Reverse osmosis for well water

Best for

Wells with a health-based drinking water problem, such as nitrate, arsenic, uranium or lead, where treating only the kitchen tap is enough.

Wrong fit

Homes whose real problem is house-wide: iron stains, hardness, sulfur smell or bacteria. Those need whole-house equipment, and bacteria need disinfection.

Tradeoff

It wastes water, produces treated water slowly, and needs regular filter and membrane changes, plus a test to confirm it is working.

How reverse osmosis works

A reverse osmosis (RO) system uses water pressure to force water through a semi-permeable membrane. Water passes. Many dissolved contaminants don’t, and they are rinsed off the membrane into a waste line.

Connecticut’s Department of Public Health (DPH) describes a complete system as an RO module, a storage tank and a separate faucet. Most units add a sediment pre-filter to protect the membrane and an activated carbon post-filter to remove organics and gases the membrane lets through.

Because RO is slow and sends part of the water to the drain, Connecticut DPH says it is not practical to treat all the water entering a home this way. On a well, RO is a point-of-use system for drinking and cooking water, usually under the kitchen sink.

What RO removes, and what it misses

The left column combines Connecticut DPH’s list with the Minnesota Department of Health’s (MDH) treatment table. Removal is never total. Connecticut DPH warns that RO membranes cannot be certified to remove 100% of anything, and performance depends on the membrane, water pressure and contaminant level.

Health problems RO is used for

These are the drinking water results that most often lead well owners to RO. The limits are EPA’s for public water systems, used as benchmarks for private wells.

For nitrate, EPA warns that infants under six months who drink water above the limit could become seriously ill. Call your state health department about any result over these limits. See nitrate, arsenic, uranium and radon and lead.

For lead and copper from corrosive water, Penn State Extension lists RO as an acceptable point-of-use option. It also recommends fixing the pH so the whole house stops dissolving metal.

RO vs other drinking-water options

RO isn’t the only point-of-use choice. The Minnesota Department of Health (MDH) compares several in its home treatment guide:

MDH estimated point-of-use RO at $300 to $1,500 and distillers at $300 to $1,200, based on 2017 to 2018 quotes. See well water filtration system cost for 2026 ranges.

For nitrate, RO is a common choice because carbon won’t remove it. For arsenic, ask for the unit’s arsenic certification and test the treated water. For lead from corrosive plumbing, fix the pH as well, so the rest of the house stops dissolving metal.

Pretreatment and treatment order

RO is the last step, at one tap. Everything house-wide happens before it:

1. Sediment filter.
2. pH neutralizer, if needed.
3. Iron, manganese and sulfur filter. See whole house well water filters.
4. Water softener or tannin filter, if needed.
5. UV, if bacteria are a concern.
6. RO at the kitchen sink.

Connecticut DPH notes high levels of particulate matter, such as iron particles, can foul or clog the membrane, and MDH says RO may require pretreatment to prevent clogging. Hardness and iron that reach the membrane shorten its life.

Water waste and septic systems

RO always sends some water to the drain. Connecticut DPH’s fact sheet says about 75% of the water fed to a typical unit is wasted: four gallons in for about one gallon of treated water. That figure is from 2009, and wastewater varies by make and model, so check the spec sheet.

If the waste line goes to a septic system, Connecticut DPH strongly recommends keeping RO waste below 30 gallons a day. On a well, wasted water also means more pump run time.

Install needs

• Under-sink space for the filter housings and, on tank models, a storage tank.
• A cold water supply line and a drain connection for the waste line.
• A separate faucet, or a faucet designed for RO.
• Adequate pressure. Connecticut DPH notes household line pressure usually drives RO, but it may need to be increased for good performance. Low-pressure wells may need a booster pump.

Maintenance

• Replace pre- and post-filters on the manufacturer’s schedule.
• Replace the membrane when output drops or test results rise. Clogged or torn membranes need replacement.
• Budget for it. MDH estimates point-of-use RO maintenance at $100 to $200 every one to two years, based on 2017 to 2018 quotes.
• Test raw and treated water. Connecticut DPH says to test both after installation at a state-certified lab, then annually, or quarterly to semi-annually if contaminant levels are high. MDH says to test arsenic-treated water periodically.

Certification and the SpringWell option

For RO, look for a listing under NSF/ANSI Standard 58, which NSF’s database lets you search. Check the specific contaminant claim you need, such as nitrate, arsenic or lead, not just the standard number.

SpringWell sells a MOEN 4-stage under-sink RO system. SpringWell’s page says it reduces contaminants including arsenic, fluoride, TDS and lead. It was listed as out of stock when checked. Its page did not show certification details, so look the model up on the official NSF, WQA or IAPMO listings before relying on a specific claim, especially for bacteria, which Connecticut DPH says RO alone should not be used to treat.

Questions to ask before you buy

• Which contaminants is it certified to reduce, and to what level? Check the listing, not just the box.
• How much treated water does it make per day?
• How much water goes to the drain for each gallon of treated water?
• What do replacement filters and membranes cost, and how often are they due?
• Does it need a booster pump at your well’s pressure, or other pre- or post-treatment?

These follow Connecticut DPH’s list of questions to ask. It also suggests being wary of exaggerated claims and confirming the system was tested and certified by a third party.

Who should skip it

• House-wide nuisance problems. Iron, hardness, sulfur odor and sediment need point-of-entry treatment.
• Bacteria. Shock chlorinate, fix the well and use UV or chlorination.
• Radon or hydrogen sulfide. Gases pass through the membrane.
• Homes with failing septic fields and high water use, unless the waste stream is small.
• Anyone who won’t retest. RO performance fades quietly as the membrane ages.

To check which treatment your numbers point to, enter them at /test-results/.