Black tank plumbing question

[Skipg]

I know I have a black water vent on the top of the roof of the MH . Why do I also have an AAV (air admittance valve) under the bathroom sink? Aren't. they used for the same function? I'm confused.

[Bobbo]

Almost right. The AAV under the sink, on either the black tank or the gray tank, vents the sink's drain pipe after the P trap. If it wasn't there, as the water drained out of the drain pipe into the waste tank, it would pull a vacuum that would suck the water out of the P trap, allowing the tank fumes to come up through the sink drain.

[harold1946]

The AAV is for the grey water tank, to help prevent the backflow of fumes through the P-trap.