Reverse Osmosis for Well Water: Why Pretreatment Comes First
The under-sink RO went in over a weekend in March. By July the water tastes fine but fills a glass like a leaky tap, and when you pull the first-stage housing the sediment cartridge comes out the color of a terracotta pot. You changed it in May. The membrane is rated for two to five years and it has been four months. Nothing is broken — the system is doing exactly what it does when it is fed raw well water.
Start with the lab report, not the TDS meter
Nothing below can be decided without numbers, and the $15 TDS meter taped to the RO box will not produce them. It reports dissolved solids as one conductivity figure: 480 ppm says nothing about whether that is calcium, iron, manganese or nitrate, and it reads zero for bacteria.
CDC and state health departments recommend testing a private well at least annually for total coliform, nitrate, TDS and pH, and again whenever taste, color or odor changes. For an RO decision, ask a state-certified lab for the panel covering iron, manganese, hardness, pH, hydrogen sulfide, turbidity, tannins and nitrate — $50-250 by analyte count, with subsidized coliform-and-nitrate screens under $40 in many states.
Two field checks fill the gaps between lab visits. Let a clear glass stand 20 minutes: cloudiness that settles to the bottom is sediment, cloudiness that clears from the top down is dissolved air, and water that turns orange while sitting is ferrous iron oxidizing — the form that looks clear at the tap and still ruins a membrane. A $10 hardness strip on a cold-tap sample then puts your grains per gallon in a band, enough to know whether a softener is in the conversation at all.
What a membrane can actually take
These are feed-water limits published across residential TFC membrane specs — the water arriving at the membrane, after whatever prefilters you installed, not the water leaving the well.
| Parameter | Membrane feed limit | Common in wells | What goes in front | Price |
|---|---|---|---|---|
| Iron | Under 0.1 mg/L | 0.3-10 mg/L | Air injection or greensand iron filter | $700-2,000 |
| Manganese | Under 0.05 mg/L | 0.05-1 mg/L | Same oxidizing filter, pH 8+ for manganese | included above |
| Hardness | Under 7-10 gpg | 10-30 gpg | Ion-exchange softener, or antiscalant | $600-2,000 |
| Hydrogen sulfide | None | 0.5-5 mg/L | Aeration or oxidizing filter, then carbon | $700-2,000 |
| Turbidity / sediment | Under 1 NTU | 1-50 NTU after rain | Spin-down 50-100 micron, then 5 micron | $60-250 |
| Tannins | Fouls organically | Yellow-tea water, shallow wells | Tannin resin (anion exchange) | $800-1,800 |
| Bacteria (coliform) | Not a barrier | Roughly 1 in 3 tested wells | UV at 40 mJ/cm², or shock chlorination | $200-800 |
| pH | Works at 3-11, best 7-8 | 5.5-6.5 in acidic regions | Calcite neutralizer | $500-1,200 |
| Feed pressure | 40-80 psi | Pump cut-in often 30-40 | Adjust pressure switch, or booster pump | $80-250 |
Iron is the line most often crossed, because 0.3 mg/L water looks perfectly clear in a glass. NSF/ANSI 58, the standard residential RO systems are certified to, tests units at 50 psi and 77°F on prepared challenge water — not on well water carrying dissolved iron that oxidizes into rust particles the moment it meets air inside the housing. That rust plates onto the membrane, and no backwash cycle exists under your sink to remove it.
Hardness fails differently. RO concentrates whatever it rejects into a thin brine film at the membrane face, so 15 gpg feed water is far harder than 15 gpg right at the surface, and calcium carbonate precipitates as scale. Softened feed is why the same membrane lasts five years in one house and eight months two miles away.
Pretreatment by problem, in the order it gets installed
Whole-house pretreatment protects the RO and everything else — water heater, softener resin, washing machine — which is why the spend lands differently than "an accessory to a $300 filter."
| Stage | What it handles | When you need it | Typical price |
|---|---|---|---|
| Spin-down sediment, 50-100 micron | Sand, grit, pump debris | Visible grit; any well with a screened intake | $60-150 |
| UV disinfection | Coliform, E. coli, viruses | Any positive coliform result | $200-800 |
| Oxidizing iron / sulfur filter | Iron over 0.3 mg/L, manganese, H₂S | Orange stains, rotten-egg smell | $700-2,000 |
| Water softener | Hardness over 7-10 gpg | Scale on fixtures, soap that won't lather | $600-2,000 |
| Under-sink RO, 4-5 stage | Nitrate, arsenic, sodium, TDS, taste | After the four rows above are satisfied | $200-600 |
Order matters as much as selection: sediment before UV (a UV lamp cannot disinfect through cloudy water — turbidity must be under 1 NTU for the dose to reach the organisms), oxidation before softening (iron fouls resin), and RO last, in the kitchen, on water that has already been cleaned up.
Why "just install an RO and skip the rest" backfires
The appeal is obvious: an under-sink RO costs $200-600, rejects 90-99% of dissolved solids, and skipping $2,000 of whole-house equipment looks like the smart move.
What it buys is a consumables treadmill. Prefilters rated for 6-12 months come out loaded at 4-8 weeks on raw well water; the membrane that should run 2-5 years at $40-90 a swap fouls inside a year. Over five years, $250-400 annually in cartridges on a fouling system approaches the cost of the iron filter that would have prevented it — while the untreated water keeps staining every fixture RO never touches.
The more serious gap is microbiological. An RO membrane rejects bacteria physically, but a residential system is not certified or designed as a disinfection barrier: no integrity monitoring, a storage tank and carbon stage sitting downstream where growth can occur, and a pinhole that is invisible from the faucet. EPA and state health departments treat a coliform-positive well as a case for disinfection — UV or chlorination — not for a point-of-use filter. If your report came back positive, that is the first purchase, ahead of anything on the taste side.
Common mistakes
- Buying the RO before the water test. The report decides whether you need $60 of sediment filtration or $2,000 of iron removal. A $50-250 panel ordered first has changed plenty of purchases; ordered after, it just explains the failure.
- Trusting a TDS meter to describe well water. Conductivity cannot separate calcium from iron and reads zero for bacteria. Two wells at 450 ppm can need completely different equipment.
- Treating clear water as iron-free. Ferrous iron is dissolved and invisible at the tap, oxidizing to visible rust only after it meets air — often inside the filter housing. The lab number is the only honest read.
- Counting on RO for a coliform-positive well. Residential RO is not a certified disinfection barrier and its downstream tank and carbon stage sit past the membrane. Shock-chlorinate, retest, and install UV.
FAQ
Can you put a reverse osmosis system on well water?
Yes, and it is the standard way to bring nitrate, arsenic and sodium down at the kitchen tap. The condition is feed-water quality: under 0.1 mg/L iron, under 0.05 mg/L manganese, no hydrogen sulfide and turbidity under 1 NTU. On water inside those limits an RO runs for years; outside them it fouls in months.
Do I need a water softener before reverse osmosis?
Above roughly 7-10 gpg, yes. RO concentrates rejected calcium and magnesium at the membrane face, so hard feed water scales the membrane far faster than the raw number suggests. Softened feed is routinely the difference between a 5-year membrane and a 1-year one; below about 7 gpg most systems cope unaided.
Does reverse osmosis remove bacteria from well water?
The membrane rejects bacteria physically, but a residential RO is not a certified disinfection barrier — no integrity monitoring, and a storage tank plus carbon stage downstream of the membrane. For a coliform-positive well, EPA and state health guidance points to disinfection: shock chlorination, then UV at 40 mJ/cm², $200-800 installed.
How long does an RO membrane last on well water?
With proper pretreatment, 2-5 years, the same as on city water; a membrane costs $40-90. Fed raw well water carrying iron, sulfur or 15+ gpg hardness, expect 3-12 months, with sediment and carbon prefilters loading up in 4-8 weeks instead of 6-12 months.
Related:
General information about home water quality and equipment, not medical advice. For bacteria, nitrates, lead, or arsenic, test through a state-certified laboratory and follow your state health department's guidance. Membrane feed-water limits, equipment sizing and prices vary by model and by water chemistry — check your manual and your own lab report before buying.