I have a Cuno SQC3 reverse osmosis system. The three obvious filters for replacement on the left are the:

  1. prefilter - 47-55706G2
  2. RO membrane - 66-4703G2
  3. postfilter - 47-55710G2

However, in this picture you see there is a fourth module inline with the water line (on the ground). It doesn't have a identifier on it. Just says "flow" to show direction of water. Does this need to be replaced? Currently my water is coming out very slowly with the filters being 2 years old.

enter image description here

  • you do not say where it is placed in the system .... if it is at the inlet, then it probably filters out large particles
    – jsotola
    Jan 25, 2018 at 19:58
  • It's either an additional prefilter as @jsotola indicates or a final post fine carbon filter "for taste", which theoretically shouldn't be needed for RO water, but the water filter companies like to sell overpriced media, and I've seen post carbon filters installed. We have a local company that services air/water filters in people's homes--Basically all replaceable media and softener salt for a re-curring monthly charge autozapped out of your checking account--they way over install water filtration, and are always up-selling add-ons.
    – Tyson
    Jan 25, 2018 at 20:13
  • What, if anything, is written on it? Looks like there's at least a bar code, obscured by the power cord, maybe more.
    – mmathis
    Jan 25, 2018 at 20:46
  • There's nothing written on it except "flow".
    – Tom
    Jan 25, 2018 at 21:59

2 Answers 2


The fourth filter is an inline carbon polishing filter for removing any taste/odor that was imparted into the water by sitting in the water storage tank.

Yes, it should be replaced at the same time as your pre- and post- carbon filters. You don't need it, so if you don't want to replace it then remove it from the system. You might not notice any difference in the taste/odor of your water.

If it is installed correctly, the blue 1/4" water line coming off the right side of the fourth filter should go directly to your RO faucet.


I'm thinking the 4th filter looks like it might go to the refrig. If so eliminate it if it's connected to the RO

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.