Modern sanitary engineering / [Gilbert Thomson].

  • Thomson, Gilbert.
Date:
1912
Catalogue details

Licence: Public Domain Mark

Credit: Modern sanitary engineering / [Gilbert Thomson]. Source: Wellcome Collection.

    58/296 (page 38)
    Discharging Capacity.—The velocity thus obtained is in feet per second, and the discharge in cube feet per second is obtained by multiplying this by the area, also in feet. A further multiplication by 64 gives (approximately) the dis- charge in gallons per second.* | Having in this way ascertained the size of pipe which would be necessary to deal with the rain-water, the question next arises whether a combined or separate system is best. It is seldom desirable to have a sewage drain from an ordinary dwelling larger than 6 inches, and when the admission of rain-water would make a larger pipe necessary, it is as a rule better to make at least a partial separation. By this means the sewage drains are kept small and are thus more easily kept clean, while the rain-water drains are not polluted by sewage at all. In such a case the latter may be made of material less costly than that which is used for the sewage drains. In the case of very large buildings and institutions it may be necessary, even when rain-water is excluded, to calculate the carrying capacity of the pipes, and a very liberal estimate of the flow must be made. The ordinary rule as to sewers— that the pipes should be able to take off all the sewage at double the average rate—is not applicable. When no provision 1s made for rain-water it would be well to allow for the maximum rate of sewage flow being five or six times the average. ‘The class of institution must be taken into account; in some it is necessary to avoid to a great extent the smallest size of pipes.t Possible extensions must be kept in view; and the separation is not always complete, and any rain-water which is admitted (or which might in the future be admitted) must be fully allowed for. There is one warning which is most important. However safe it may be to calculate the maximum discharging power in the manner described, the velocity thus calculated is entirely misleading when applied to the ordinary flow of sewage when the pipe is far from being filled. This will be more fully considered in the next chapter. * See Appendix, note B. + For example, in lunatic asylums where patients put all sorts of material into the closets. See p. 197.