SEWERAGE. 23 SEWERAGE. sumption of 200 gallons, of which 150 gallons would reach the sewers. Under very unfavor- able circumstances infiltration of ground water has been estimated as equal to the flow of sewage proper, but design and constructiipii permitting such a condition should never be tolerated. Un- der normal conditions of both consumption and infiltration the extra volume on the latter ac- count may be taken at 15 per cent, of the as- sumed sewage flow. In round numbers, then, the capacity of separate, sanitary sewers should be 175 gallons per capita per day. The amount of rainfall for which provision must be made is a more dillicult problem than might appear at first thought. There must be determined, first of all, the maximum rate of rainfall during comparatively brief periods, and next the percentage of the total which will reach the sewers at the same moment. As to the percentage of rainfall reaching the sewers in a given time much will depend upon the per- meability of the soil, the proportion of roofed and paved to the total area of the district, and the slopes of the area. The general practice is to base the calculations on the rate of rainfall per hour. An old rule for populous districts was to make the sewers large enough to carry away a rainfall of one inch per hour. The more re- cent short-period observations show that far higher rates may reach the sewers. Material, Size, and Slope of Conduits depend largely upon whether the separate or combined system is adopted. Vitrified clay or terracotta sewer pipes (see Pipe) are now almost universal- ly vised for all sewers up to 24 inches, and some- OUTLINES OF VARIOrS SHAPES OR CROSS-SECTIONS OF SBWEES AND DRAI.NS. times up to 30 inches in diameter, whether the system be separate or combined, but in the com- bined .system there is comparatively little oppor- tunity for using the smaller sizes of conduit. Cement pipe is also used in a few cities. Where vitrified clay pipe cannot be used, and iron pipe is not required for its greater strength or tighter joints, brick is the material most commonly em- ployed. The size of pipe sewers, in the separate sanitary system, ranges from 6 to 30 inches, but 8 to 24 inches is a more common range. The C or 8 inch pipe is used for laterals and for conduits receiving the sewage from a few laterals. Iron sewers may be used up to 5 feet or more in diameter, but they rarely go above 3 feet, and are not often employed in any size, because of cost. There is practically no limit to the size of brick sewers. In the large combined sewers stone masonry is occasionally and concrete fre- quently substituted for brick, particularly in the lower part, or invert, of the sewer, and on heavy grades, where the scour due to high velocities and street sand and other dirt is likely to wear the brick. Both wood and steel have been used for large outfall sewers, especially for submerged pipe. Crossings beneath streams are frequently made by means of .so-called inverted siphons. ■■Iron Coyer For Pipe Sewer. For Brick Sewer Manholes. Terra-cotta, iron, and wooden pipe are generally round; brick and concrete are given various shapes, depending largely upon the available grade. A'here feasible, all large sewers, other than iron, are smaller at the bottom than the top, in order to concentrate the dry-weather flow and diminish the chances for stoppage. AccESSORiE.s include manholes, or chambers giving access to the sewers from the street ; lampholes and hand- holes, for inspecting WA feii^rt/rffcOn'e*' and cleaning separate sanitary sewers; flush tanks for suddenly re- leasing a supplementary volume of water; catch or inlet basins, for the admission of surface drainage to combined or storm sewers. The lat- ter are generally at the curb line. Such deposits as cannot be flushed out of the sewers must be removed from time to time by passing a ball, scrajier, or other device from manhole to manhole. Automatic Flush Tanks are provided in many sewers of the separate type. They are Catch or Inlet Basin, 'nitsnm'tui!' fT^nmimTW!^:- ArTOMATIC FLUSH TANK. chambers for the storage of water, with means for its sudden discharge down the sewer. The discharge is generally effected by means of a
