About 1850 the separate system was intro duced in several English towns. In 1875-76 a separate system of sewerage was built at Lenox, .Alass., and in 1880 a more extensive one was constructed at Memphis, Tenn. Both these were designed by the late Col. Geo. E. Waring, Jr. The Memphis system attracted great attention, owing largely to the yellow 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 necessary. 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 miaow 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 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 construetion permitting suet] 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 difficult 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.