Alternating or Sequential Siphons are required when a septic system has multiple fields (aka, beds). Dual alternating siphons can alternate automatically without any kind of controller. Sequential siphoning with three or more siphons requires an electronic controller.
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1 Two siphons are set in a single tank at the same elevation. Both traps are primed with water (figure 3a).
2 As the tank fills, the siphons are sealed and the air in the long leg of each trap descends towards the invert of its trap (figure 3b; please see the 'Operation of Single Automatic Siphons'). The air displaces nearly half of the water in the trap.
3 If the siphons were set perfectly, they would both go into operation at the same time. Slight variations, however, are inevitable, and will bring one siphon into operation before the other (figure 3c).
4 The first siphon to operate will empty the tank and shut off with its trap fully primed with liquid. The siphon that did not operate will have lost nearly half of the liquid in it's trap as described in stage 2. Since it did not operate, its liquid was not replaced (figure 3d). The extra air in this siphon will cause it to go into operation next, resulting in alternating operation.