Add Softener Drain to Drain Stack

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I need to add some water treatment to my house (water softener/iron filter) and the only good place for the equipment is in the basement.

I have a septic system and the waste stack exits out of the basement to the septic about 2 feet above the finished floor.

There is a clean-out just above the exit, then the stack continues vertically another 5 feet to the floor joists where it transitions to horizontal.

There is no floor drain to direct the softener drain to.

From what I understand reading through similar threads, and the NJ plumbing code, I should be able to add in a 3x3x2 sanitary tee followed by an AAV, trap, standpipe, and appropriate air gap. I would add it between the ell at the ceiling and the two 45s before the cleanout. There is enough room to cut out a section of the stack, lift it up, and place the new tee.

I think I could also use a threaded adapter on the existing clean-out and build out the standpipe as long as I kept a clean-out. Though option 1 is likely the better approach.

Is my thinking accurate? Or am I missing something?

I do understand this would now be the lowest drain in the system, unfortunately, I don't see any good alternatives. The laundry room is two floors up.

I could potentially add a drain to the horizontal part of the main stack, but I would only have roughly 10" of space for the trap, vent, standpipe, and airgap in the joist space. Which wouldn't allow the minimum of 18" for the standpipe.


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Is my thinking accurate? Or am I missing something?
I think you have covered it nicely. You may want to support the standpipe with a clamp in case the drain line gets tugged.

I think 1.5 inch PVC would be good for the standpipe. They sell air gaps that will fit into 1.5 inch.
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