clock menu more-arrow no yes mobile

How to Install a Reverse Osmosis Water Filter

This Old House plumbing and heating expert Richard Trethewey removes the impurities, VOCs, and excess minerals from water with a simple reverse osmosis water filter system.

Q: Our town water department just informed us that the water supply is contaminated with too high a level of PFAS. Would a reverse-osmosis water filter be a good idea? —KRIS DHUPAR, NORTH ATTLEBORO, MA

You’re right to be concerned: Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a family of toxic, human-made chemicals that are hazardous to ingest.

One of the best ways to remove these contaminants from drinking water is with a point-of-use, reverse-osmosis (RO) filtration system. Some units, like the GE one shown above (, are compact enough to fit under a kitchen sink. (I don’t recommend whole-house RO systems because of their size, expense, and maintenance requirements.)

Reverse Osmosis Water Filter Components

Richard Trethewey Reverse Osmosis Aleese Gooden

Reverse osmosis has three stages of filtration. A prefilter (A) traps sediment that can make water cloudy and odorous. Activated charcoal (B) takes out most of the volatile organic compounds (VOCs), heavy metals like lead and cadmium, PFAS, and other chemical contaminants.

A third filter (C) contains layers of semipermeable membranes that trap whatever gets past the charcoal, so only purified H2O comes through. Because it takes time for water to make its way through all these filters, the system shown here has a small holding tank (D) to keep some liquid ready for use.

RO filtration also removes beneficial minerals, which give water its taste. But those can be added back with a remineralizing cartridge (E), which restores water’s flavor before it reaches your glass.

How to Install a Reverse Osmosis Water Filter

  1. Start by shutting the valve to the cold water under the sink. Use the adjustable wrench to remove the hose stretching from the valve to the faucet (only remove it from the shut-off valve).
  2. Install the brass tee fitting that came in the kit. Use the wrench to tighten it onto the shut-off valve and reconnect the sink hose to the top side of the tee. Install the smaller plastic tubing to the tee using the included nut, compression fitting, and sleeve. Tighten the hose onto the tee with the adjustable wrench.
  3. Install the manifold. Hold it against the side of the cabinet and mark the holes with a pencil or marker. Pre-drill holes at these marks and thread screws into the holes with the screwdriver. Repeat the same process for the remineralizer, offset so that it won’t interfere with the manifold. Hang both the manifold and the remineralizer brackets from the screws and tighten them in place before installing the filters.
  4. Wrap the threaded nipple on the top of the tank with 4 or 5 passes of Teflon tape before installing the tank connector (the white plastic elbow). Hand tighten this fitting over the Teflon before pushing the larger white plastic tubing all the way into the nut before tightening with the wrench. Place the tank in the corner of the cabinet.
  5. Insert the rubber gasket in the bottom of the faucet base. Attach the larger red hose to the larger barb fitting, and the smaller red hose to the smaller fitting. Feed the tubing through a hole in the sink or cabinet, and place the spacer onto the faucet from underneath before threading the nut and tightening with a wrench. Attach the included hose connecter to the threads and push the large white hose into the fitting.
  6. Place the foam seal on the drain pipe and place the saddle over the hole, tightening with the wrench. Drill a 7/32-inch hole into the pipe, but do not drill through both sides.
  7. Connect all the tubing according to the diagram, turn the water on, and check for leaks.


Richard installs a water filter (OptimH2O® Reverse Osmosis + Claryum®) under a sink. After turning off the cold water by turning the valve under the sink, he disconnects the cold-water line with a wrench. Richard then installs the brass “T” by hand screwing it to the cold-water line. The system comes with white ¼” and ⅜” tubing to connect the filtration cartridges and water tank to the water supply. Using a hand drill, Richard makes holes on the manifold to add mounting screws.

Richard removes an old sprayer hose from the sink to install a dedicated system faucet. He uses the ⅜” and ¼” red tubing to connect to the cartridges and the water tank. Richard positions the connector above the trap and drills a 7/32” hole into the drainpipe. He checks that all tubes are fully pushed in before turning the cold water supply back on.

Click here to watch the install video from Aquesana.