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I have a single hole sink with the dishwasher drain going into the disposal

The wall plumbing has the P trap coming from the disposal into the wall drain. There is an air vent with a P trap entering the wall drain directly BELOW the disposal p trap in the wall drain.

I'm constantly getting back flow from the lower vent trap. Originally there was a double sink and the dishwasher drain hooked into this vent trap. I'm wondering if this vent trap shouldn't be above the disposal trap since I now only have a single sink and there isn't a vent above the disposal trap. It seems it would have gotten air from the second sink before and now there is no air above. Does this sound logical?

When I get back flow it's when I run the disposal and there is some water running.

pic of plumbing

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    The unused ptrap should be removed and the pipe capped. There should be a vent for the upper drain coming off of the sanitary tee in the wall, is there ?
    – Alaska Man
    Jun 2, 2020 at 17:16
  • I don't know what's behind the wall but I don't see a vent, I was thinking the other sink with two drain was the vent. It just doesn't seem right that the vent here is the lower p trap. I feel that it should be capped but not sure.
    – Danny Nabors
    Jun 2, 2020 at 18:52

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My question would be how do you keep water in the trap for the vent? , I would add an air admittance valve then you won’t have to worry about water in the trap and won’t get back flow through the vent. The air admittance valve on the open end of the pipe.

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    I don't think it is good idea to let water stagnate in the old unused Ptrap. If it is down stream of the sanitary tee than dirty water can flow into it. But we can not see in the wall.
    – Alaska Man
    Jun 2, 2020 at 17:18
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    Plus, it's not a vent if it has a trap on it.
    – isherwood
    Jun 2, 2020 at 18:23
  • I agree with both + but I was thinking the trap was drying out and an AAV glued in up there would Stop the issue of back flow.
    – Ed Beal
    Jun 2, 2020 at 18:41

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