Fig. 42.
Fig. 43.—View of Diverting Manhole.
In large installations for cities and villages it is usual to install either plural alternating siphons or apparatus known as sewage feeds, by means of which the contents of the dosing chamber are discharged upon the different beds in rotation, there generally being four or five beds constructed in each unit. This requires a separate siphon or sewage feed for each bed, and entails considerable expense. However, for smaller plants such as are now being considered, two ordinary siphons may be placed in the same dosing chamber as described in Chapter III, and so primed as to discharge alternately. Then, by means of a diverting manhole or chamber through which the dose must pass, the effluent may be diverted onto two beds in rotation, allowing a third bed to rest, or, if there are five beds, it may be diverted onto two pairs of beds in rotation, allowing a fifth bed to rest. For instance, in the case of five beds, a diverting manhole may be constructed as shown in Fig. [45], and arrangements may be made to couple bed No. 3 with No. 2 or No. 4, allowing bed No. 1 or No. 5 to rest by means of the stop-plank to cut off the flow to either of these beds, as shown in the illustration. Then, when bed No. 3 is to be rested, stop-planks A and B are both closed, and the stop-planks against all pipe outlets are raised. If it is desired to throw bed No. 1 out of use, the stop-plank is placed against the end of the pipe leading to this bed, stop-plank A is raised, and stop-plank B is lowered. One siphon will then discharge onto beds Nos. 4 and 5, and with the next filling of the siphon chamber the second siphon will discharge onto beds Nos. 2 and 3. By a proper combination of the stop-plank positions, any two sets of two beds each may receive alternately the discharge from the siphon chamber while the remaining single bed may be left resting. The method for operating the beds in rotation described above may, of course, be easily applied when only three beds are constructed. A provision for allowing one bed to be thrown out of use for a week or so at a time is very necessary for the reasons stated above.
Fig. 44.—Plan of Diverting Manhole.
At intervals of several weeks it will be found necessary to break up the surface of each bed by raking or else to remove a thin coating of clogging material. This should be done after the bed has been rested and dried out, when the surface matting may be taken off without removing much sand. To provide for operating the beds in winter, in the late fall, before the ground has frozen, ridges and furrows should be formed on the surface of the beds, similar to those shown in Fig. [51]. The furrows should be two or three feet apart and eight to twelve inches deep. Then when effluent is discharged onto the beds in freezing weather, as it fills the furrows, an ice roof will gradually form, spanning the furrows and protecting the sides and bottoms of the furrows from freezing, especially if a snowfall occurs before severe weather sets in. It will sometimes be found necessary, especially with small beds that are well underdrained, to provide board coverings for the furrows to take the place of the natural ice roofs.
Fig. 45.—Five-way Diverting Manhole.
The effluent from the tank should be discharged in such quantities as to flood the entire bed to a depth of from one to two inches, except that some of this effluent will immediately begin to seep into the bed.