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. A_{1} is ready to discharge if bed No. 4 was the last one to fill, since, when that bed filled, the small bell D_{4} forced the siphon A_{1} open. Sewage therefore flows through siphon A_{1} into bed No. 1. As the sewage level rises in bed No. 1, the outlet siphon from bed No. 2, G_{2}, is locked by the air pipe from B_{1} so that bed No. 2 will be ready for the next dose. Also the air pipe from the bell H_{1} opens the siphon G_{4}, and allows bed No. 4 to drain into the outlet drain. Also bell D_{1}, when bed No. 1 is full, opens the siphon A_{2} through the connecting air pipe so that bed No. 2 begins to fill as soon as bed No. 1 is full. And finally bell C_{1} locks the siphon A_{1}, 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.