Reverse Osmosis Does Remove Sodium, But Not Completely

Yes, reverse osmosis (RO) removes most of the sodium that a water softener adds. A standard RO system can remove 95 to 98 percent of dissolved sodium, bringing levels down significantly from what comes out of a softened tap. However, "most" is not the same as "all"—a small amount of sodium typically remains in the water that passes through the membrane.

The reason this matters is that water softeners work by trading calcium and magnesium ions (which cause hardness) for sodium ions. This trade-off solves the hardness problem but leaves your water noticeably saltier. If you are on a sodium-restricted diet or concerned about sodium intake, that extra salt becomes a real issue. Reverse osmosis addresses this by forcing water through a semipermeable membrane so fine that most dissolved minerals—including sodium—cannot pass through.

The catch is that RO is not a perfect barrier. A small percentage of sodium molecules are small enough or move through in ways that let them slip past the membrane. The exact amount depends on the membrane's age, water pressure, and how much sodium was in the water to begin with. A newer membrane in good condition will perform better than an older one.

Key Takeaways

  • Reverse osmosis removes 95 to 98 percent of sodium from softened water, but a trace amount typically remains in the filtered water.
  • Water softeners add sodium because they swap it for hardness minerals, so if you want low-sodium water, RO is the next step after softening.
  • RO membrane age and water pressure affect how much sodium gets through, so older systems or low-pressure conditions leave more sodium behind.
  • The water that RO rejects (called reject water or brine) contains the sodium and minerals that did not pass through the membrane.

How Reverse Osmosis Filters Out Sodium

Reverse osmosis works by explore pressure to push water through a membrane with pores so small (around 0.0001 microns) that most dissolved substances cannot fit through. Sodium ions are tiny—much smaller than bacteria or viruses—but they are still larger than water molecules. The membrane lets water through while blocking most sodium and other minerals on the other side.

The system creates two streams: the filtered water (called permeate) that comes out of your tap, and the reject water (called concentrate or brine) that carries away what did not make it through. That reject water is where most of the sodium ends up. In a typical RO system, about 25 to 50 percent of the water that enters becomes permeate, and the rest becomes reject water that drains away.

The reason RO is so effective at sodium removal is that it does not rely on chemistry or ion exchange the way a softener does. It is purely physical—the membrane is a barrier that lets through only the smallest molecules. This is why RO can remove not just sodium but also other dissolved minerals, fluoride, and some contaminants that a softener leaves behind.

Why Softened Water Has Sodium in the First Place

A water softener removes hardness by running water through resin beads that are coated with sodium ions. When hard water (containing calcium and magnesium) passes through, those minerals stick to the resin and sodium ions are released into the water in their place. The result is soft water, but water that now contains more sodium than it did before.

The amount of sodium added depends on how hard your water is. Very hard water requires more ion exchanges, so more sodium ends up in the softened water. A person on a low-sodium diet or with high blood pressure may find that softened water adds an unacceptable amount of sodium to their daily intake, especially if they drink a lot of tap water or use it for cooking.

This is where reverse osmosis becomes useful. Because RO removes sodium so effectively, it can be installed as a point-of-use system (usually under the kitchen sink) to provide low-sodium drinking and cooking water while the rest of the house still benefits from the softened water for bathing and cleaning.

Combining a Water Softener and Reverse Osmosis System

Many households use both a softener and an RO system together. The softener treats all the water in the house, and the RO system treats only the water at one tap—usually the kitchen sink. This setup gives you soft water everywhere while keeping sodium out of the water you drink and cook with.

The order matters: softened water goes into the RO system. Because the softener has already removed the hardness minerals, the RO membrane does not get clogged or damaged by calcium and magnesium buildup. If you ran hard water directly through an RO system without softening first, the membrane would foul much faster and need replacement more often.

Some people install the RO system before the softener instead, which also works but is less common. In that setup, the RO removes most minerals (including hardness minerals), and the softener handles any remaining hardness. This approach uses less water in the RO reject stream, but it requires more frequent RO membrane changes because hard water is harder on the membrane.

What Happens to the Sodium That RO Removes

The sodium that reverse osmosis filters out goes into the reject water, which typically drains down the sink or into a drain line. For every gallon of drinking water an RO system produces, it usually sends 2 to 4 gallons of reject water down the drain. That reject water contains the sodium and other minerals that the membrane blocked.

This is one reason RO systems use more water than they produce—the reject stream carries away the contaminants. If water conservation is a concern, you can install a storage tank under the sink so the system runs during off-peak hours, or you can use the reject water for plants or cleaning rather than letting it drain away entirely.

Testing Sodium Levels in Your Water

If you want to know exactly how much sodium your softened water contains, or how much an RO system removes, a water test is the most reliable way to find out. You can buy a basic test kit at a hardware store that measures sodium, or you can send a sample to a lab for a full analysis.

Most water softener companies will test your water for free when they install or service the system. They can tell you the hardness level before softening and the sodium level after. If you already have an RO system, testing the water before and after it passes through the membrane shows you how much sodium is being removed.

Sodium levels in softened water typically range from 50 to 300 milligrams per liter, depending on how hard the original water was. After reverse osmosis, that number usually drops to 5 to 10 milligrams per liter or lower. For someone on a sodium-restricted diet, that difference can be significant over the course of a day or week.

Common Mistakes When Using RO With Softened Water

One frequent mistake is installing an RO system on hard water without softening first. The mineral-heavy water will clog or damage the RO membrane much faster than softened water would, and you will end up replacing the membrane every 6 to 12 months instead of every 2 to 3 years. Always soften the water before it reaches the RO system.

Another mistake is assuming that RO removes everything. While it removes sodium very effectively, it does not remove all contaminants equally. Some volatile organic compounds and certain pesticides can pass through an RO membrane. If you have specific water quality concerns beyond sodium, a water test will tell you whether RO alone is enough or whether you need additional filtration.

A third mistake is neglecting RO membrane maintenance. The membrane needs a new cartridge every 2 to 3 years under normal use, and more often if the water is very hard or the reject water is not draining properly. A clogged or worn membrane will not remove sodium as effectively, and the system may start producing water that tastes salty again.

Frequently Asked Questions

Can I use a potassium-based water softener instead to avoid sodium?

Yes, potassium chloride softeners add potassium instead of sodium, which is better for people on sodium-restricted diets. However, potassium-based resin costs more than sodium-based resin, and the softener itself is more expensive. Reverse osmosis is usually cheaper than switching to a potassium softener if your main concern is removing sodium from drinking water.

How often do I need to replace the RO membrane?

A reverse osmosis membrane typically lasts 2 to 3 years with softened water. If your water is harder than expected or the system is used heavily, it may need replacement sooner. The manufacturer's instructions will tell you the expected lifespan, and you can test the system's output to see when performance drops.

Will reverse osmosis remove other minerals I might want to keep?

Yes, RO removes most dissolved minerals, including some beneficial ones like calcium and magnesium. If mineral content is important to you, you can install a remineralization cartridge after the RO membrane to add back small amounts of minerals. Many point-of-use RO systems include this as an optional step.

Does the reject water from RO go to waste?

Most reject water drains away, but you can capture it for watering plants, cleaning, or other non-drinking uses. Some systems include a storage tank or diverter valve to make this easier. Capturing reject water reduces overall water use, though it requires extra plumbing.

Can I drink water that still has a small amount of sodium in it?

That depends on your doctor's recommendation. If you are on a strict low-sodium diet, even the small amount that passes through RO (typically 5 to 10 mg/L) may add up over time. A water test will show you the exact level, and your doctor can tell you whether it fits within your daily sodium limit.