Fig. 25.—Air-lock Siphon for Admitting and Releasing Sewage from each one of Four Beds in Regular Order.
Fig. [25] shows a method of securing the alternate discharge of sewage by siphons whose action depends upon an air trap, each siphon being of the type shown in Fig. [24]. The installation of the figure is further complicated by the fact that it is arranged to discharge sewage from the four contact beds as well as to discharge sewage onto the beds. The compartments, and the piping connected therewith, at the four corners operate to admit the sewage from the central channel onto the four beds in rotation. The four square wells between the corner wells operate to empty those beds in turn into the pipe shown at the centre of the drawing, the pipe leading to the nearest stream. The operation may be described as follows: Sewage enters at the top of the drawing, and from the inlet channel flows into the siphon channels marked A. A1 is ready to discharge if bed No. 4 was the last one to fill, since, when that bed filled, the small bell D4 forced the siphon A1 open. Sewage therefore flows through siphon A1 into bed No. 1. As the sewage level rises in bed No. 1, the outlet siphon from bed No. 2, G2, is locked by the air pipe from B1 so that bed No. 2 will be ready for the next dose. Also the air pipe from the bell H1 opens the siphon G4, and allows bed No. 4 to drain into the outlet drain. Also bell D1, when bed No. 1 is full, opens the siphon A2 through the connecting air pipe so that bed No. 2 begins to fill as soon as bed No. 1 is full. And finally bell C1 locks the siphon A1, and stops further flow into bed No. 1. The other beds operate in the same way in turn.
The manufacturers of siphons are always glad to advise prospective buyers of the proper arrangement of siphons and the details of placing, with dimension sketches.
As a summary, it may be pointed out that in any installation, one of the three methods above described may be adopted.
1. A simple valve worked by hand may be adopted and the alternate distribution of the sewage regulated by choice of the several valves placed at the head of the several discharge pipes.
2. An automatic discharge mechanism may be installed which will operate regularly and intermittently, but lacking any automatic selection of the bed onto which the discharge is to be made. These siphons will discharge as often as the tank fills, but the particular valve must be opened in order that the discharge may take place onto any one of the several beds.
3. An apparatus may be installed which will both discharge intermittently and will also automatically select different beds in turn onto which the discharge shall take place. It may even discharge onto contact beds and also empty those beds, entirely automatically.
Which of these mechanisms shall be selected depends upon the amount of money available and on the value to be placed on the freedom from constant care which an automatic installation gives. Not that a sewage-disposal plant may be ignored because an automatic mechanism has been installed. No machine is infallible, and sewerage machinery may give out or stop working just as that for any other purpose. But instead of a daily routine of duties which may not be interrupted, by means of automatic apparatus one may avoid everything except casual inspections and periodic cleaning.
CHAPTER IV
SUB-SURFACE IRRIGATION
The disposal of sewage by the method of sub-surface irrigation, sometimes known as the Waring system, consists in its distribution by means of open jointed tiling over a comparatively large area of soil and at a depth of a few inches beneath the ground surface. The sewage should first be passed through settling tanks to remove as much as practicable of the suspended matters contained in it, as explained in the chapter on settling tanks. The partially clarified effluent from the settling tank should then be collected in a dosing chamber, or separate compartment of the settling tank, and discharged intermittently, preferably by means of a siphon, into the sub-surface irrigation system. This intermittency of discharge of accumulated quantities of effluent is necessary for an even distribution of the effluent throughout the entire system of sub-surface tiling, and for a continuous and successful operation of the system as a whole. It has been found necessary, also, to alternate the discharge of effluent from the dosing or siphon chamber over different portions of the irrigation area. One siphon is all that is necessary to install in the siphon chamber for sub-surface irrigation systems, and if the settling tank has two compartments, as shown in Fig. [3], Chapter II, the single siphon would be placed in the centre of the chamber.