I have purchased a Whirpool Reverse Osmosis Water Filter which installs under the sink. I am trying to install it and have run into an issue with the drain: the revers osmosis filter has a red tube which needs to be connected to the drain of the sink, so that the waste water from the RO system can be disposed of. The RO filter kit comes with an adaptor to install the drain, and the manual says that for a double-barrel sink (which is what I have, the drain should be installed as follows:

Scan of Whirpool Manual

However, I don't have enough room to install the drain adapter in the location they indicate (I have added a freehand circle for clarity). Here is an actual picture from under my sink:

Picture from under my sink

The inset in the bottom right shows the drain adapter (on perfect proportion to the rest of the image). As seen in the indicated areas, the drain adapter, if installed under the right sink, is too tall and I will be unable to connect the right sink's drain to the P-trap.

How else can I install the drain for my reverse osmosis water filter? Can I somehow hook it up to the drain from the dishwasher? What is the cheapest and easiest way to install this RO filter without hiring a plumber?

EDIT: To clarify, I want to do this in a non-destructive way if possible. I.E. no cutting the existing pipes :-) I need to be able to remove the filter and restore the plumbing back the way it was if I move out.

4 Answers 4


I don't see any problem with installing it horizontally on the run between the two sinks. You'll probably need a new slip nut (the part that goes on top of the trap) but otherwise it can probably connect into the existing union that's there now. Just keep the piece that goes to the RO on the top (facing up).

  • Doesn't this presume that the wall thicknesses are the same? What will it mean if they are not?
    – jcolebrand
    Commented May 2, 2011 at 2:05
  • The wall thickness of the pipe doesn't really matter; just the outside dimensions (or inside dimensions, for couplings/elbows/etc). If the thickness does increase at some point along the downstream length of the pipe, in theory a small amount of water could pool up behind it. In general, this is not enough to be a problem, and in this case it most definitely isn't because the run is so short.
    – gregmac
    Commented May 3, 2011 at 16:25

If you can find a way to lower the trap and remount it closer to the floor, then you should have room to install the new piece. An alternate approach is to install a second trap that leads to the right (it can share the same vent as the existing by splicing in under the existing trap with a 90° piece that's about 2 or 3 inches tall) and then you can put a cap on the top of your new p-trap after you put this piece in. That would definitely require the least amount of remodeling, but would be the safest as far as knowing that it would work, and would also cost the most (probably) since you would have to buy all the parts for a new p-trap.

I would probably go that way since it would be easier.

Example photo to demonstrate what I mean for the alternative option


There are drain line adapters that provide an air gap and allow you to add an RO system without drilling or cutting into any pipes. image https://www.premierh2o.com/products/premier__164020_dishwasher-_garbage_disposal_drain_line_adapter

I have used them in previous apartments where I could not change the plumbing. The long end of the barb is trimmed with a hacksaw or coping saw to fit your dishwasher drain line and the RO tubing goes into the quick connect fitting on the top.

Make sure to size the port correctly, as RO tubes come in different sizes depending on the brand. I have made the mistake of buying a 3/8" adapter when my RO kit uses 1/4" hose. They're usually around $20 but install easily and can go with you when you leave, as opposed to drilling a hole in the plumbing and having to seal it or leave the whole filter behind.


My reverse osmosis kit came with a drain adapter that looks like this:

drain adapter

It is kind of a cuff that wraps around the down pipe and you drill a small hole through the pipe, then slide the drain line in. There is a compression fitting at the hole to make it tight. This is by far the easiest way to get the drain tube installed.

I know it involves drilling a hole, but in all honesty to get the drain adapter you show in your pictures on and off the plumbing you are going to do at least as much messing around with the pipes and a lot more work. Drilling the hole is the easy solution. If you remove the system you just plug the hole.

  • Tyler, I wish I had seen that adapter yesterday BEFORE I rebuilt my entire sink drain, LOL! Trying to fit the PVC arm adapter provided with the kit to existing black ABS, I uncovered previous owner's half-assed repairs (what a mess!) so I wound up scrapping the whole ABS lot, all the way up to the gunked-up, wrench-shredded sink drain itself. The compression-fitted adapter you show would have taken 10 minutes, compared to all day/multiple trips to hardware store/$$$ as I uncovered each new glitch. Ah, well, we now have a nice, easy-to-work-with PVC drain, and I learned more about plumbing than
    – user34167
    Commented Feb 27, 2015 at 12:50

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