, it is highly desirable that the separate system be installed, both on account of the extra cost involved in handling the surface water and of the great disadvantages and difficulties inci- dent to sudden and marked changes in the vol- ume of sewage to be treated at purification works. Another advantage of separate sewers is that they render it unnecessary to place the storm sewers deep enough to serve the bottom of the cellars, thus often saving very heavy deep trenching. The smaller cities and towns find it highly advantageous to adopt the separate sys- tem of sewerage, and to construct the sanitary sewers, only, at the outset. The volume of sewage for which provision must be made is dependent on water consumption and rainfall. In the separate system of sanitary sewers rainfall need not be considered, since it is excluded, but some allowance must be made for the leakage of ground water into the sewers. In fixing the capacity of the combined system of sewers the house sewage scarcely needs be con- sidered except on the laterals serving single short streets, since the maximum surface or storm water to be carried is so far in excess of the house wastes. Ordinarily it is safe to assume that the maximum water consumption is double the average flow, and that 75 per cent, of the latter reaches the sewers, the remainder being used for lawn-sprinkling and for houses not connected with the sewers. On this basis, a city with an average daily water consumption of 100 gallons per capita would have a maximum con-