Reverse Osmosis for Well Water: When You Need It & How to Set It Up

Yes, reverse osmosis works on well water, as long as it sits under the kitchen sink for drinking and cooking water and the main water supply is cleaned up first. A caller in Florida had a whole-house RO that was not keeping up with his well water. I told him, "once you clean everything up in the main water supply, you can put a reverse osmosis under the kitchen sink for your drinking water, cooking water, water for coffee, whatever."

Two things decide whether it works on your well. The first is what goes in ahead of it, in the order shown in the picture. The second is whether your well pressure is enough to push water through the membrane, which on a well set at 30/50 is a fair question.

What to install ahead of the RO

An RO on a well needs clean water going into it, so the equipment your test calls for goes in ahead of it. The MAW-50's maker allows no more than 0.3 ppm of iron in the water going in, and none of the rotten egg gas. Most rows in the table below start from a number on a lab report, so the test comes first.

No lab report yet? Start here

One test shows what goes ahead

One lab test, mailed to you, run by an accredited lab, reports the iron, manganese, hardness, cloudiness and bacteria most of this table keys off. If you already have a lab report, skip this.

Get the well water test

$199. One sample, 53 results.

The full list of what each of our two units allows, side by side, is in what ruins an RO membrane.

The order is the one in the picture at the top. After the well's pressure tank, the sediment filter goes first, then the iron filter, then the softener, and the UV light goes last on the main line. The RO goes after all of it, under the kitchen sink. If your pH is low, an acid neutralizer goes in too, and I set its place from your test. My guide to the correct order for well water treatment explains why each one sits where it does.

Does reverse osmosis remove iron?

An RO membrane is not an iron filter: the MAW-50's maker allows only 0.3 ppm of iron in the water going in, and the Pure-75's maker 1 ppm. A caller whose RO kept clogging asked me what he was doing wrong. I told him, "You can't take iron out with reverse osmosis. You have to use a dedicated iron filter. Iron will clog the membrane of a reverse osmosis system."

An iron filter on the main line also takes the iron out of every other faucet, the washer and the water heater, not just the kitchen tap. My guide on how to remove iron from well water compares the ways to do it.

Have your lab report?

Send Aidan your numbers

Text a photo of the report. Aidan will tell you what goes ahead of your RO, and in what order.

Text Aidan443-277-2204 Call800-460-5810Aidan

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Is your well pressure enough for an RO?

A well on a 30/50 switch gives an RO the low end of the pressure it is rated for. Both of our under-sink units are rated for 30 to 100 psi, and the MAW-50's 50 gallons a day is measured at 60 psi. A 30/50 switch turns the well pump on at 30 psi and off at 50 (the example setting in Schneider Electric's switch catalog), so the gauge at your pressure tank swings between those two numbers all day, and every filter or softener after the tank takes a few psi more off what the sink sees while water runs. The gauge shows where yours runs.

Pressure is what pushes water through the membrane. The University of Nebraska's guide to home RO says the feed pressure "must be adequate to overcome the osmotic pressure and any back pressure generated from the storage tank." DuPont, which makes FilmTec membranes, rates its 50 gallon-a-day home membrane at 50 psi, and its own formula has the output rising and falling with the pressure across the membrane. I told an RO buyer in Maryland the same thing in plainer words: "It all depends on the temperature of the well water and the pressure going into the system."

The tank pushes back, too. Pentair's manual for its own under-sink RO says its shut-off valve stops the unit when the tank reaches 2/3 of your household pressure, which works out to about 20 to 33 psi in the tank on a 30/50 well. Watts, another RO maker, says in its manual that "A booster pump is needed for incoming water pressure under 40psi," and our own MAW-50 page recommends 40 psi or more. So on a 30/50 well an RO still works, but it makes less water than its rating and refills the tank more slowly.

The first thing to try is the switch itself. If you already have a filter or softener behind the pressure tank, my guide to the well water pressure switch explains moving a 30/50 switch to 40/60 and resetting the tank air to match, as long as your pump can make 60 psi. If it cannot, or the RO is still slow, there is a pump for it.

Which pump fixes low RO pressure

There are two kinds of pump for this. A permeate pump has no motor: the water the RO sends to the drain runs it. A booster pump is electric. Aquatec, which makes both kinds, puts the difference in one line: "The Booster Pump increases the feed water pressure to the membrane, while the Permeate Pump is located after the membrane and eliminates the performance robbing back pressure created by a full, or filling, air charged storage tank."

Read together, the makers' numbers point one way. Under 30 psi, our two units and Aquatec's permeate pump are all below their makers' minimum, so that well needs a booster pump. Between 30 and 40 psi, either pump helps, and only the booster needs an outlet under the sink. We do not sell either pump. Aquatec says its permeate pumps can be added to an RO you already have. Text me your switch setting before you buy one for ours.

Should you put RO on the whole house?

A whole-house RO needs a big storage tank and its own pump, and we do not sell one. The tank holds 80 to 500 gallons, a pump sends the water back into the house, and the drains have to take what it sends away. A caller in Maine with a 10-person rental asked me whether his treated water would go into a holding tank. I told him, "They only do that after a whole house reverse osmosis system, which is a nightmare, and you have to have a lot of room for it."

The University of Nebraska puts the cost side plainly: whole-house units are "more costly to purchase, install and operate" than under-sink ones. On a well there is one more cost. If your house is on a septic system, the water an RO sends down the drain goes into it, which Nebraska says adds to its load, and the EPA says a typical under-sink unit sends five gallons or more to the drain for every gallon it makes. Here is what the makers' own whole-house systems list:

It does not skip the pretreatment either: Crystal Quest builds prefilters into every system, and US Water Systems says well water needs a lab test before it recommends a system. If you want treated water at every faucet, treat the whole house for what the test finds and put the RO at the kitchen sink, the setup I gave the Florida caller in the opening. One buyer who did it that way wrote, "I also put a small reverse osmosis under the sink just to make sure that the drinking water is perfect." (Stuart S., March 2020, an acid neutralizer and softener package Verified buyer)

Which of our two ROs fits a well

Either of our two under-sink units works on a well once the treated water meets its maker's limits. The Pure-75 takes more iron: 1 ppm, against 0.3 for the MAW-50. Neither has a pump or a system certification, and both run on your water pressure alone.

The MAW-50 is $275 with 4 stages or $325 with 5, and is rated at 50 gallons a day at 60 psi. The Pure-75 is $595, with cartridges that twist off. When a caller with our iron filter on his well asked about one, I told him, "the Pure 75 is a good model, and it's easy to service 'cause you don't have to shut the water off." For what either one takes out, see what reverse osmosis removes.

What ships: the RO unit with its cartridges, the 3.2 gallon tank, the faucet, the tubing, the drain saddle and the valve that taps your cold water line.

Setting up an RO on a well

Setting one up on a well is the same job as anywhere else, with two checks first: the lab results and the pressure.

1. Test the well, and put in what the test calls for on the main line. See what to install ahead of the RO.
2. Read the gauge at your pressure tank through one pump cycle. If it runs 30 to 50, read the pressure section before you choose.
3. Pick the unit whose limits your treated water meets. See which of our two fits.
4. Install it under the sink. The valve taps the cold water line, the drain saddle goes on the sink drain, the faucet goes in a hole in the sink or counter, and the tank sits in the cabinet. It can also go in the basement with extra 3/8 or 1/4 inch tubing up to the faucet, and you can tee it to the fridge.
5. Test the water from the RO faucet after it goes in, then check it now and then with a TDS meter.

I recommend having a plumber do the install, and I'm a phone call away for your plumber. If you'd rather do it yourself, text me with any questions. Filters and the membrane are covered in RO maintenance.

Frequently asked questions