Reverse Osmosis vs Water Softener: Which Do You Need?

What is the difference between reverse osmosis and a water softener?

The difference is what each treats and where: a water softener removes hardness from all the water entering the house, and reverse osmosis filters a small stream of drinking water at one tap. A softener uses ion exchange, swapping calcium and magnesium for sodium on resin beads, as how a water softener works explains. Reverse osmosis pushes water through a membrane that holds back most dissolved substances.

The Water Quality Association's glossary describes reverse osmosis as using "pressure to force the water molecules through a semipermeable membrane", and says RO "removes ionized salts, colloids, and organic molecules". Under-sink systems usually pair the membrane with carbon and sediment cartridges, a small storage tank or tankless pump, and a separate drinking faucet. The membrane's reject water goes to the drain.

What does reverse osmosis remove?

Reverse osmosis removes many dissolved chemicals from drinking water, but exactly which ones depends on the certified model. The CDC says RO filters "will remove some types of chemicals from water, including lead, copper, chromium, chloride, and sodium", may also reduce arsenic, fluoride and other substances, and adds that buyers should check the label for specifics. Its advice is to look for NSF certification on the label and to look products up in the NSF database.

RO also lowers total dissolved solids, which is what most people notice as taste. Palmdale Water District's 2025 report lists total dissolved solids averaging 250 mg/L in its surface water and 254 mg/L in its wells, well under California's secondary standard of 1,000 mg/L, so on city water RO is mostly a taste and preference choice. CCRs print these values in "ppm" and trace metals in "ppb"; ppm equals mg/L and ppb equals µg/L.

Upkeep is part of the deal. The CDC says any filter must be maintained "to keep it working properly and prevent germs from growing in it", which means changing filters on the maker's schedule and washing hands when doing it. A neglected RO unit is not doing what its label says.

What does a water softener do that RO can't?

A water softener protects the whole plumbing system from scale, which an under-sink RO unit cannot do. Showers, the water heater, the dishwasher and the washing machine all get softened water, so scale on fixtures and inside the heater slows. Nebraska Extension notes that heat drives scale buildup in water heaters, which "can then slow the heating process and increase energy usage".

On Antelope Valley city water that benefit is moderate, because the supply is moderately hard: Palmdale Water District reported 6.5 gpg for treated surface water in 2025, and Lancaster's District 40 well average was 5.7 gpg (98 mg/L). A softener there is a comfort and scale choice, as the softener decision guide explains. On a well with very hard water, the whole-house benefit is larger.

Do you need both?

Most city-water homes need neither, and some choose one; both make sense mainly where hardness is high and there is a specific drinking-water concern. A common pairing is a softener for the house and an RO tap in the kitchen, which also takes out much of the sodium the softener adds. Nebraska Extension notes the alternative of leaving the kitchen cold line unsoftened.

The order of decisions matters more than the equipment. On a private well, start with a lab test from a state-accredited (ELAP) lab covering hardness, iron and the health items, then treat what the results show; see testing a well and then treating well water. On city water, start with your supplier's latest report and decide whether hardness, taste or a specific contaminant is the issue.

Does reverse osmosis remove arsenic and chromium 6?

Some reverse osmosis systems are certified to reduce arsenic or hexavalent chromium, but only a specific model's certification tells you whether that one does. The State Water Board names reverse osmosis among the best available technologies for hexavalent chromium and keeps a list of certified residential point-of-use and point-of-entry devices for it; the CDC lists RO among treatments for arsenic and says to find a system certified to remove it. A softener is not a treatment for either.

Locally, each system says something different. Rosamond CSD's notice distributed in September 2026 says "Our water system recently violated a drinking water standard", reporting running annual averages of 14 and 15 µg/L at wells 8 and 9, and adds that customers "do not need to use an alternative water supply". Quartz Hill Water District reported hexavalent chromium averaging 12 µg/L (11 to 13) and says a system of its size has until after October 1, 2027 before the limit counts as a violation. Cal Water's Lancaster system reported its wells between 6.1 and 11 µg/L (8.7 on average), with blending treatment planned before its 2028 date. The hexavalent chromium guide and the arsenic in well water page cover the details.

The Waterdrop G3P600 (sold by SoftPro Water Systems, a store we own) is an option for drinking-water taste and dissolved solids. Waterdrop states the G3P600 is certified by IAPMO R&T to NSF/ANSI 58 for the reduction claims on its performance data sheet, which lists hexavalent chromium. That test's pass level for hexavalent chromium is 0.1 mg/L (100 µg/L), far above California's 10 µg/L limit, so the listing does not show the unit brings water under the state limit. The data sheet makes no arsenic claim. For either contaminant, choose from the State Water Board's certified device list and confirm with a lab test afterward.

What does each cost to own?

A softener costs more to buy and run; an under-sink RO unit costs less up front but needs regular cartridge and membrane changes. SoftPro listed its ECO City Water Softener from $967 (32,000-grain, softener only) and the Waterdrop G3P600 at $429 on October 26, 2026; installation is extra for both. The store's listing describes the G3P600 as producing two parts filtered water to one part drain water; that ratio is the store's and maker's figure.

Running costs differ in kind. A softener uses salt and about 50 gallons of water per regeneration (Penn State Extension), plus yearly brine valve and float care; the water softener cost page goes through those drivers. RO running costs are replacement cartridges on the maker's schedule; this guide did not find a sourced yearly figure and does not estimate one.

Common questions

Related guides

• Tap waterHexavalent chromium in drinking water
• Hard water & softenersHow does a water softener work?
• WellsArsenic in well water