[mtlakejim]

[QUOTE=BB_TX;1206595]That is not true. The purpose of an anti siphon atmospheric vent is to break the vacuum to prevent the siphoning of water back thru a pipe.



The problem with the theory above is your forgetting that the black tank flush line is supplied via its own separate hose fitting. It is only connected (via a special hose I hope) to the supply water for the few minutes it takes to wash the tank. The line is only used for a few minutes of MONITORED use. And at all the parks I have ever visited, the flush water was on a dedicated line with a sign indicating non potable water which means it doesn't matter if the black tank water did siphon back to it. And that is the reason you should have a dedicated hose for flushing the tank (I even got one that was black IE: color coded).

Additionally for the water to siphon back out of the black tank it must be above the level of the spray nozzle in the black tank (which is usually mounted very high in the tank).

The two above facts make it all but completely impossible for the black water to ever contaminate your fresh water supply. You should be more concerned about being struck by lighting a mile deep in a cave.

But if you want to be silly about it, it makes more sense to add a check valve on the back of the hose fitting than to worry about the anti-siphon valve. Considering the very real possibility that the lowest bidder cheap plastic anti-siphon valve will fail and lead to serious water damage I for one am in the bypass it camp!!