Well water treatment: which filter or system for which problem
Choose treatment from a lab test, not from a sales pitch: each technology handles a specific set of problems. Reverse osmosis removes dissolved salts and nitrate, UV kills microbes but removes no chemicals, and a softener removes calcium and magnesium but adds sodium [1] [2]. The model plumbing code requires treatment units certified to NSF/ANSI standards and an air gap on reverse-osmosis drains [3].
Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
What does each type of treatment remove?
| Technology | Removes | Does not remove / watch out |
|---|---|---|
| Sediment (mechanical) filter | Sand and turbidity; iron and manganese only after they are oxidized [2] | Dissolved salts; microfiltration does not stop viruses or chemicals [1] |
| Water softener (ion exchange) | Hardness; dissolved iron and manganese [2] | Adds sodium; does not lower TDS [4]; no protection against microbes [1] |
| Activated carbon | Chlorine, chloramine, tastes and odors, pesticides, organics, radon [2] | Salts and hardness; an overdue cartridge lets contaminants break through [2] |
| Reverse osmosis (RO) | Bacteria, viruses, protozoa and salts such as sodium, chloride, copper and lead; partly arsenic, fluoride, sulfate, nitrate [1] | Only 20–30% of the water becomes product [5] |
| UV disinfection | Microbes, with a prefilter [1] | Any chemicals |
| Aeration plus filter | Iron, manganese, part of hydrogen sulfide and radon [2] | Needs a backwashing filter and a disinfected aerator [6] |
How do you remove iron and manganese?
- Low iron: catalytic carbon works for dissolved iron below 1.0 mg/L if the water has at least 4.0 mg/L of dissolved oxygen; polyphosphate only hides iron below 2 mg/L [6].
- Birm-type catalytic media: needs dissolved oxygen and a pH of at least 6.8 for iron and 7.5 for manganese; it removes manganese unreliably and needs backwashing [7].
- Softener: removes dissolved iron incidentally when pH is above 6.7, hardness 3–20 grains per gallon and dissolved iron below 5 mg/L; rusty (oxidized) iron fouls the resin and must be filtered out first [8].
- Manganese greensand filter: regenerated with potassium permanganate, for combined iron and manganese of 3–10 mg/L [9].
- Above 10 mg/L combined: feed an oxidant (chlorine, permanganate or peroxide) with a metering pump, give it contact time in a tank, then a backwashing filter and, if needed, carbon to remove the chlorine taste [10].
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What removes a rotten-egg smell?
That smell is hydrogen sulfide, and the fix depends on the level: below about 1 mg/L activated carbon; up to 2–3 mg/L an oxidizing filter; aeration up to about 2 mg/L; above 6 mg/L chlorination or potassium permanganate with filtration. If only the hot water smells, replace the magnesium anode in the water heater [11].
Under-sink or whole-house reverse osmosis?
Most household RO units sit under the kitchen sink with their own faucet and make about 10–34 gallons (38–130 L) a day. Whole-house RO costs much more to buy, install and run, and RO water is more aggressive toward pipes. On hard water, a softener ahead of the membrane extends its life, and thin-film membranes need a carbon prefilter because chlorine destroys them [5].
Approximate prices, as of September 2026: under-sink RO $150–$1,300 installed, whole-house $1,000–$4,800 or more [12].
When do you need UV disinfection?
When bacteria show up in the well. NSF/ANSI 55 Class A units deliver 40 mJ/cm² and must have a UV sensor and alarm; they are meant for contaminated well or surface water. Class B (16 mJ/cm²) is only a supplement for water that is already safe [13].
UV needs clear water. Chautauqua County, New York, requires iron of no more than 0.3 mg/L, manganese 0.05 mg/L, hardness 300 mg/L, hydrogen sulfide 1 mg/L and turbidity 1 NTU ahead of the lamp, plus a 5-micron prefilter, an alarm, a raw-water tap and a spare lamp; a softener is usually needed first [14].
Do magnetic or salt-free conditioners soften water?
They are sold as alternatives: salt-free units at about $800–$4,000 and magnetic ones at $200–$600 (approximate, as of September 2026) [15]. We found no US test data to cite for them. Germany’s Bavarian environment agency reports that magnetic, electric-field and galvanic devices do not change hardness — calcium and magnesium stay in the water — and that the effect of magnetic and electric-field devices is not proven; only some electrochemical and catalytic devices hold a German DVGW certificate [16].
In what order should treatment go, and what fits your test?
Remove heavy iron before the softener [17], soften before an RO membrane on hard water, put carbon ahead of the membrane [5], and put UV last, after a 5-micron filter, with a raw-water tap before any treatment [14].
| Problem in your test | Typical choice |
|---|---|
| Sand, turbidity | Sediment filter [2] |
| Hardness 3–20 gpg, dissolved iron below 5 mg/L, pH above 6.7 | Softener [8] |
| Dissolved iron below 1 mg/L with oxygen in the water | Catalytic carbon [6] |
| Iron plus manganese 3–10 mg/L | Greensand filter [9] |
| Iron plus manganese above 10 mg/L | Oxidant feed plus filtration [10] |
| Coliform bacteria | UV Class A after pretreatment [13] |
| High TDS, salty taste, nitrate, sodium, arsenic | RO or distillation [2] |
| Chlorine, taste and odor, organics | Activated carbon [2] |
What certification does treatment equipment need?
Where the International Residential Code is adopted, drinking water treatment units must meet NSF 42, 44, 53 or 62 or CSA B483.1; under-sink RO must meet NSF 58 or CSA B483.1 and drain through an air gap [3].
Read the certificate, not just the logo: NSF 42 covers aesthetic effects such as chlorine taste, NSF 53 specific health contaminants, NSF 58 reverse osmosis and NSF 44 softeners — and a certificate covers only the substances it names [18].
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Frequently asked questions
What is the first filter to put on well water?
The one your test calls for. Sediment is caught by a mechanical filter, heavy iron goes ahead of the softener, and UV goes last after a 5-micron filter [17] [14].
Does a water softener remove iron?
Some dissolved iron, below 5 mg/L, when pH is above 6.7; rusty iron clogs the resin and needs a filter first [8].
Does UV remove chemicals from water?
No. UV treats microbes only; CDC states that UV systems are not effective in removing chemicals [1].
Does my filter need NSF certification?
Under the International Residential Code, yes: NSF 42, 44, 53, 62 or CSA B483.1, and NSF 58 for RO [3].
How much does reverse osmosis cost?
About $150–$1,300 installed under a sink and $1,000–$4,800 or more for a whole house (approximate, as of September 2026) [12].
Sources
- CDC — A Guide to Drinking Water Treatment Technologies for Household Use (2012), Reverse Osmosis; Ultraviolet; Water Softeners; Filtration (US) · recommendation · checked 2026-09-29 · source
- University of Nebraska–Lincoln Extension EC703 «Drinking Water Treatment: An Overview» (rev. 2014), Table I (problem → treatment); Table II (methods) (US) · recommendation · checked 2026-09-29 · source
- International Residential Code 2024 (ICC), Chapter 29 Water Supply and Distribution — P2909 (via UpCodes, GSA adoption), P2909.1–P2909.3 (US) · regulation · checked 2026-09-29 · source
- Montana State University Extension Water Quality — Hardness fact sheet, Treating Hard Water (US) · recommendation · checked 2026-09-29 · source
- University of Nebraska–Lincoln Extension G1490 «Drinking Water Treatment: Reverse Osmosis» (rev. 2014), Treatment principles; Equipment (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Iron and Manganese in Private Water Systems, Other Options (aeration, catalytic carbon, polyphosphate) (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Iron and Manganese in Private Water Systems, Oxidizing Filters (Birm) (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Iron and Manganese in Private Water Systems, Water Softeners (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Iron and Manganese in Private Water Systems, Oxidizing Filters (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Iron and Manganese in Private Water Systems, Oxidation followed by filtration (US) · recommendation · checked 2026-09-29 · source
- Penn State Extension — Hydrogen Sulfide (Rotten Egg Odor) in Water Wells, Treatment options (US) · recommendation · checked 2026-09-29 · source
- This Old House — How Much Does a Reverse Osmosis System Cost in 2026?, System type (US) · price · checked 2026-09-29 · ✓ · source
- wcponline.com, Class A / Class B (US) · regulation · checked 2026-09-29 · source
- Chautauqua County (NY) Environmental Health Services — Private Drinking Water Standard for Ultra Violet (UV) Light Disinfection System, Water Quality; installer checklist (US) · regulation · checked 2026-09-29 · source
- thisoldhouse.com (US) · price · checked 2026-09-29 · ✓ · source
- Bayerisches Landesamt für Umwelt, Merkblatt Nr. 1.6/5 «Alternative Wasserbehandlungsgeräte zur Verminderung der Steinbildung in Hausinstallationen» (Stand Juli 2010), 1 Hintergrund; 4 Schlussfolgerungen (DE) · recommendation · checked 2026-09-29 · source
- Pentair Fleck 5600SXT Upflow Service Manual, Installation: Water Pressure, Electrical Facilities, Existing Plumbing, Location Of Softener And Drain, Bypass Valves (US) · recommendation · checked 2026-09-29 · source
- nsf.org, — (US) · regulation · checked 2026-09-29 · source
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