What Is Reverse Osmosis and How Does It Work? (+ RO Water Guide)

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If you are into home water treatment or just pay attention to the quality of water that you drink then you have probably heard about reverse osmosis. You might even have a filter system installed under the kitchen sink in your home.

But how does reverse osmosis work, actually?

Although the name sounds complicated, it really isn’t. Are you ready to go back to chemistry class?

What Is Reverse Osmosis?

A Definition

According to Merriam-Webster, reverse osmosis is “the movement of fresh water through a semipermeable membrane when pressure is applied to a solution (such as seawater) on one side of it”.

In other words, reverse osmosis (RO) is a technology to treat water and remove almost all impurities from it.

Furthermore, it is considered one of the safest water filtration methods which is why many bottlers in the U.S. rely on it. (Next time you go grocery shopping take 5 minutes of your time and check out some of the bottled water labels. Look for hints such as “Purified by Reverse Osmosis”.

RO is an abbreviation and stands for Reverse Osmosis.

What Is Reverse Osmosis Water?

Simply put, reverse osmosis water, or RO water, is water that was purified by the use of reverse osmosis. Like we said, it’s almost pure H2O with only tiny bits of foreign particles.

Pureness is also the reason why RO water is being used in medical applications (e.g. dialysis and injections) and other industry processes, and why it’s suited for drinking and cooking.

In the end, how pure reverse osmosis water really is depends on a variety of factors, such as the treatment equipment and the condition and quality of the feed water. As a general rule, feed water that contains more impurities prior to the purification will also contain higher amounts afterwards, compared to feed water that was cleaner in the first place.

It is also relevant what types of contaminants are present. For instance, large bacteria and viruses can be removed much easier and more effectively than small ions like fluoride.

How about actual numbers for purity levels? Somewhere between 85 and 98 percent contaminant removal is realistic. Thus, RO water is not 100 percent pure, but that doesn’t mean that it’s unsafe. The remaining solutes are unlikely to cause health problems.

By the way, these numbers are for the reverse osmosis process alone. A reverse osmosis water system for home use comes with additional filter stages that allow for up to 99% contaminant reduction rates.

Pro tip: Look for models with reduction rates certified by independent 3rd party organizations such as NSF. Need help? View our reverse osmosis system reviews.

How Does Reverse Osmosis Work?

Reverse osmosis is most commonly used to desalinate sea water in areas where fresh drinking water is hard to access. It is also used to recycle commercial wastewater. There are RO systems for homes, for commercial use, and large plants for industrial applications. But how exactly does reverse osmosis work?

Osmosis & Reverse Osmosis Process Diagram

Osmosis Process Diagram

Reverse Osmosis Process Diagram

Reverse osmosis is the opposite of a natural process called “osmosis”. If you remember your chemistry, osmosis occurs when a solvent moves from a low concentrate solution to a high concentrate solution through a semipermeable membrane; semipermeable meaning that only some substances can pass through it.

This process continues until both solute concentrations on either side of the membrane are equal. The movement of a solvent to equalize solute concentrations creates osmotic pressure.

In reverse osmosis the exact opposite happens. External hydraulic pressure is used to overcome the osmotic pressure and the flow of the solvent, usually water, is reversed. The solvent molecules now move from the high concentrate solution (feed water) to the low concentrate solution (permeate). The aim is to separate the solvent from the dissolved solids resulting in purified water.

However, not all water can be purified. No matter how efficient, every reverse osmosis application produces wastewater. The remaining concentrate a.k.a. reject which now contains all the impurities either goes down the drain or is fed back into the water supply for recycling.

Not sure if you fully understood how this works? Try this 90-second YouTube video:

About RO Membranes

A reverse osmosis membrane rejects contaminants based on their size and ionic charge – the larger or higher, the better.

Compared to micro and ultrafiltration membranes, an RO membrane has much smaller pores that water molecules can diffuse through. However, the majority of impurities can’t and are flushed down the drain. Thin-Film Composite (TFC) RO membranes that are commonly found in drinking water purification system reject particles – that includes salts, heavy metals, bacteria, viruses, chemicals and organics – down to a size of 0.0001 microns.

RO membrane

That being said, there is a key difference between reverse osmosis and “regular” filtration and that is the predominant removal mechanism. Regular filtration works by size exclusion. This simply means that if a solvent is too large it will be filtered out. Theoretically speaking, you can achieve perfect exclusion here given that membrane pores are small enough.

Not so with reverse osmosis, because it involves a diffusive mechanism so that filtration efficiency also depends on solute concentration, water pressure and water temperature. Higher water pressure means higher filtration efficiency, while lower water temperature results in lower efficiency.

FYI: The ideal pressure for operating a point-of-use home RO system is around 60 psi. In case pressure drops below 30 psi, it’s generally considered insufficient and should be increased using a pressure pump.

Membrane Types & Material

The majority of commercially manufactured RO membranes are TFC, cellulose acetate (CA) or cellulose triacetate (CTA) membranes.

On the one hand, TFC membranes are more durable than CA and CTA membranes and also have higher rejection rates, 98% on average for the common contaminants.

On the other hand, CA and CTA membranes are better at tolerating chlorine, but more susceptible to fouling from bacteria.

Each membrane is made up of a flat sheet rolled around a perforated core tube where the permeate water is channeled into. A membrane flat sheet has 3 layers: First, the active barrier skin about 0.2 microns in thickness, followed by 2 support layers about 100 microns in thickness that strengthen the very thin barrier layer.

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Questions? Don’t hesitate to ask – just leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald has been with BOS since the very beginning. She is head of content creation and has fully immersed herself into the home water treatment industry only to become an expert herself. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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