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How Does Reverse Osmosis Work? 

Edited by Sensible Digs
What you should know before drinking reverse osmosis water.

Have you heard the term reverse osmosis and are curious about what it means? Perhaps you’ve seen one in action, and you’re eager to learn how it works. You’re not alone.

Knowing how reverse osmosis works can play a huge part when looking for a filter. Whether the filter is for your whole house or drinking water from the kitchen tap, it’s worth deliberating.

Transitioning from one way of drinking water to another isn’t a decision to take lightly. You want to be sure that you’re making the best and most eco-friendly choice for your home.

What Is Reverse Osmosis?

What Is Reverse Osmosis? Icon

Regular osmosis involves liquids — for instance, water — passing through a semipermeable membrane.

Think of this membrane as similar to mosquito netting — it only allows some compounds to pass through it. For instance, air can pass through the mesh, but mosquitoes and other insects can’t.

Dissolved substances in the water get caught in this membrane, whereas the water itself can pass through it.

The problem with osmosis is that the water will always move from the lower concentration side of the membrane to the higher concentration part (1).

For example, say you have dirty water on one end and slightly cleaner water on the other. The cleaner water will gravitate to the dirtier side to dilute it. That won’t do much for purifying the water.

Regular vs. Reverse Osmosis

With reverse osmosis, the semipermeable membrane is under pressure. The pressure gets applied from the higher-concentration side, where all the unwanted compounds are (2).

When enough pressure is applied, osmosis will occur in reverse. The water can’t naturally gravitate towards the higher-concentration side. Instead, it’s forced through the membrane, leaving the undesirable compounds behind.

How Does Reverse Osmosis Work?

How Does Reverse Osmosis Work? Icon

There are many facets to a reverse osmosis system. It includes everything from the mechanisms that make it work to the phases the water passes through on its journey to you.

To understand how it functions as a whole, you should be aware of:

Components of a Reverse Osmosis System

A reverse osmosis filter isn’t a single-piece device. Many separate parts work alongside each other to purify your water.

Some are vital — meaning any RO filter should have them. Others, though, will differ based on the model you buy. These are the mechanisms that make up these systems: (3)

Intake Valve

This is the valve that connects your main supply to let water directly flow into the reverse osmosis system.


The pre-filter removes large-particle debris such as sand and sediment. Some systems have more than one of these. Your unit may also have an activated carbon pre-filter to block other organic compounds.

Pressure Vessels

These hollow tubes contain the RO membrane. They are capable of withstanding the pressure needed to overcome regular osmosis.

Reverse Osmosis Membrane

The soul of the system — the reverse osmosis membrane — traps most of the contaminants you’re aiming to avoid. Some systems have several RO membranes to ensure purification.

The pores or holes of an RO membrane need to be extremely small for maximum purification. Remember the mosquito netting example? You want to keep all the unwanted organisms out of your water.

For this reason, the pores are 0.0001 microns. If that doesn’t sound impressive, consider that other filtration methods such as ultrafiltration and nanofiltration have pore sizes of 0.01 microns and 0.001 microns, respectively (4).

Pressure System

There may be one or more electrical pumps for large-scale systems. Smaller residential systems don’t usually use electricity, relying wholly on water pressure to work, instead.


Valves correct and control water flow through the system to keep things moving smoothly. This ensures there is no back-flow or blockages.

Storage Tank

The purified water, known as permeate, is stored in a tank. In industrial RO systems, the tanks can hold thousands of gallons of water. Residential systems are typically far smaller, usually 2–5 gallons (5).

Drain Line

The drain line carries concentrate, or dirty water, away. It prevents a build-up of pollutants in the system that can affect performance.


There can be more than one post-filter in higher-end systems. These filters are typically activated carbon and serve as a final purification measure to remove micropollutants.


Your RO system will likely come with its own faucet. That’s the last phase of the system that your water will pass through before emerging into your glass.

The Difference Between Stages and Passes

You’ll see two frequently-mentioned terms where it concerns reverse osmosis systems — stages and passes. Although they may sound similar, they’re not.

If you see RO systems advertised as four, five, or six-stage systems, this usually refers to the number of additional filters it has.

One-Stage vs. Two-Stage Reverse Osmosis

In a one-stage system, water can only go in one of two places. Either it’s drained away as concentrate (impure water) or it emerges as permeate (pure water) from the faucet.

Two-stage systems give the concentrate the chance to pass through the system to become permeate. This allows for improved water output (6).

One-Pass vs. Two-Pass Reverse Osmosis

One-pass setups are much like their single-stage counterparts. The water enters as concentrate and emerges as permeate. This leaves the wastewater or concentrate going down the drain.

With a two-pass system, though, you essentially have two RO systems in one (7). The permeate from the first pass goes through a second treatment. Concentrate, at the last stage of treatment, is recycled back to the feed rather than being wasted.

How Does an RO System Work?

Now that you’re familiar with the RO basics, we can explain how these filtration units operate. This is the journey your water will take:

  1. Entry: Water flows through your pipe into the RO system.
  2. Pre-filtration: One or more filters clear the water of physical debris and, depending on your unit, certain chemicals such as chlorine. These filters ease the burden from the RO membranes.
  3. Reverse osmosis: The water gets forced through the reverse osmosis membrane(s) at sufficient pressure to keep it moving forward. The concentrate is either fed back into the system or drained away, contingent on the type you have.
  4. Different paths: At this point, the water will either continue to emerge from the faucet or make a second pass through your RO system. The former will occur in single-stage and single-pass units, and the latter will happen if it’s a double-pass model.
  5. Post-filtration: The water may pass through more filters, as we discussed above, for final purification. If you have a simpler system, this step won’t apply.
  6. Exit: Whichever type of RO system you have, the water will come out in the same place, purified, from the faucet.

What Does Reverse Osmosis Remove?

The most vital feature of any filtration system is how effective it is at purifying your water. Most household units filter out bigger forms of debris, such as sediment and rust particles.

The question when it comes to water filters isn’t about the small stuff. Rather, we want to know what unseen hazards it can block out, like viruses or bacteria.

First, you should know that the legal definition of a contaminant is anything in your water that’s not, well, water. That doesn’t necessarily mean all sorts are dangerous, but some can be (8).

The Centers for Disease Control and Prevention (CDC) rates reverse osmosis as highly effective at removing or reducing the following (9):

  • Common chemical contaminants — metal ions and aqueous salts.
  • Certain biological contaminants — bacteria, protozoa, viruses.

Common Chemical Contaminants

Trace minerals in your water can make it metallic, salty, or otherwise odd-tasting. Some compounds, such as lead, can even be harmful in excessive quantities (10).

These are the compounds that reverse osmosis can get rid of (11):

  • Chloride.
  • Copper.
  • Chromium.
  • Lead.
  • Sodium.

Reverse osmosis systems are capable of reducing levels of:

  • Arsenic.
  • Calcium.
  • Fluoride.
  • Magnesium.
  • Nitrate.
  • Phosphorus.
  • Potassium.
  • Radium.
  • Sodium.
  • Sulfate.
  • Volatile organic chemicals (VOCs) — only with a post-filter.
Does Reverse Osmosis Remove Useful Chemicals?

It can at least reduce their quantities. As you may have realized, not every item above is considered dangerous. For example, calcium, potassium, and fluoride are all beneficial for our health. The long-term effects of drinking demineralized water aren’t yet known (12).

Biological Contaminants

A biological contaminant is defined as any living organism found in your water. That includes diverse species of bacteria, viruses, and parasites such as microscopic worms.

According to the CDC, these are examples of biological contaminants that reverse osmosis is highly effective against: (13)

  • Bacteria: E. coli, salmonella, campylobacter, and shigella.
  • Protozoa: Giardia and cryptosporidium.
  • Viruses: Norovirus, rotavirus, hepatitis A, and enteric.

Bacteria are found almost everywhere, including in the water we drink. Most of these species are harmless, but some — like E. coli — can make us sick.

Shigella, a group of bacteria that can grow in private wells, is another example. Shigellosis can result in contagious diarrhea, fever, and other nasty symptoms.

On a positive note, reverse osmosis is capable of eliminating up to 99 percent of bacterial cells in tap water (14).


Parasites giardia and cryptosporidium are two common culprits in waterborne outbreaks of diarrhea. These protozoa are less sensitive to standard water treatments than bacteria and viruses, meaning they’re harder to kill.

But with the minuscule pore size of the filter, reverse osmosis can block both giardia and cryptosporidium from flowing through with your water (15).


Reverse osmosis membranes are one of the few types of filters capable of stopping the passage of ultra-small viruses (16).

Waterborne viruses can range from mild to severe, causing horrible symptoms like diarrhea. Worse yet, in the case of Hepatitis A and E, they can cause potentially serious infections.

Although the United States boasts one of the safest drinking water supplies worldwide, contamination can still occur.

Reverse Osmosis Filter Specifications

It is wise to understand the specifications of a reverse osmosis filter. This can help you decide whether or not it’s effective and right for you.

Not every reverse osmosis unit is created equally. NSF International, formerly the National Sanitation Foundation, sets the standards on such devices.

The NSF codes show you what an RO filter can protect against. These are the common certification numbers and what they mean (17):

  • NSF 41: Refers to taste and odor treatments. That means your water should emerge without smell or noticeable taste.
  • NSF 53: This code means cyst reduction, which refers to the spores or oocysts parasites, such as giardia, that exist in water.
  • NSF 58: You want to be sure the reverse osmosis filter you buy has NSF 58 on it. That means it’s up to the reverse osmosis standards set by the organization.

Why Use Reverse Osmosis Filters?

Why Use Reverse Osmosis Filters? Icon

Overhauling your current method of filtering water or cutting out bottled water isn’t an easy decision to take.

The advantages of reverse osmosis filters include:

Cleaner, Better-Tasting Water

As long as you buy a quality RO system from a reputable manufacturer, you can rest assured that your drinking water is clean.

Certain minerals can affect how your water tastes.

If a bitter or metallic undertaste puts you off drinking from your tap, a reverse osmosis system could solve your problem (18). Your unit can also remove unpleasant or suspicious odors and colors from your water.

Automatic Filtration

Frustrated with refilling a pitcher-style filter every time you get a drink? Then you may be intrigued by reverse osmosis systems.

Once the unit is set up and installed, your job is more or less done. You’ll have to keep up with maintaining your system, but the filtration process is automated.

Reduce Your Plastic Use

Plastic, in general, is a major source of marine debris. It also presents a hazard to wildlife. Although recycling habits are improving, there are still millions of tons of plastic that end up in landfills.

With a reverse osmosis system, you can opt for reusable bottles to cut back on your plastic use. Drinking from your tap is the most eco-friendly choice.

Lots of Options

Reverse osmosis filters come in all sizes and styles. You can opt for a small unit that fits under your sink. Or, if you’re committed to purified water everywhere, you can use a whole-house system.

Search for a model that meets your demands, e.g., one that eliminates odors and tastes.

Maintaining a Reverse Osmosis System

Maintaining a Reverse Osmosis System Icon

Although reverse osmosis systems don’t require much hands-on effort to operate, they do need maintenance.

That translates to sanitization and replacing filters and membranes when they get dirty, as fouling can impact performance.

Specific maintenance guidelines will differ based on the size and type of unit you’ve purchased.


Always read the instructions that came with your system. This ensures you learn how often essential components such as filters and membranes need replacing. Don’t guess or assume.

How to Clean a Reverse Osmosis System

Any water treatment system requires disinfecting periodically. You should take the time to sanitize your unit at least once a year or as directed by the manufacturer.

We recommend cleaning it out at the same time as you’re ready to install new filters. That way, you can get it all done at once. Here’s how to get your reverse osmosis system spick and span again:

  1. Switch off the water: Cut off the water supply to your unit. If you have anything else connected to your RO system, such as an ice-maker, disconnect them.
  2. Depressurize: Open the water faucet attached to your tank and let it run so that your storage tank drains completely.
  3. Remove all membranes and filters: Take out all of the RO membranes and filters in your unit. If you’re unsure how many there are and how to pull them out, consult the manual.
  4. Apply sanitation solution: Use bleach or a product like Sani-System, which is approved by the EPA and NSF. Pour the contents into the portion of your unit that water travels through first, which is usually the filter housing marked “sediment.”
  5. Replace housing and tubes: Put the filter housings back in place, and reconnect any tubes. Don’t put the new filters in yet.
  6. Reconnect the system: Set the system up again, connecting it to your water supply line.
  7. Switch water on: Turn the water back on. Your cleaning agent of choice will distribute itself throughout.
  8. Wait: Let the solution sit for at least 10 minutes.
  9. First flush: Open up your faucet and allow the water to run for another 10 minutes or so. This time may vary based on your specific model.
  10. Let system rest: Shut off your faucet and leave the RO system where it is. Take a break for 10 minutes.
  11. Second flush: Repeat the same process, letting the water run from your faucet for 10 minutes.
  12. Shut off water supply: Follow the same steps as you did at the start. Cut off the water supply and depressurize by running water from your faucet.
  13. Install filters and membranes: Place your new membranes and filters in the correct housings.
  14. Set up system: Replace your reverse osmosis system, and enjoy the knowledge that it’s running like new.

Expected Lifespan of an RO System

The expected lifespan of your reverse osmosis system is dependent on the maintenance and your water conditions. They can stay with you for 10 to 15 years, if cared for properly.

  • Maintenance: Poor maintenance doesn’t bode well for the longevity of any gadget. Keep up with filter and membrane replacements, and don’t neglect full-system sanitization. If a part such as tubing or filter housing breaks, replace it quickly. This would be better than letting it compromise the rest of the system.
  • Water conditions: Harsh water conditions can wear out your filters and membranes faster (19). If your system has to process water contaminated by heavy-duty pollutants, it may require more frequent maintenance to stay running.


FAQs Icon

Still not sure?

These are the most common questions people ask about the reverse osmosis process.

Reverse Osmosis Water vs. Bottled Water?

Bottled water is not the same as reverse osmosis water, even if companies use RO treatments for their water. For instance, Dasani water has undergone reverse osmosis, but it contains added minerals (20).

Are you wondering whether or not you’ll still need to buy bottled water if you have an RO system? Before you decide, you should know the pros and cons of each.

Bottled Water

You may be under the impression that bottled water has additional health benefits compared to other sources. These purported advantages are usually about the water being extra-pure or having certain wellness-boosting minerals (21).

There is some truth to these claims. Mineral water has demonstrated health benefits. For example, calcium-rich water can enhance bone density.

Perhaps a wariness of your local water system has kept you on the bottle. At least you know that what you’re drinking is regulated by the Food and Drug Administration (FDA) (22). Plus, bottled water is convenient. You don’t have to fill anything up, or carry a pitcher to and from your fridge to the tap.

The downside is that you’re using plastic, and you’re spending money. Certain brands of water aren’t cheap, and it can get expensive over time.


  • Convenient.
  • Some brands contain added minerals and nutrients.
  • FDA-regulated.


  • Can be costly.
  • Not eco-friendly.

Reverse Osmosis Water

As we’ve discussed, reverse osmosis water can taste just as fresh as the bottled version. Filters can take out foul odors and tastes. Better yet, they also remove other contaminants such as bacteria, viruses, and parasites.

Choosing to use RO filtered water can save you cash. If you’re drinking large quantities of bottled water per day, you’ll be saving trips to the grocery store to buy more.

You’ll also save on time. The filtration is automated to avoid inconvenience, and you won’t be producing plastic waste every time you use it.

If you’re concerned about digging deep for your DIY skills, maintenance isn’t too effort-intensive. All you’re obliged to do is switch out filters and sanitize the system, neither of which is too tough.

Residential systems rely on pressure rather than electricity to work, meaning they’re energy-saving. The caveat is that if you drain your storage tank, it won’t refill instantaneously; the process takes time.

Another downside is that you don’t get the same guarantee of water quality as with bottled water. Over time, if the RO membranes or filters grow foul and aren’t replaced, purity and other factors will be affected (23).


  • No plastic.
  • Money-saving for high-consumption households.
  • Automatic filtration.
  • Simple to maintain.


  • Tank can take time to refill.
  • Water quality can deteriorate if the unit isn’t maintained.

How Much Water Can RO Systems Produce Daily?

A typical unit will produce 10–35 gallons of water per day. Now, the typical U.S. family goes through roughly 300 gallons of water per day. However, only 19 percent — 57 gallons — of that is from the faucet (24).

You probably won’t be washing your hands or brushing your teeth with reverse osmosis water. So you can shrink that figure even further. If you use your system for only drinking water and cooking, the water it produces should be more than sufficient.

Do Reverse Osmosis Systems Waste Water?

The average household model has a 20–30 percent recovery rate. That means that out of 100 gallons of water passing through your RO unit, only 20–30 gallons turns to treated water (25). That might sound overwhelmingly wasteful, but if you factor in the water waste per average household — roughly 88 gallons per day — it isn’t too bad.

Bear in mind that with other appliances such as washing machines and dishwashers, all of the water is wasted. You’re not reusing any of it.

Are Reverse Osmosis Systems Expensive?

Yes, the cost of a reverse osmosis system can be hundreds of dollars or more. But it all depends on the size of the one you buy.

Do Reverse Osmosis Systems Need Electricity?

Not always. Large-scale reverse osmosis systems may use electrical pumps. However, standard residential systems operate off water pressure alone.

Are Reverse Osmosis Filters Recyclable?

Yes, you should be able to recycle your used reverse osmosis water filters. Most companies have recycling programs with no extra cost besides shipping fees if needed. Otherwise, it’s safe to dispose of them with your regular garbage.

The Lowdown on Reverse Osmosis

The Lowdown on Reverse Osmosis Icon

There you go — you’re all clued up on how reverse osmosis works. By now, you should feel well-informed enough to take a stance on reverse osmosis filtration.

You’re also more informed on the bottled water vs. reverse osmosis debate.

There are hundreds of thousands of distinct public water systems across the United States. Remember to check that the RO system you purchase tackles your unique water issues, e.g., removing protozoa from well-water.

Although these filtration gadgets don’t use all the water they process, at least you aren’t contributing to plastic waste in the environment. Additionally, they’re more conservative than other appliances, such as your dishwasher and washing machine.

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