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How Does Reverse Osmosis Work? All You Need to Know

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

If you are looking for a water filter, you have probably seen the term “reverse osmosis” more than once. There are numerous reverse osmosis filters to choose from, made by some of the leading names in filtration.

Reverse osmosis is currently the most popular filtration option and with good reason, as it will remove the vast majority of contaminants from water. Despite being used in millions of homes, you might not be familiar with what it means.

In this article, we will explain how reverse osmosis works, the different stages of the filtration process, and the contaminants it can remove from your water. We will also answer some of the most popular questions people have about reverse osmosis filters.

Key Takeaways

  • Reverse osmosis filters effectively remove contaminants, providing cleaner and better-tasting water.
  • These systems are automated, requiring minimal effort to use and maintain.
  • Using a reverse osmosis system can reduce plastic waste from single-use water bottles.
  • There are various options available, from under-sink models to whole-house systems, to suit different needs.

What Is Reverse Osmosis?

What Is Reverse Osmosis? Icon

Regular osmosis is a process that filters liquids — most commonly water — by having them flow through a semipermeable membrane.

This membrane functions like a mosquito net — it only allows certain materials through it. For example, air can pass through the mesh of a mosquito net, but mosquitoes and other insects can’t.

In the case of an osmosis filter, water molecules can flow through but solid contaminants and dissolved materials won’t be able to.

The issue with this type of osmosis is that it can’t keep the filtered water and contaminated water separate after filtration. The clean water will naturally be drawn towards the dirty water, taking in some of the contaminants and leaving both sides dirty (1). This makes it ineffective as a method of purifying your drinking water.

Regular vs. Reverse Osmosis

In reverse osmosis, the semipermeable membrane has pressure applied to it from the higher-concentration side, where the contaminants are (2).

If enough pressure is applied, osmosis will occur in reverse. The purified water can’t be drawn back towards the higher-concentration side. Instead, it is forced away from the membrane after passing through it, leaving the unwanted compounds behind.

How Does Reverse Osmosis Work?

How Does Reverse Osmosis Work? Icon

Reverse osmosis units are complex systems. They contain everything from the mechanisms that encourage the water through the system to the various phases the water passes through to ensure it is as clean as possible.

To have a deeper understanding of how reverse osmosis systems work, there are some important parts you should be aware of:

Components of a Reverse Osmosis System

A reverse osmosis filter isn’t a single piece. There are many distinct parts that work together to purify your water.

Some of these components are essential, so every RO filter should have them. However, some other parts will differ depending on which model you buy. These are the mechanisms that make up reverse osmosis systems: (3)

Intake Valve

This valve is connected to your home’s main water supply to purify it before it reaches your outlets such as faucets.


The pre-filter removes large particles and debris such as sand and sediment. Some RO systems have more than one pre-filter. Your unit might 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 required to overcome regular osmosis.

Reverse Osmosis Membrane

The key part of these systems is the reverse osmosis membrane, which traps most of the contaminants you want to remove from your water. Some systems have several RO membranes to ensure the water is purified.

The pores or holes of an RO membrane need to be tiny for maximum purification. The smaller these holes are, the fewer contaminants will be able to get through.

To thoroughly filter the water, these membrane pores are 0.0001 microns. For comparison, other filtration methods such as ultrafiltration and nanofiltration have pore sizes of 0.01 microns and 0.001 microns respectively (4).

Pressure System

Depending on the size of your system, there may be one or more electrical pumps to encourage water through your home. Smaller residential systems rarely use electricity, instead relying on water pressure to function.


Valves control the flow of water through the system to ensure the filter runs smoothly. This is mainly a matter of ensuring there is no backflow or blockage in your pipes.

Storage Tank

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

Drain Line

The drain line carries concentrate, or dirty water, away from the purified water. This prevents a buildup of pollutants in the system that would otherwise affect its performance.


Post-filters are usually activated carbon filters and serve as an extra purification stage to remove any micropollutants left in the water. High-end systems can have more than one post-filter for more comprehensive filtration.


An RO system will often include a dedicated faucet. This is the final phase of the system that your comprehensively filtered water will pass through before it emerges and pours into your glass.

The Difference Between Stages and Passes

You will probably see two terms used frequently when reading about reverse osmosis systems — stages and passes. These terms sound similar but they have important differences in this context.

If you see an RO system described as being a four, five, or six-stage system, this usually refers to the number of filters it has. Generally speaking, the more stages, the better.

One-Stage vs. Two-Stage Reverse Osmosis

In a one-stage system, water can only go to one of two places. It is either drained away as concentrate (impure water) or emerges as permeate (pure water) from your faucet.

Two-stage systems allow the concentrate to pass through another stage of the system and possibly become permeate before it is discarded. This can increase your 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. The wastewater or concentrate is then drained and discarded.

However, a two-pass system is essentially two RO systems in one (7). The permeate from the first pass undergoes a second treatment. At the last stage of treatment, the concentrate is recycled back into the feed rather than being wasted.

How Does an RO System Work?

Now that you are familiar with the basics of reverse osmosis, we can take a closer look at how these filtration units operate. This is the process your water will undergo before it reaches your glass:

  1. Entry: Water flows through your pipe into the RO system.
  2. Pre-filtration: One or more pre-filters clear the water of physical debris and, depending on your unit, some chemicals such as chlorine. These filters ease the burden on the RO membranes.
  3. Reverse osmosis: The water is forced through the reverse osmosis membrane or membranes at sufficient pressure to ensure it keeps moving forward. The concentrate or wastewater is either fed back into the system for a second pass or drained away, depending on your system.
  4. Different paths: At this point, the filtered water will either continue to your faucet or make a second pass through your RO system. In single-stage and single-pass units, the water will be ready for use but a double-pass model will complete the process twice.
  5. Post-filtration: The water may pass through some extra filters to remove any micro contaminants still left in the water. This usually only happens in high-end systems.
  6. Exit: With the water filtered, it can now exit your system via an outlet, often a dedicated RO system faucet.

What Does Reverse Osmosis Remove?

The most important aspect of any filtration system is how effective it is at purifying your water. Most residential water filters can remove larger debris, such as sediment and rust particles.

However, if you are considering a reverse osmosis system, you are probably looking for more thorough filtration. This can include removing unseen hazards such as viruses and bacteria.

The legal definition of a contaminant is anything in your water other than H2O. A very low percentage of contaminants are dangerous, but some of these can be very harmful (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 such as metal ions and aqueous salts
  • Certain biological contaminants such as bacteria, protozoa, and viruses

Common Chemical Contaminants

Trace chemical contaminants in your water can make it taste metallic, salty, or otherwise unpleasant. Some compounds, such as lead, can be very harmful if ingested regularly (10).

Here are some of the compounds that reverse osmosis can remove from your water (11):

  • Chloride
  • Copper
  • Chromium
  • Lead
  • Sodium

Reverse osmosis systems are also capable of reducing the 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 Beneficial Material?

Reverse osmosis is very effective at removing harmful material, but this means it can also remove beneficial minerals and chemicals. For example, calcium, potassium, and fluoride can all have health benefits (12). Some filter systems overcome this by adding minerals to the water after filtering it, so you can enjoy the benefits of purified water with healthy mineral content.

Biological Contaminants

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

According to the CDC, here are some 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 our drinking water. Most bacteria are harmless, but some — such as E. coli — can make us very ill.

Shigella, a group of bacteria that can grow in private wells, can be a major issue for people living off-grid. Shigellosis can cause contagious diarrhea, fever, and other unpleasant symptoms.

Thankfully, reverse osmosis is capable of eliminating up to 99% of bacterial cells found in tap water (14).


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

However, the minuscule pore size of the filter membrane means reverse osmosis can remove both giardia and cryptosporidium from your drinking water (15).


Reverse osmosis membranes are one of the few filter types capable of stopping microscopic virus cells (16).

Waterborne viruses can range from mild to severe, causing illnesses such as diarrhea. They can also cause serious infections such as Hepatitis A and E.

Although the United States boasts among the safest drinking water supplies in the world, contamination can still occur locally. This is why it is worth protecting yourself with a home filter.

Reverse Osmosis Filter Specifications

Not all reverse osmosis units are created equal. NSF International, formerly the National Sanitation Foundation, sets the industry safety standards for water filtration devices.

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

  • NSF 41: The filter can treat taste and odor. This means your water should emerge without an unpleasant smell or taste.
  • NSF 53: This code refers to cyst reduction, specifically spores or oocysts parasites such as giardia, that exist in water.
  • NSF 58: If you are choosing a reverse osmosis filter, you want one that is NSF 58 compliant. This means it meets the NSF’s reverse osmosis safety standards.

Why Use Reverse Osmosis Filters?

Why Use Reverse Osmosis Filters? Icon

Replacing your current filtration system or leaving behind bottled water can be a difficult decision. However, there are many benefits of reverse osmosis systems, including:

Cleaner, Better-Tasting Water

As long as you buy a quality reverse osmosis system from a reputable manufacturer, you can rest assured that your drinking water will be much cleaner.

If the water from your faucet tastes bitter or metallic, a reverse osmosis system should remove the contaminants causing the issue (18). Your unit can also remove unpleasant odors and colors from your water.

Automatic Filtration

A pitcher-style filter might seem cheap and convenient, but you will need to refill it multiple times per day. In contrast, a reverse osmosis system is largely automated.

Once the unit is installed, it will automatically filter the water from your supply. You will need to replace its filters, but far less often than the smaller filters in pitchers.

Reduce Your Plastic Use

Plastic is a major source of marine debris. It is also hazardous to wildlife on land. Although recycling habits are improving, in the United States alone, millions of tons of plastic still end up in landfill every year.

One of the main contributors to this is single-use plastic water bottles. A reverse osmosis system will bring your home’s drinking water up to a similar standard to bottled water, allowing you to use reusable bottles instead. This makes an RO system an eco-friendly choice if you previously found your water supply to be undrinkable.

Lots of Options

Reverse osmosis filters are available in all sizes and styles. You can opt for a small unit that fits under your sink. Or, if you want purified water throughout your home, you can install a whole-house system.

Look for a model that meets your specific demands. If you know your water is safe but it tastes unpleasant, look for a system that eliminates bad odors and tastes.

Maintaining a Reverse Osmosis System

Maintaining a Reverse Osmosis System Icon

Although reverse osmosis systems don’t require any effort to use, they do need maintenance from time to time. Specifically,  this means sanitizing and replacing filters and membranes when they get dirty, as they won’t perform optimally when they are covered in contaminants.

Specific maintenance guidelines will differ based on the size and type of your unit.


Always read the instructions or user manual included with your system. This should walk you through replacing essential components such as filters and membranes. If you don’t have the instructions, you should be able to find them on the manufacturer’s website.

How to Clean a Reverse Osmosis System

Any water treatment system will require disinfecting periodically. You should sanitize your unit at least once a year or however often the manufacturer recommends.

We recommend cleaning it at the same time as you install new filters. This means you won’t have to perform your maintenance in two sessions. You should always follow the manufacturer’s instructions but here is a general guide on how to clean your reverse osmosis system:

  1. Switch off the water: Shut off the water supply to your unit. If you have any other appliances connected to your RO system, such as an ice maker, disconnect them.
  2. Depressurize: Open the water faucet attached to your RO tank and run it until 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 are unsure how many there are and how to remove them, consult the manual.
  4. Apply sanitation solution: Apply bleach or a product such as Sani-System, which is approved by the EPA and NSF. Pour the solution into the part 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 lines. 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 chosen cleaning agent will be distributed throughout the system.
  8. Wait: Leave the solution to work for at least 10 minutes.
  9. First flush: Open the faucet and allow the water to run for another 10 minutes or so. This time may vary depending on your specific model.
  10. Let the system rest: Shut off your faucet and leave the RO system as 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 the water supply: Repeat the initial steps to shut off the water supply. Then, depressurize by running water from your faucet.
  13. Install filters and membranes: Place your new membranes and filters in their correct housings.
  14. Set up system: You can start using your reverse osmosis system, safe in the knowledge that it will stay hygienic and provide safe water.

Expected Lifespan of an RO System

The expected lifespan of a reverse osmosis system depends on its quality, maintenance, and how contaminated your water supply is. A properly maintained RO system from a reputable company should serve you for 10 to 15 years.

  • Maintenance: Poor maintenance will shorten the lifespan of any technology. Replace the filters and membranes when necessary and don’t forget to perform a full-system sanitization when required. If a part such as tubing or filter housing breaks, replace it quickly. Leaving a damaged part in place can lead to a wider issue with the system.
  • Water conditions: If your water supply is of particularly low quality, your filters and membranes will likely wear out faster (19). If your system has to process more severe pollutants, you will probably need to replace the membranes and sanitize the system more frequently.


FAQs Icon

Reverse Osmosis Water vs. Bottled Water?

Bottled water is not identical to reverse osmosis water, although bottled water companies use RO treatments during their production. For example, Dasani water has undergone reverse osmosis, but it also contains added minerals (20).

If you are wondering if you will still need to buy bottled water or whether an RO system will be enough, there are some key points to keep in mind. Let’s explore the pros and cons of each type of water.

Bottled Water

There are many claims that bottled water has additional health benefits compared to tap water. These advantages are usually related to the water being extra-pure or containing certain beneficial minerals (21).

Mineral water has proven health benefits. For example, water with high calcium content can improve bone strength. This is why hard water supplies are also considered healthier than soft, even if they are worse for your plumbing.

In the United States, bottled water is regulated by the Food and Drug Administration (FDA) (22). It is also convenient as you won’t need to fill anything up, carry a pitcher, or perform any maintenance.

On the other hand, you will be spending far more money and producing a large amount of plastic waste. Leading bottled water brands can be relatively expensive, which will soon add up.


  • Convenient
  • Some brands contain added minerals and nutrients
  • FDA-regulated
  • No maintenance is required


  • Can be very expensive over time
  • Produces a huge amount of plastic waste

Reverse Osmosis Water

Reverse osmosis water can taste just as fresh and clean as bottled water, as filters can eliminate foul odors and tastes. More importantly, they also remove other contaminants such as bacteria, viruses, and parasites.

If you currently drink a lot of bottled water, switching to RO-filtered water can save you a lot of money in the long run. You won’t need to drive to your local store to buy more, helping you save time.

Despite requiring occasional maintenance, a reverse osmosis system is automated, supplying filtered water on demand. You also won’t produce plastic waste every time you use it, as you can use your own bottles.

If you are concerned about your DIY skills, maintaining an RO system is relatively straightforward. All you will need to do is occasionally replace the filters and sanitize the system, both of which are simple tasks.

Residential systems usually rely on pressure rather than electrical pumps to function, so they won’t consume energy. The downside of this is that if you drain your storage tank, it won’t refill immediately; the process takes time.

Another downside is that you don’t receive the same guarantee of water quality as you do from bottled water. If the RO membranes or filters aren’t replaced when required, your water purity will be affected (23).


  • No plastic waste
  • Economic for large households
  • Automatic filtration
  • Simple to maintain


  • The tank can take time to refill
  • Water quality will deteriorate without proper maintenance

How Much Water Can RO Systems Produce Daily?

An average reverse osmosis unit can produce 10 to 35 gallons of water per day. Given that an average family in the United States uses about 300 gallons of water per day, this might not seem like much. However, only 19% — 57 gallons — of that water consumption comes from the faucet (24).

You probably won’t wash your hands or brush your teeth using reverse osmosis water, reducing that figure even further. If you only use your RO system’s water for drinking and cooking, the volume it produces should be plenty.

Do Reverse Osmosis Systems Waste Water?

The average reverse osmosis unit for residential use has a 20% to 30% recovery rate. This means that out of 100 gallons of water that enters your RO unit, only 20 to 30 gallons will emerge as treated water (25). This sounds very wasteful but if you consider that the average household wastes roughly 88 gallons of water per day, it is less alarming.

Some RO systems will reduce the amount of water wasted by sending the dirty water through the filter membrane again, so you will waste as little of it as possible.

Are Reverse Osmosis Systems Expensive?

A reverse osmosis system can cost a few hundred dollars, or much more to buy a whole-house system. If you are only concerned with having clean drinking water in your home, a small RO unit and faucet should fulfill your needs. However, if you want all the water in your home to be treated, you will need to invest in a larger, more complex system that will usually cost thousands.

Do Reverse Osmosis Systems Need Electricity?

The majority of small reverse osmosis systems for residential use do not require electricity and depend on water pressure to function. However, larger RO systems such as whole-house filters and commercial units might feature electrical pumps as the water will need to travel further to reach the various outlets.

Are Reverse Osmosis Filters Recyclable?

If you buy a reputable brand, you should be able to recycle your reverse osmosis water filters. Most companies have recycling programs at no cost other than paying shipping fees. Otherwise, you should be able to recycle or dispose of them locally.

Do I Need a Reverse Osmosis System?

Having access to safe, clean drinking water is essential for our health and overall well-being. Water quality can vary dramatically depending on where we live and how our water is sourced. Although the majority of people in the United States have a municipal water supply, many in remote areas rely on well water, which may not be regulated.

While most municipal supplies and wells are perfectly safe, others can be affected by issues such as lead contamination or bacteria buildup.

If you are concerned that your drinking water might be contaminated and unsafe, a reverse osmosis system could be the ideal solution. These systems use advanced filtration technology to remove impurities and improve your water, providing water that is much safer and free from foul tastes and odors.

Does Reverse Osmosis Water Dehydrate You?

It has been claimed that drinking reverse osmosis water regularly can cause dehydration. While it is true that reverse osmosis filtration removes many minerals from water, there is no evidence that drinking treated water will negatively affect your hydration level.

Additionally, as reverse osmosis removes excess sodium from the water, it reduces excess fluid retention in the body, which can cause feelings of bloatedness and swollen hands and feet.

If you eat a healthy diet and exercise regularly, there is no reason to worry about becoming dehydrated due to drinking purer forms of water such as that created by reverse osmosis.

How Long Do Reverse Osmosis Filters Last?

The filters in reverse osmosis units can last up to 12 months, depending on the type of filter, the water quality, and how much it processes. When choosing an RO filter, keep in mind that there are several different configurations with different benefits and drawbacks, so take the time to explore each of them.

Many filters feature multiple stages that work together to trap small particles and large contaminants. Simpler models will be less expensive but can also be less effective at removing some types of impurities.

Is Reverse Osmosis the Best Way to Purify Water?

Reverse osmosis is currently one of the most popular ways to purify water, usually using multiple strong filters to remove contaminants from water. While this process is very effective, it can also be slow and costly.

For these reasons, many experts believe that RO is often not the best option to cleanse water. Some viable alternatives include filtration, distillation, and ozonation.

These methods are often more sustainable and affordable than reverse osmosis, making them better choices if you want clean water quickly, without breaking the bank.

What Is Better: Alkaline Water or Reverse Osmosis?

Alkaline water has supposed health benefits, including improving digestion, boosting energy levels, and reducing some of the signs of aging. It has a higher pH level than regular tap water, which can benefit those with acid reflux or digestive issues.

Additionally, alkaline water allegedly contains antioxidants that can neutralize free radicals in the body and boost overall health. However, it is important to note that relatively little research has been done into alkaline water’s benefits, so some of these claims might be unfounded.

In contrast, reverse osmosis is proven to be one of the most effective methods for removing impurities from tap water. Due to its multi-stage filtration system and specialized membranes that remove tiny particles, reverse osmosis produces high-quality H2O without an unusual taste or odor.

The only drawback is that this also removes beneficial materials such as fluoride and minerals from the pure water. To overcome this, some filters will add minerals back into the water after treating it.

If you don’t replace these minerals elsewhere in your diet, drinking reverse osmosis water might be counterproductive. You have other options, such as filtered tap water or using regular bottled distilled water.

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