Choosing well water treatment in Washington: softeners, iron filters, UV, RO, arsenic and nitrate

Water treatment is easy to oversell. The equipment is real and often worth it, but only when it is chosen for a contaminant you have measured. No single unit removes everything, and every unit needs upkeep. This guide walks through the order to do things in: test, match treatment to the results, check the certification, then plan for maintenance and retesting.

Test first

Start with a lab report, not a salesperson’s in-home demo. Our testing guide covers what to test and how to sample. In short, Washington DOH’s guidance for private wells says:

• Coliform bacteria and nitrate at a minimum, because of their acute health risk.
• Arsenic, iron and manganese as initial tests. DOH calls them naturally occurring contaminants “found in many areas of the state”.
• Chloride for wells near salt water.
• Uranium in northeast Washington and anywhere local government knows it occurs.

Use an Ecology-accredited lab (lab search, or our testing directory). For treatment planning, also ask for hardness and pH. Both decide whether a softener or an iron filter will work (see below).

Penn State Extension notes that dissolved iron and manganese are most common in groundwater with a pH below 7.0. Minnesota’s health department says sulfate can affect which treatment removes arsenic. Give the installer the whole report, not one number.

Match the treatment to the result

Sources: Minnesota Department of Health’s home treatment table, CDC, EPA WaterSense and Washington DOH, as listed below. Sand is covered in our sand guide.

Whole house or one tap? A point-of-entry (POE) system treats all the water where it enters the house. A point-of-use (POU) unit treats one tap, usually the kitchen sink. EPA says POU suits water for drinking and cooking. It says POE suits a source with “consistent and significant” contamination, and usually costs several thousand dollars to install. Hardness, staining and bacteria affect every tap, so they usually need POE. Arsenic and nitrate matter mainly in water you drink and cook with. DOH’s arsenic page describes both kinds of system.

Order matters. Systems are usually combined into a train. CDC says UV works better when the water is filtered first. Minnesota’s health department says UV may need pre-filtration if turbidity is above 1 NTU. DOH’s nitrate guidance says ion exchange resin may need pretreatment where iron, manganese and other metals together exceed 0.1 mg/L. It also says reverse osmosis membranes need pretreatment to prevent fouling and scaling. Ask the installer to explain the order of each stage and why.

Hardness and water softeners

USGS classes water by hardness as calcium carbonate:

• soft: 0–60 mg/L;
• moderately hard: 61–120 mg/L;
• hard: 121–180 mg/L;
• very hard: more than 180 mg/L.

Hardness causes scale on fixtures and appliances and makes soap hard to lather. EPA says some hardness is generally not a health hazard.

• Certification: NSF/ANSI 44, or IAPMO/ANSI Z601 for scale-reduction devices (EPA).
• Efficiency: NSF/ANSI 44 has a voluntary efficiency rating of 5.0 gallons of water or less per 1,000 grains of hardness removed. EPA suggests demand-initiated regeneration and comparing water use and salt efficiency between models.
• Sodium: Minnesota’s health department says softening with sodium chloride adds sodium to the water. That may be a health issue for some people. Potassium chloride is an alternative salt.
• Corrosion: the same source says softening can change how corrosive the water is.
• Discharge: EPA notes that softeners discharge salt and that some local jurisdictions restrict them. Ask where the regeneration water will go, especially on a septic system, and check with your county.

A softener is not a general-purpose filter. CDC says softeners don’t remove parasites, bacteria or viruses.

Iron and manganese

EPA’s secondary (aesthetic) limits are:

• Iron, 0.3 mg/L: rusty color, sediment, metallic taste, reddish or orange staining.
• Manganese, 0.05 mg/L: black to brown color, black staining, bitter metallic taste.

Manganese is also a health question. DOH’s December 2023 fact sheet, written for public water systems and their customers, says infants are the most sensitive group. It recommends manganese treatment at the source above 0.05 mg/L. At or above 0.3 mg/L (EPA’s health advisory level), it says to use another water source, such as bottled water, for baby formula. DOH says boiling may raise manganese, and that you can’t rely on the look or taste of the water, because dissolved manganese may not be noticeable.

Penn State Extension’s guide gives rough ranges for choosing treatment:

Minnesota’s health department notes the trade-offs:

• Oxidizing media needs periodic regeneration with chemicals that must be handled and stored carefully.
• Aeration can make water corrosive.
• Chlorination can form by-products and may need a carbon filter for taste.

Penn State notes that one non-regenerating media, Birm, needs a pH of at least 6.8 for iron and 7.5 for manganese. Find providers under iron and manganese treatment.

Bacteria and UV disinfection

A positive coliform result is first a question of how contamination is getting into the well. Follow the steps in our testing guide: look for a way in, shock chlorinate and retest.

If you then want ongoing disinfection:

• What UV does: EPA says UV inactivates bacteria, viruses and protozoa. It is “not effective against chemicals” such as metals, and adds no chemicals to the water.
• Certification: NSF/ANSI 55.
• Pretreatment: CDC says UV works better with filtration first. Minnesota’s health department says pre-filtration may be needed above 1 NTU turbidity. Ask how iron, manganese, hardness and sediment will be handled ahead of the UV unit.
• Upkeep: follow the manufacturer’s schedule for the lamp and sleeve. Keep testing for bacteria every year.

See UV disinfection providers.

Arsenic

DOH recommends no more than 10 ppb of arsenic in water for drinking or cooking. It says levels up to 33,000 ppb have been found in some Washington wells.

• What works: DOH says reverse osmosis, distillation or special filter media can reduce arsenic. Standard taste-and-odor filters “do not remove arsenic”. Minnesota’s health department also lists anion exchange and adsorptive media. It lists oxidation filtration only where iron is also high.
• Two forms: arsenic III and arsenic V. Minnesota’s health department says pre-oxidation may be needed so the treatment removes the type present in your water. A 2020 USGS study of treated private wells in Maine and New Jersey found more failures where untreated arsenic and the share of arsenic III were higher. Ask the lab whether it can measure arsenic III.
• Who maintains it matters. In the same study, whole-house systems failed less often than single-tap ones. Systems installed and maintained by vendors also failed less than those installed and maintained by homeowners.
• Certification: DOH recommends NSF-certified equipment. Minnesota’s health department says to check that the unit is certified for arsenic reduction by NSF, UL or the Water Quality Association.
• Retest: DOH says to test the treated water after installation and after routine maintenance.

See arsenic treatment and reverse osmosis providers.

Nitrate

DOH’s drinking water standard is 10 mg/L. Its nitrate guidance says Washington’s nitrate contamination occurs most often in agricultural areas of eastern Washington and in Whatcom County. Don’t boil high-nitrate water: DOH says boiling can raise nitrate.

• What works: reverse osmosis, distillation and anion exchange (Minnesota’s health department; EPA WaterSense). EPA notes that some RO systems can reduce nitrate, and that carbon filtration and UV don’t.
• Anion exchange risk: DOH’s nitrate treatment guidance, written for small public systems, warns that resin which isn’t regenerated often enough can “dump” nitrate. Treated water can then spike above the untreated level, because sulfate displaces captured nitrate. Minnesota’s health department gives the same warning for home units.
• Retest on a schedule. Nitrate is an acute risk, so a failed unit matters quickly. Test the treated water, not just the well.
• Funding: DOH says your local health department may offer funding for nitrate problems in private or Group B wells, usually as reimbursement.

Salty water near the coast

Ecology says seawater intrusion has made some coastal Washington wells unusable. It names Island and Jefferson counties in its local resources. It asks well owners to regularly check water levels, chloride and conductivity. If intrusion is indicated, it suggests pumping less or scheduling pumping at low tide, and asks owners to contact Ecology and the county planning department. EPA’s secondary limit for chloride is 250 mg/L, above which water tastes salty.

Minnesota’s health department lists reverse osmosis and distillation as treatments that remove chloride and sodium. A softener doesn’t, and adds sodium. Treatment doesn’t stop intrusion, so report it rather than only treating it.

Reverse osmosis and distillation

These two remove the widest range of dissolved contaminants.

Reverse osmosis (NSF/ANSI 58):

• Wastewater: EPA estimates that a typical under-sink unit wastes at least 5 gallons for every gallon it produces. WaterSense-labeled models waste 2.3 gallons or less. DOH notes that RO sends the filtered contaminants to your septic or sewer system.
• Pretreatment: membranes need protection from fouling and scaling.
• Corrosion: RO strips minerals, which DOH says can make treated water more corrosive.

Distillation (NSF/ANSI 62):

• Energy: EPA and CDC say distillers use a lot of energy.
• Hygiene: CDC says bacteria can grow on the cooling coils when a distiller isn’t in use.

Systems may combine stages. EPA notes that an RO unit can also carry NSF/ANSI 42, 53 or 55 certification.

Certification: what to look for

Source: EPA WaterSense (November 2024).

A standard number on the box is not enough. EPA says to check the system’s performance data sheet, or the certifier’s website, to confirm it is certified for your contaminant. Minnesota’s health department says NSF, UL and the Water Quality Association certify units, but not for every contaminant.

DOH’s 2026 PFAS fact sheet adds a well-specific catch. Certified filters are tested on municipal water, and arsenic, iron and manganese in well water can stop store-bought whole-house filters from working.

Maintenance and retesting

EPA says all systems need ongoing maintenance and that “unmaintained systems may make water quality worse”. CDC says to change filters on the manufacturer’s schedule, partly to stop germs growing in them. Typical jobs:

• replacing cartridges, membranes and UV lamps;
• adding softener or regeneration salt;
• backwashing and regenerating media;
• disinfecting units;
• removing scale.

Retest the treated water:

• after installation;
• after routine maintenance (DOH’s advice for arsenic);
• at least yearly for bacteria and nitrate, as DOH recommends for every well.

Keep the reports with the system’s manuals.

What it costs (indicative only)

There is no Washington price survey we can cite. These figures are from other agencies and are dated; use them only to sanity-check quotes.

Source: Minnesota Department of Health, based on 2017 quotes and 2018 research. Low ends are for homeowner installs and high ends for professional installs. EPA’s November 2024 guide says RO and distillers “tend to cost a few hundred dollars” and whole-house systems “can cost thousands”.

Getting quotes

Get at least two written quotes, each based on the same lab report. Washington’s plumbing law (RCW 18.106.010) lists treatment and filtration equipment as part of the domestic water pumping specialty. Check the installer’s L&I registration; the treatment directory shows it on each listing.

Questions to ask:

1. Which results in my lab report is this system treating, and what is it certified for (standard and contaminant)?
2. Does my water need pretreatment for iron, manganese, hardness, sediment or arsenic III? In what order do the stages go?
3. Is it whole-house or single-tap, and why?
4. How much water does it use for backwash, regeneration or RO reject, and where does that water go? Is my septic system OK with it?
5. What maintenance does it need, how often, and what will it cost each year? Do you offer service?
6. Will you test the treated water after installation? If not, which accredited lab should I use?
7. What does the warranty cover, and who do I call if a retest fails?

Red flags

Minnesota’s health department lists these signs of deceptive sales:

• claiming to work with the water utility or the health department;
• in-home “tests” that supposedly show contamination;
• misstating drinking water standards, or saying water should contain no contaminants at all;
• one-time-only discounts;
• units sold as “maintenance-free”.

Also watch for:

• a system quoted before you have an accredited lab report;
• a softener or carbon filter sold for arsenic, nitrate or salty water;
• UV sold as the fix for a well that keeps testing positive, with no look at the wellhead;
• no plan to retest the treated water.