Skip to content
WellWise
Treatment plannerSymptomsContaminantsEquipmentTesting

Reverse osmosis for well water

Updated 2026-09-15

Pushes water through a membrane fine enough to reject dissolved salts and most contaminants. Under the kitchen sink it is the most capable treatment available for water that is going to be drunk.

Why
Some contaminants, nitrate and arsenic among them, are not removed by any of the whole-house stages. RO is the practical residential answer for them.
Watch out
It depends entirely on the stages before it. Hardness and iron foul membranes quickly, so RO is the end of a train rather than a substitute for one. Where the driver is a health contaminant, match the system to that specific contaminant and confirm with a certified lab.
Typical cost
$250–$900
Maintenance
Prefilters and postfilters a few times a year, and a membrane every two to five years depending on feed water. It also sends several gallons to drain for every gallon produced.

What it removes

  • Nitrate
  • Arsenic
  • Dissolved solids and salts
  • Fluoride
  • Many dissolved contaminants

When it does not work

Every one of these is a condition under which this equipment underperforms or fails outright, however well it is installed.

  • Feed water is hard or iron-bearing and untreated. Scale and iron foul the membrane, and a fouled membrane is an expensive consumable rather than a cleanable part.
  • It is scaled up to whole-house duty without good reason. The waste ratio, the pressure requirements and the storage needed make it a poor fit for laundry and bathing water.
  • It is expected to treat bacteria. A membrane is a physical barrier and not a disinfection step, and the certifications are separate.
  • It is chosen for a health contaminant without matching the certification. Not every membrane is tested against every contaminant, and arsenic in particular depends on which chemical form is present.
  • The prefilters are neglected. They exist to protect the membrane, and skipping them shifts the cost onto the expensive part.

The one stage that treats water rather than the house

Everything else in a well treatment train is plumbed where water enters the building, because staining, scale and smell affect every fixture. Reverse osmosis is usually different, and deliberately so.

The contaminants RO is bought for are almost always health contaminants, and health contaminants matter at the point water is swallowed. Nitrate in the laundry is not a problem. Nitrate in a glass is. Treating the small volume that gets drunk, at the tap where it gets drunk, costs a fraction of treating everything and does the same job for the same risk.

The exceptions are real but narrow: very high dissolved solids affecting appliances, or a specific contaminant that damages plumbing. Neither describes most wells.

Why it goes last

A membrane is a precision component with a hard requirement: the water reaching it has to be reasonably clean already. Hardness scales it. Iron fouls it. Chlorine, on some membrane types, damages it outright, so a carbon stage normally sits directly in front.

This makes RO the end of a train and never a replacement for one. A system installed on raw well water with high hardness and iron will work for a while and then need an expensive membrane, repeatedly, and the owner will conclude that reverse osmosis is unreliable. It was asked to do the job of four other stages.

Two of the usual reasons to buy RO need a laboratory before they need a membrane. Arsenic removal depends on which chemical form is present, and establishing that needs a laboratory. Nitrate above the federal standard usually means something has changed about the well or its surroundings. Both are worth confirming with a certified lab and raising with a county or state health department before any money is spent, because the right purchase depends on an answer a web form cannot produce.

Read next

  • Treatment

    Activated carbon filters for well water

    Adsorbs organic compounds, chlorine and the things that make water taste and smell wrong, on a bed of carbon with an enormous internal surface area. It is the polishing stage, and it is mandatory behind any chemical injection.

  • Treatment

    Water softeners for well water

    Trades the calcium and magnesium that cause scale for sodium, on a resin bed that regenerates itself with brine. It is the right answer for hard water and the wrong answer for an iron problem, though it is sold for both.

  • Contaminant

    Coliform and E. coli in well water

    The one result that is about the well's physical condition rather than the water's chemistry. Bacteria in a well mean something is getting in, and disinfection treats the consequence rather than the cause.

  • Contaminant

    Nitrate in well water

    A genuine health standard rather than a staining threshold, and the population at risk is infants under six months. It is also a signal about the well itself, because nitrate is not naturally abundant in deep groundwater.

Not sure what is in your water yet?See which tests to order, thenplan the treatment from your results.

Questions

Why not just put RO on the whole house?
Cost and waste, mostly. RO sends several gallons to drain for each gallon it produces, needs pressure and storage, and strips minerals from water that is about to be used for flushing toilets and washing clothes. Treating the two gallons a day that get drunk is a far better trade than treating the three hundred that do not.
Does it remove the minerals I want?
It removes most dissolved minerals, yes. Drinking water is a minor contributor to mineral intake compared with food, so for most people this is not a practical concern. Remineralising cartridges exist for those who dislike the flat taste.
Will it fix my nitrate problem?
RO is one of the recognised treatments for nitrate, and it is a point where the advice has to stop being about equipment. Nitrate above the federal standard usually means surface contamination is reaching the aquifer, which is worth understanding before spending anything, and it is a conversation for a certified lab and your health department.
How much water does it waste?
Older units commonly ran around four gallons to drain per gallon produced. Better modern units improve on that considerably. It is worth checking the recovery ratio before buying, particularly on a well with limited yield.

Sources