Reverse Osmosis Drinking Water: How It Works and What to Expect

A softener treats hardness. Reverse osmosis does a different job at your drinking-water faucet.

If you want to reduce dissolved solids in the water you drink and cook with, an under-sink reverse osmosis system may be a useful next step. It treats a smaller drinking-water supply rather than every gallon used by your showers, laundry, and toilets.

For whole-house pretreatment, see carbon filtration and sediment filter selection.

What happens inside an RO system?

1Prepare the feed water

Prefilters protect the membrane. The required treatment depends on the source water and system design.

2Separate at the membrane

Pressure drives part of the water through the membrane. The product stream has fewer dissolved solids.

3Store and dispense

A typical tank-style drinking-water system stores product water and supplies it through a finishing filter to the faucet.

The other stream goes to the drain.

Concentrate carries away rejected dissolved material. Drain flow is part of normal RO production; the amount depends on the system and operating conditions.

Conceptual process for a typical tank-style system, not an installation diagram. Follow the exact model’s plumbing instructions.

Do I need RO if I already have a softener?

Possibly. A conventional water softener does not reduce total dissolved solids (TDS). It exchanges calcium and magnesium for sodium ions, or potassium ions when potassium chloride is used. The dissolved minerals are exchanged rather than removed without replacement, so TDS stays roughly the same and can increase slightly. That is why a TDS meter cannot tell you whether your softener is working—use a hardness test instead.

RO reduces many dissolved substances at the drinking-water outlet. A softener and an RO system can complement each other: softened feed water can reduce hardness scaling at the membrane, while the RO serves drinking and cooking needs.

Neither device should be selected as a universal treatment for every contaminant. For a specific health-related concern, check the exact system’s documented reduction claims and relevant certification, then verify performance with appropriate testing. A basic RO installation is not a substitute for addressing an unsafe well supply.

Well water needs a pretreatment check

Iron, manganese, suspended material, and hardness can foul or scale a membrane. Hydrogen sulfide also needs attention. Do not ask a small under-sink system to take the place of appropriate whole-house treatment for those problems.

On a chlorinated supply, the membrane’s chlorine tolerance and the prefilter maintenance matter. Follow the model’s requirements rather than assuming every membrane uses the same pretreatment.

Start with your water test and include the conditions at the RO inlet, after any existing treatment.

What does a gallons-per-day rating mean?

An RO production rating describes output under specified test conditions. Actual production changes with water temperature, pressure at the membrane, dissolved-solids concentration, and tank backpressure. Colder water or less available pressure can mean slower refill.

Busy household? Choose the 14-gallon tank for a basement setup.

If you use a lot of drinking water and are installing the RO in a basement with lines running up to the fixtures, we recommend selecting the 14-gallon tank option. Families with several kids filling water bottles, topping off pet bowls, and filling pitchers often benefit from having more water ready when they need it.

The larger tank provides more stored water; it does not make the membrane produce water faster. Its 14-gallon rating is the nominal tank size, not 14 gallons of usable drinking water. Actual drawdown depends on operating pressure and tank settings.

For a basement installation, measure the line length and elevation to the fixtures and count the outlets you plan to serve. Follow the model’s tubing and pressure requirements; line length, elevation, and available pressure affect delivery at the faucet. If the manual does not cover your planned run or pressure requirements, contact us about that specific installation question.

Plan for filters, checks, and sanitation

Replace cartridges on the schedule specified for your system and water conditions. Use the correct replacements, follow the sanitation procedure, and check for leaks after service. Membrane replacement depends on its condition and performance; it is not necessarily due every time prefilters are changed.

Comparing feed-water and product-water TDS can help track membrane performance over time. A low TDS reading does not prove that every contaminant is absent. If you are treating a specific concern, test for that concern.

Which RO system do we recommend?

Higher demand or more than one fixture?

If you want higher flow or plan to serve multiple fixtures, the PROformance 5-Stage 100 GPD system may be a better choice. Compare its production rating, storage options, and installation requirements with your planned demand. A higher gallons-per-day rating alone does not guarantee more flow at a distant faucet.

For a busy household with a basement installation, select the 14-gallon tank option described above. The PROformance faucet is included only when selected from the product options.

View the PROformance RO

Related: Shop reverse osmosis · Hardness treatment · Iron pretreatment