Reverse osmosis vs a water softener: two different jobs

What a water softener does

Hard water is water with a lot of calcium and magnesium in it. The USGS grades it by milligrams per liter as calcium carbonate: 0 to 60 soft, 61 to 120 moderately hard, 121 to 180 hard, and more than 180 very hard. Hard water is why soap forms scum and why more soap or detergent is needed to get things clean; water utilities add that its deposits build up on dishes, fixtures and basins and can cause scale on pipes.

A softener removes the calcium and magnesium. As water utilities put it, in home softeners, sodium is added to the water in exchange for the hardness removed. Ask which standard a softener is certified to. A softener is usually fitted to treat the water for the whole house.

What reverse osmosis does

The EPA’s WaterSense program describes reverse osmosis as the process by which pressure forces water through a semi-permeable membrane, making treated water and reject water that goes down the drain. It says RO systems can potentially remove contaminants such as lead, volatile organic compounds, PFAS, arsenic, bacteria and viruses. An under-sink RO system is what the EPA calls point-of-use: connected to a single fixture, such as under the kitchen sink. The EPA points to NSF/ANSI 58 for reverse osmosis systems.

Side by side

	Water softener	Reverse osmosis
Treats	Hardness: calcium and magnesium	Drinking water; can potentially remove contaminants such as lead, VOCs, PFAS and arsenic
Where	All the water entering the house	Under-sink: one fixture. Whole-house: a bigger installation
How	Exchanges sodium for hardness	Pressure through a semi-permeable membrane
Standard	Ask which standard it is certified to	NSF/ANSI 58
Goes to the drain	Ask the installer about regeneration	EPA: five gallons or more per gallon treated for a typical point-of-use unit; 2.3 or less for WaterSense labeled

When you might want both

Because a softener adds sodium in exchange for the hardness it removes, some households with a softener want an RO system at the kitchen tap for drinking and cooking water. Water utilities note that softeners increase the sodium content of water, and suggests that people on a sodium-restricted diet consult their physician. That is a question for your doctor, not this site. Ask a pro whether an RO system on softened water suits your house.

Health, and what utilities say

Neither a softener nor an RO system is a health promise. Water utilities say hard water is not a health hazard and that there is no health-based drinking water standard for hardness. Utilities also note that some softening systems can increase the corrosivity of water, which can damage the anodes in a water heater: one more thing to ask an installer about.

What about salt-free conditioners?

Salt-free conditioners are a different technology from ion exchange. This site has no primary source on what they do, so ask the installer what a particular unit is certified for.

How to decide

1. Read your utility’s yearly water quality report, or test your well with a certified laboratory.
2. If hardness is the problem, talk about a softener. If something else in the water is, talk about what it is certified to remove.
3. If you want both, ask how they fit together and in what order.
4. Ask for each part of the quote in writing.

Signs that point one way or the other

• Scale on fixtures, spots on dishes and soap that will not lather point toward hardness, which is a softener’s job.
• A contaminant on your water report or well test, such as lead or arsenic, points toward treatment certified for that contaminant, which may be RO.
• A taste you dislike at the kitchen tap is worth showing to a pro along with your report.

The EPA’s private well page lists a water softener as the treatment for hardness, next to scaly deposits and soap that will not lather. For anything else, the test result decides.

For RO, start with RO system installation or whole-house reverse osmosis. Send the form and an independent local water treatment pro can contact you.

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