How Reverse Osmosis Removes Salt From Softened Water

Most people in the United States are concerned about the quality of their drinking water. As a result, many use reverse osmosis to remove various contaminants from their drinking water. Some impurities, like salt in softened water, aren’t actually pollutants, yet they should be removed due to a variety of reasons, including underlying health issues.

Reverse osmosis systems can remove salts from softened water, with the percentage removed dependent on the design of the membrane in each model. Most semipermeable membrane have pore sizes of 0.0001 micron, which removes the larger molecules of salt (0.0007 microns) and sodium ions (0.00023 microns).

This article will go over the meaning of softened water, explain why softened water contains salt and why some people may need to eliminate these salts from softened water. The article will also explain how reverse osmosis removes salt from softened water.

What is softened water?

Softened water is water that has had the calcium, magnesium, and other ions that produce water hardness removed.

There are two types of hard water: temporary and permanent.

Water containing dissolved calcium bicarbonate is considered to be temporarily hard and can be softened by simply boiling it. Permanent Hard water, on the other hand, contains dissolved calcium sulfate that is resistant to boiling.

The main method that you can use to soften water is chemically, using the ion-exchange method.

Softening water using ion exchange resin involves placing ion exchange beads in special ion-exchange columns. When hard water passes through, its calcium and magnesium ions contents are replaced with sodium and potassium salts, generating softened water.

Ion-exchange columns usually have two tanks. There is a resin tank and a brine tank.

The brine tank supplies enough sodium to replace the hard water ions (calcium and magnesium) in the resin tank, completing the water softening process.

Why softened water contains salt

Softened water contains salt because salt (sodium chloride) is typically used in the softening process.

Sodium is used to cleanse the resin that traps the undesirable mineral ions of calcium and magnesium that cause water hardness. According to Environmental Protection Agency (EPA), a tiny quantity of sodium is exchanged for magnesium and calcium in this ‘ion-exchange process’.

Ultimately, this simply means that softened water contains some sodium.

The amount of salt in softened water is to some degree determined by the hardness of your water, which in turn, is influenced by your geographic location.

If your water hardness is high, it means that there is a greater concentration of calcium and magnesium ions in it, and the more calcium and magnesium in the water, the more sodium is required to soften it.

Why you should remove salt from water

Sodium is generally safe for human ingestion. In fact, sodium is even regarded by nutritionists as a necessary component in the human diet. For instance, sodium is an electrolyte that helps your body maintain fluid and blood volume so that it can function normally.

The human body needs it to function properly.

Sodium is not regarded as hazardous at usual levels of food and drinking water intake. However, if you have certain medical issues, you may be advised to reduce your sodium/salt intake or to completely avoid it.

According to the Dietary Guidelines for Americans, if you want to follow a healthy eating pattern, you should not ingest more than 2,300 milligrams (mg) of sodium per day. Even healthy people can develop certain complications if they take excessive sodium.

The Centers for Disease Control and Prevention (CDC), outlines the following effects that sodium can have on certain people: 

1. High blood pressure

Consuming too much sodium can result in more salt being absorbed into the human body. Blood volume can rise, and as a result, cause blood pressure to rise.

According to CDC, in most cases, uncontrolled high blood pressure can lead to heart attacks, cardiac failure, strokes, renal damage, and blindness. Which is why people with high blood pressure typically try to limit the salt content of their food and water.

2. Heart disease

As we have already established, consuming high amounts of sodium can raise blood pressure. If this continues for an extended length of time, it can lead to heart disease and stroke – a primary reason why heart disease sufferers are recommended to avoid salt.

3. Stroke

According to CDC, stroke is not only a leading cause of death in the United States but is also a major cause of substantial disability in adults. A high salt intake has repeatedly been linked to an increased risk of stroke.

Statistics from the Northern Manhattan Study (ref 1), a study that involved food frequency questionnaires, and intense participant monitoring, suggests that people who ingested less than 4000 mg of sodium per day had a higher risk of the stroke.

4. Liver problems

Research indicates excessive salt intake may cause liver problems (ref 2).

Excessive salt consumption is thought to produce a variety of changes in the liver, including malformed cells, increased cell death, and slowed cell division. All of these changes can contribute to hepatic fibrosis, a disease where excessive scar tissue builds up in the liver.

5. Kidney issues

Several studies show links between sodium intake, high blood pressure and chronic kidney diseases. People with kidney disease typically have a reduced ability to eliminate salt, or “sodium sensitivity”.

This affects their salt balance and makes them more susceptible to the detrimental effects of salt in softened water.

6. Influence on people losing weight

People who have lost a large amount of weight or who desire to lose weight can be greatly affected by salt. Excessive salt should especially be avoided by people who have lost a lot of weight as it can create water retention, a condition that is hazardous for your health.

How reverse osmosis removes salt from softened water.

Reverse osmosis can remove salt from softened water. Reverse Osmosis systems use pressure to force the softened water through a semi-permeable membrane. When water passes through the semipermeable membrane, all total dissolved solids, including sodium and potassium salts and mineral ions, are removed, leaving just pure water.

A reverse osmosis system has between three and five filtration stages, including a sediment pre-filter stage, carbon pre-filter, RO membrane, post-filter, and the final stage (or ‘post’) filter.

Sediment pre-filter:

Sediment pre-filters are the first stage of filtration and remove pollutants as small as 5 microns in diameter.

Contaminants such as dust, debris, and dirt are removed from the water in this initial stage. These larger particles are simply stuck, while the smaller ones pass through with the water to the next stage.

Carbon pre-filter:

The activated carbon filter absorbs contaminants, including chlorine, volatile organic compounds (VOCs), foul odors, and PFAS, while also improving the water’s flavor.

Semi-permeable membrane:

This is actually where a reverse osmosis system removes the salts from softened water. The semi-permeable membrane is the most crucial step, when an RO system eliminates up to 98 percent of total dissolved solids (TDS) from any type of water, including salt and metals.

The membrane in most reverse osmosis systems has pore sizes of 0.0001 microns, so it can easily remove the larger molecules of sodium chloride (0.0007 microns) and sodium ions (0.00023 microns).


Many RO systems have storage tanks. After all the impurities have been removed from the water, it is stored in a storage tank until it will be consumed.

Some newer RO systems are tankless, which are more water and waste efficient.

Post filter:

The filtered water flows through another carbon filter between the storage tank and the tap. This stage guarantees that the RO faucet produces the best-tasting quality drinking water free of all contaminants.

Should you soften water before reverse osmosis?

If you live in a region with hard water, the water should ideally be softened before running it through a reverse osmosis system. Overtime, the minerals in hard water degrade the filter and components of the system. Normal tap water does not need to be softened.

Hard water, in most cases, causes scale build-up, making it extremely difficult for the RO membrane to function well for long periods, resulting in poor water quality. Water softening does not remove contaminants or impurities from the water, but simply reduces water hardness.

However, softening water before running it through an RO water system is not necessary, especially if you do not come from a hard water area. 

Any hard water run through an RO system will simply result in the filters and membrane needing to be replaced more regularly to cope with the mineral content.

Water Purification Guide has a comprehensive article to help you determine if you need a water softener available here.

Can you run soft water through a reverse osmosis?

Soft water can be run through a reverse osmosis system. Water softening is usually advised because it protects the reverse osmosis membrane and extends its life by reducing scale build-up.

Water softening reduces the levels of calcium and magnesium in water by replacing it with sodium. Using softened water in a reverse osmosis system prevents mineral build-up on the membranes and piping. Ensuring the reverse osmosis system can remove impurities like chlorine and Total Dissolved Solids (TDS).

Is reverse osmosis the same as soft water?

Reverse osmosis is not the same as soft water. Reverse osmosis is a water purification system that filters drinking water of available impurities or contaminants using the semipermeable membrane and several pre and post filters. On the other hand, soft water is simply water with a low concentration of calcium and magnesium ions.


1. Gardener, H., Rundek, T., Wright, C.B., Elkind, M.S. and Sacco, R.L., 2012. Dietary sodium and risk of stroke in the Northern Manhattan study. Stroke43(5), pp.1200-1205. <>

2. Wang, G., Yeung, C.K., Wong, W.Y., Zhang, N., Wei, Y.F., Zhang, J.L., Yan, Y., Wong, C.Y., Tang, J.J., Chuai, M. and Lee, K.K.H., 2016. Liver fibrosis can be induced by high salt intake through excess reactive oxygen species (ROS) production. Journal of agricultural and food chemistry64(7), pp.1610-1617. <>

Russell Singleton

Russell has a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

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