I have an RO system that I need to drain wastewater from - so this will only be water, not gray/blackwater. The closest place to drain it into is an unidentified rough-in pipe that I'm trying to determine the viability of.

In the following images I'm relatively confident in A/B/C being sink/toilet/shower. The unknown, and the subject of this question, is 10+ft away from these and only a couple inches off the wall.

Ideally I can just drill a 1/4" hole in the cap of the unknown white pipe and run the 1/4" drain hose into it...

rough in plumbing

rough in plumbing

  • 1
    "Ideally I can just drill a 1/4" hole in the cap of the unknown white pipe and run the 1/4" drain hose into it..." That's a recipe for sewer gas. You need trap between your RO drain and the sewer. It's not optional. Or you can drain it into a sump, if you have one.
    – Ecnerwal
    Commented Aug 26, 2023 at 17:41
  • 2
    Perhaps you should identify the pipe's destination, positively first. Commented Aug 26, 2023 at 21:41
  • @RohitGupta at the risk of sounding confrontational, what do you think I'm trying to do with this post if not exactly that?
    – mac
    Commented Aug 27, 2023 at 23:40
  • @Ecnerwal this pipe doesn't seem to be connected to sewer/waste/drain. That is all the black ABS pipe labeled in the A/B/C image. This pipe is a different material, a different size, and located in a different spot. It is also clean inside with no foul smell (I popped the cap off and took a look).
    – mac
    Commented Aug 27, 2023 at 23:42

1 Answer 1


That 3"/4" white PVC pipe in your first picture looks like it's a rough in for a radon mitigation system. If so, that means it just extends below the basement slab a bit, and is not meant to carry water.

  • radon was my best guess so far as well. I popped the cap off of it and it looks like there's a "T" at the bottom with one leg going down the wall towards the sump pit and the other going out the exterior wall to somewhere unknown. Interestingly, the sump pit has a white pipe about the same size popping out the side of it into the pit. I had hoped these were connected, and they may still be, but I dumped a gallon of water down it and it DID NOT come out into the sump pit.
    – mac
    Commented Aug 27, 2023 at 23:35
  • "going down the wall to the sump pit". That would make sense for a radon mitigation system, as sump pits tend to be high sources of radon gas, if your ground is prone to that.
    – SteveSh
    Commented Aug 28, 2023 at 0:22
  • It is a radon prone area, and in fact I have a radon mitigation system in place that's ran into the sump pit itself. Still though I find it odd that water didn't reach the pit when poured in the opposite end, if this is in fact what it is.
    – mac
    Commented Aug 28, 2023 at 4:22

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