CHAPTER II
THE SETTLING TANK AND ITS CONSTRUCTION
As has been stated, a most effective preliminary step in the treatment of sewage is to pass it through a properly designed settling tank in order that the grosser solids and suspended matters as far as possible may be deposited there and finally disposed of separately from the liquid sewage. This partial removal of the suspended matters, amounting to about fifty per cent. in well-designed and carefully operated tanks, very materially aids in the final treatment of sewage on filters or on sub-surface irrigation areas by preventing clogging of the filters or of the piping in the irrigation system.
In connection with the larger settling tanks for hotels or institutions, it is sometimes advisable to pass the sewage first through a screen chamber before it is discharged into the settling tank, in order that the grosser suspended solids may be collected more easily than from the tank; but, as has been pointed out, screening of sewage is not necessary at small disposal plants, and in fact is not generally advisable owing to the continual labor involved in removing and disposing of the screenings, and no description of screening plants will therefore be given.
The old method of discharging sewage and house wastes into loose-walled cesspools on all occasions and under all sorts of conditions is rapidly changing, as is desirable. True, in certain locations, where ample area is available, where the soil is dry and porous, and where neither springs nor wells nor the soil near dwellings will be contaminated thereby, cesspools may be safely used. In other locations a small expenditure of time and money will provide the means by which nature’s processes of reduction of the organic matter in sewage may be carried on much more efficiently and satisfactorily than ever can be the case in a cesspool.
Fig. 1.—Plan of Settling Tank.
The scheme for properly disposing of sewage at any point should therefore include its sedimentation in a settling tank of proper construction and ample capacity, whether its final treatment is to be effected by sub-surface irrigation, intermittent sand filtration, contact beds, or sprinkling filters. Where the sewage effluent is to be discharged into a stream or body of water of comparatively large flow or volume, and where that stream is not subsequently used as a potable water supply, it is sometimes permissible to subject the sewage to settling tank treatment only. Such partial treatment, however, should be arranged for only as a temporary measure, and the tank should be so constructed with respect to the elevation of adjacent areas that works for final treatment of sewage, when required, may be constructed as advantageously as possible. Moreover, in the more progressive States, as noted in Chapter I, the purity of streams is being carefully safeguarded, and the general tendency of public health officials is to require more complete treatment of sewage before its discharge into a watercourse than is accomplished by settling tanks.
The settling tank for residences and institutions, as shown in Fig. [1], should have a capacity of from five to fifteen cubic feet for each person served by the sewer in order that proper time of detention in the tank may be allowed for the sedimentation of the suspended matters in the sewage. The depth of the tank should be from five to eight feet, and its width should generally be from one-third to one-half the length. Fig. [2] shows a longitudinal section of the settling tank and siphon chamber.
Fig. 2.—Longitudinal Section of Settling Tank.