drsteve wrote:
BB_TX wrote:
While remotely possible for a backflow to happen, the likelyhood of such an event is very very low. Would take a set of highly unlikely conditions for it to happen. As long as there is water pressure at the park faucet, it simply can not happen. If the shutoff valve to the flush connection was closed it can not happen. You could only get backflow if park water pressure were lost while you were in the process of flushing resulting in that loss pulling a vacuum on the water line. Even then the flush head would not be submerged in the black tank water and at most there could possibly be a bit of black water on the flush head. And then only if you had not yet actually run any water thru that head.
Having said that, many of us have removed those failure prone anti siphon valves, but have added an external anti siphon valve to the outside flush connection to maintain the integrity of the system.
The tank is vented, so the flush valve does not have to be open for backflow to take place.
I agree, the likelihood is still very low, but when you have millions of tank dumping events, it's going to happen.
Backflow means black water flowing backward into the public water source. And that can not happen unless you have the flush water valve open to run the flush. And then it can only flow backward if there is a loss of water pressure from the souce causing the water line to pull a vacuum on the line connected to it. As long as there is pressure at the public water source, that water can not flow backward valve open or not. Vent is immaterial.
