What is a Reverse Osmosis Water Filter and How Does it Work?

14 min read

If you want to know what a reverse osmosis (RO) filter is and how it works, you’ve come to the right place.

A high-quality reverse osmosis system can remove up to 99.9% of the contaminants found in tap water.

Some under-the-sink RO units even add essential trace minerals back into the water.

When it’s all said and done, your family can enjoy toxin-free, nutrient-rich, perfectly alkalized water every time.

In this ultimate guide, we dive deep into the science behind reverse osmosis systems, including how they can improve your health.

By the time you’re done reading, you’ll have a PhD in Hydrationology.

Let’s dive in!

What Is Reverse Osmosis?

Reverse osmosis is a water purification process that exists in a water filter that removes unwanted ions, molecules, and all sorts of unsavory pollutants from drinking water.

How does it work?

RO uses pressure to overcome a scientific principle called osmotic pressure. In osmosis, water becomes more concentrated as it passes through a membrane in order to maintain equilibrium on both sides. Reverse osmosis, however, blocks pollutants from entering the less concentrated side. 


The nasty chemicals and bacteria are trapped, leaving nothing but world-class, drinkalicious water for you and your family. 

What Is a Reverse Osmosis Filter?

A reverse osmosis water filter leverages more than just reverse osmosis — it uses a combination of sediment filters, carbon filters, and ion exchange resins to polish and shine your water until it’s sparkly-clean. 

Later on in this guide, you’ll learn exactly how these different pieces work together. 

For now, let’s take a closer look at the most important piece: the reverse osmosis membrane.

Using concentrated pressure, water is pushed through the RO membrane: a semi-permeable, compact membrane that even micro-sized heavy metals, pesticides and volatile organic compounds (VOCs) can’t slip through. 

Heck, even fluoride and lead are out of luck. They all have a one-way ticket to the waste drain. 

Benefits of Reverse Osmosis Water

When you break it down, the benefits of a reverse osmosis filter really start to stack up. 

Some of the top reasons to drink RO water include:

  • Improves taste
  • Protects your pipes from “hard” water
  • Removes chemical disinfectants, like chlorine
  • Makes perfectly alkalized, balanced pH water every time
  • Removes contaminants like lead, arsenic, pesticides and nuclear waste
  • Remineralizes your water with magnesium, calcium and other electrolytes
  • Limits plastic water bottle consumption and reduces your carbon footprint

In the end, the biggest benefit is the peace of mind you’ll get knowing that your family is safe and healthy. Even in areas where public water sources are closely regulated, heavy metals have been known to make their way into drinking water.

Fortunately, under-the-sink RO systems remove these contaminants before they have a chance to undermine your health. The best part, however, is that some RO systems also remineralize your water.  Filtration strips the water of beneficial nutrients, but remineralization puts them back in. 

At the same time, it alkalizes your water and balances pH. Ideally, you want your water to be slightly basic (not acidic).  Although more evidence is needed, early studies suggest that drinking alkaline water may have benefits for diabetes, high cholesterol, and high blood pressure. A separate 2012 study found that alkaline water with a pH of 8.8 may help reduce acid reflux.

Many of the unmonitored contaminants in our tap water can throw our body's pH levels out of wack. Check out this article to learn more about the benefits of alkaline water for pH balance.

Later on in this article, we’ll talk more about the health benefits of remineralized water. 

For now, let’s take a closer look at the shocking health risks associated with municipal water, then we’ll dive deep into the science of reverse osmosis systems.

Problems with Municipal Water

Over 286 million Americans are tapped into the community water supply. Despite pledging to keep this water squeaky-clean, the federal government has fallen short. Nearly 20% of the ponds, lakes, and reservoirs in the U.S. contain contaminants that violate their own standards. Most of the time, if levels are high, the federal government just looks the other way. Despite taking measures to ensure safe water consumption, there are still lingering amounts of hazardous contaminants. 

Some of the most common sources of water contamination include:

  • Arsenic
  • Copper
  • Fluoride
  • Chlorine
  • Radon
  • Uranium
  • Pesticides
  • Bisphenol-a (BPAs)
  • Manufacturing operation byproducts
  • Sewer overflows and wastewater releases
  • Runoff from concentrated animal farming operations

Ultimately, these contaminants have been linked to a host of illnesses and outbreaks, including: 

  • Giardia
  • Shigella
  • Norovirus
  • Legionella
  • Salmonella
  • Hepatitis A
  • Campylobacter
  • Cryptosporidium
  • E.coli (linked to excess fluoride)

Studies show that contaminated drinking water can have devastating effects on the gut and brain. Yikes!

Here’s a closer look at the health risks associated with common water pollutants:

1. Nuclear Waste

If radon and uranium sound familiar, that’s because they’re common types of nuclear waste, and yes, they’re found in home water supplies across the country. It doesn’t take a rocket scientist to know how awful these substances are for your health.

Are you ready for the shock of your life? 170 million Americans drink water that’s contaminated with nuclear waste, including 64% of Californians and a jaw-dropping 80% of Texans.

2. BPAs

BPAs from plastic belong to a class of pollutants called endocrine disruptors. They act as estrogen hormones in the body and can trigger all sorts of hormone imbalances. 

Plastic contaminants are a rapidly growing concern, and BPAs are just one of many endocrine disruptors that enter the water supply from pesticides, cosmetics and the pharmaceutical industry. 

3. Fluoride

Since 1945, the government has been adding fluoride to community water supplies. Although some experts argue that moderate amounts of fluoride can safely prevent cavities, studies also show that long-term exposure can contribute to learning and memory deficits in children.

Roughly 3 out of 4 households have fluoride in their tap water.

4. Chlorine

Chlorine is used in the municipal water supply to kill bacteria and prevent outbreaks. However, treating water with chemical disinfectants can lead to even more health problems. 

Disinfectants, like chlorine, leave a bitter taste and wreak havoc on the skin, hair, and gut. To make matters worse, several studies have linked chlorinated water to an increased risk for cancer, including colorectal and bladder cancer.

5. Wastewater Releases and Animal Farm Runoff

The bacteria and germs that fester in wastewater and animal farm runoff can cause illnesses and outbreaks. Infants, children, and the elderly are the most at risk, as well as people with compromised immune systems due to AIDS and other illnesses. 

Want to know what's in your tap water?

Enter your zip code to see the levels of these harmful contaminants in your city's water.

Fortunately, reverse osmosis can neutralize almost all of the health risks associated with polluted water.

Next, we’ll take a look at the different stages of reverse osmosis filters...

Stages of Reverse Osmosis Water Filtration Systems

Reverse osmosis filter systems can have anywhere from 3 to 6 stages of water filtration, including:

  1. Sediment filter to trap dirt, debris, decomposing vegetation rust, and calcium carbonate
  2. Second sediment filter with smaller pores to filter out tiny particles
  3. Activated carbon filter to trap organic chemicals like chlorine
  4. Ion exchange resins to target specific minerals and impurities
  5. Reverse osmosis membrane to remove hard-to-trap contaminants like arsenic, lead, and nuclear waste
  6. Polishing filter to remove any impurities that were not caught by the RO membrane

You’ll find these filters in different orders depending on the make and model of your RO system.

Here’s a look at some of the most common types of RO systems:

3-Stage Reverse Osmosis System

  1. Carbon pre-filter
  2. RO membrane
  3. Carbon post-filter

4-Stage Reverse Osmosis System

  1. Sediment pre-filter
  2. Carbon pre-filter
  3. RO membrane
  4. Polishing filter

5-Stage Reverse Osmosis System

  1. Sediment pre-filter
  2. Carbon pre-filter
  3. Second carbon pre-filter
  4. RO membrane
  5. Polishing filter

However, if you want the best-of-the-best, you should consider the next type of RO system:

5-Stage Reverse Osmosis System with Ion Exchange Resins

  1. Sediment pre-filter
  2. Carbon pre-filter
  3. Ion exchange resins
  4. RO membrane
  5. Polishing filter

In the next section, we’ll give you a detailed breakdown of how this fancy-shmancy RO system works.

All RO systems, however, share the same key elements:

  1. A pre-filter that catches the big stuff
  2. An RO membrane that traps notoriously hard-to-remove pollutants
  3. A polishing filter that finishes it off

Across the country, under-the-sink RO systems protect families from dangerous pollutants while improving the taste and smell of their water for drinking and cooking.

Next, we’ll take an in-depth look at the entire reverse osmosis water filter system in action…

How a Reverse Osmosis Water Filter Works: The Nitty-Gritty Details

We have to admit, we’re definitely biased towards the type of RO system we’re about to cover, but that’s because it’s one of the most effective RO designs out there. The combination of a sediment filter, carbon filter, ion exchange resins, RO membrane, and polish filter leaves your water perfectly clean and crystal clear. Plus, the final remineralization process alkalizes the water with important nutrients. By the time it hits your faucet, it’s the healthiest water money can buy.

Now, it’s time to seriously nerd-out on the science of RO water filtration. Let’s do it!

Step #1: Sediment Filtration

Your water’s journey begins with a sediment pre-filter to trap any loose dirt, rust, and other solid debris from stormwater runoff. It also removes the cloudiness and discoloration that can turn your water brown, orange, or yellow.

Sediment filters work through a process called mechanical filtration, which is just a fancy word for physically blocking unwanted particles. Think of it like an epic coffee filter or a screen door on steroids. The bottom line is, you want the fresh air to blow through, not the leaves. In the same way, sediment filters allow the water to flow through while leaving the contaminants behind. 

However, sediment filters do not trap dissolved solid matter, heavy metals, chemicals or bacteria — that’s for the other filters to take care of later on. 

The sediment filter also functions as a pre-treatment that protects the rest of the RO system. More specifically, it protects the flow restrictor, RO membrane, and carbon filter from clogging. Although carbon filters are able to filter out some sediment, larger dirt particles can still quickly clog it. 

Ultimately, the sediment filter helps the carbon filter maintain water pressure and run efficiently. In the long run, this extends the lifespan of the system and saves you money by reducing replacement costs. 

A sediment filter’s ability to remove certain sized particles is rated in microns, or one-millionth of a meter. To give you a frame of reference, a human hair is between 75 and 80 microns, and anything less than 35 microns you need a microscope to see. Residential RO systems work best with a sediment filter that’s roughly 5 microns.

After the water passes through the sediment pre-filter, it moves on to the carbon pre-filter…

Step #2: Carbon Filtration

Carbon-based filters are used at least once in all types of water filters. They capture compounds that give your water a bad smell and taste, like chlorine and other volatile organic compounds (VOCs). Similar to sediment filters, carbon filters protect the RO membrane from excessive wear and tear. In fact, charcoal and sediment pre-filters can extend the life of your RO membrane by 1 to 2 years. 

The environment will thank you too, since carbon filters cut down on waste by reducing the number of toxins in the RO membrane. Whereas sediment filters capture particles through mechanical filtration, carbon filters remove particles through absorption.

You might hear carbon filters referred to as charcoal filters, but it’s not the same charcoal that you used in your barbeque. Instead, they contain activated carbon, a type of carbon made by oxygenating charcoal. This process opens up millions of tiny pores that act as bonding sites for contaminants to stick to. 

How does it work?

Step #1: Activated charcoal alters the chlorine molecules and turns them into chloride. 

Step #2: The chloride clings to the filter’s pores and the clean water passes through.

It also works with chloramines, a compound used by the government to dissipate chlorine in the water supply.

Some of the most common types of carbon filters are:

  • Carbon block 
  • Radial flow carbon
  • Coconut shell carbon
  • Granular activated carbon

Coconut shell carbon, by the way, is the most environmentally friendly.

On its own, a charcoal filter can do a lot to clean and purify your water. 

However, in order to make the cleanest water possible, it has to join forces with ion exchange resins, an RO membrane, and a post-filter... 

Step #3: Ion Exchange Resins

The ion exchange process softens the water, improves the taste, and prevents mineral deposits from building up in your pipes. 

Ion exchange resins are chemically-engineered beads that replace unwanted elements in your water. In other words, ion exchange resins “exchange” the bad molecules for the good ones. 

How does it work?

In the manufacturing process, resin beads are charged with either positive or negative ions. Positive resin beads are able to remove negatively-charged elements from your water, and negative resin beads remove positively-charged ones. As the water flows over the beads, they attract and replace the elements you want to remove. So, in order to remove a negatively-charged pollutant, positively-charged resin beads are added to the system.

Reverse osmosis water filters use a wide variety of resin beads to attract and remove specific chemicals and minerals, such as:

  • PFAs
  • Heavy metals
  • Calcium 
  • Magnesium

For example, ion exchange resins can soften your water by removing excess calcium and magnesium. Calcium and magnesium are both positively-charged ions, so in order to remove them, the resin beads are embedded with negatively-charged sodium ions. When the hard water runs over the beads, the negatively-charged sodium is displaced in the water and the calcium and magnesium are removed. The end result is softer water that causes less buildup in your pipes.

Next, it’s time for the RO membrane…

Step #4: Reverse Osmosis Membrane

It's time for the RO membrane to work its magic!

By now, the sediment, chlorine, and excess minerals have all been removed, and it’s time to target the nastiest, most stubborn pollutants. Even though these pollutants are too small to even be seen with an electron microscope, they’re still no match for reverse osmosis.

Unlike a simple carbon water filter, a reverse osmosis membrane can separate and dissolve inorganic contaminants, like:

  • Lead
  • Arsenic 
  • Radium
  • Fluoride
  • Bacteria
  • Herbicides
  • Pesticides
  • Pharmaceuticals
  • Nuclear particles
  • Polyfluorinated substances (PFAs)
  • Volatile organic compounds (VOCs)


Lead is one of the most dangerous contaminants found in tap water, especially for infants and children. It can cause growth and developmental issues when ingested at an early age, which is why it's important to make sure it's being properly filtered out of your family's drinking water.

Plus, it catches any remaining chlorine that the carbon filter may have missed. 

How does it work?

1. A flow restrictor pressurizes the water and forces it through a compact, semi-permeable RO membrane. 

2. The water flows from the more concentrated side to the less concentrated side. 

3. The contaminants remain trapped on the more concentrated side. 

4. The leftover water drains as waste.

And now for the grand finale...

Step #5: Post Filter

Last but not least, the post filter uses another round of carbon and ion exchange resins to remove any remaining contaminants.

A reverse osmosis filter system takes roughly one minute to filter 2 to 3 ounces of water, gradually storing it until you’re thirsty. Once you turn on the tap to fill your glass, water flows from the storage tank and passes through the post filter before reaching your faucet.

High-end RO systems, however, add an extra step: they remineralize the water with essential minerals. Remineralization media, like Corosex (magnesium) and Calcite (calcium), balance pH and give your water a silky-smooth feel.

What happens if your RO unit skips this important final step?

As it turns out, drinking demineralized water can negatively affect digestion, hydration and mental and physical health.

Next, we’ll discuss why remineralization is so important…

Health Risks of RO Filters Without Remineralization

Most RO systems remove iron, calcium and magnesium during the filtration process. The only problem is that your body needs these minerals in order to stay healthy, and drinking demineralized water increases the risk of vitamin and mineral deficiencies. 

RO units that don’t remineralize their water have been linked to vitamin B12 deficiencies. This is because low mineral levels make it hard for the body to absorb vitamins from food. Research shows that drinking water low in magnesium and calcium can increase the risk of premature birth, heart disease, and bone fractures.

The good news, however, is that if you eat a well-rounded diet and take supplements, you should get all the vitamins and minerals you need. However, a lot of Americans have poor eating habits, and this makes water quality even more important. 

To make matters worse, cooking with demineralized water reduces the nutritional content of food. In fact, studies show that cooking can reduce magnesium and calcium levels by 60%, cobalt by 86%, copper by 66%, and manganese by up to 70%.

Next, we’ll take a closer look at the health benefits of these essential minerals...

Health Benefits of Remineralization

Natural spring water contains trace minerals from the rocks and soil, and your body is designed to soak them up like a sponge. Remineralization re-introduces the essential minerals that support nearly every organ in the body. 

Some of the most important minerals for your health are:

  • Magnesium
  • Calcium
  • Potassium
  • Sodium
  • Chloride
  • Manganese
  • Bicarbonate
  • Phosphate
  • Zinc
  • Iron

Your cells use them to produce energy, prevent muscle cramps, improve digestion, and support brain health. If your filter doesn’t have a remineralization step, you’re selling your body short. 

Fortunately, the under-the-sink RO units from Cloud Water Filters have you covered. Plus, they have the environment covered too...

Is Reverse Osmosis Environmentally Friendly?

A reverse osmosis system helps the environment in two ways:

  1. It makes waste water treatment more efficient.
  2. It reduces the impact of bottled water on the environment. 

After the wastewater drains from your RO system, it travels to a local water treatment plant. Since your water has already been treated with reverse osmosis, it’s much easier for them to process. 

The biggest impact that RO systems have, however, is that they reduce water bottle consumption. Plastic water bottles not only pollute the environment with BPAs, they also require a lot of petroleum to produce and ship. 


Over 80% of the plastic water bottles we use end up in landfills, where they take up to 1,000 years to decompose and leak harmful chemicals into our soil and water supply. Not to mention, they're expensive! The average household that buys bottled water regularly spends about $3,000 a year on plastic water bottles. By switching to a RO filter system, your family can save serious dough and help save the planet at the same time.

The bottom line is, by purifying your water at home, you can significantly reduce your carbon footprint.

Is Reverse Osmosis Water Right for You?

Let me ask you this, who isn’t an RO system right for?

If you live in a rural area, it can protect your family from pesticides, herbicides, and other farm runoff. And if we learned anything from the Flint Water Crisis, city dwellers are equally at risk. By 2016, an estimated 6,000 to 12,000 children were exposed to lead poisoning after corroded pipes leached into the water supply. Chances are, these kids would have fared much better if they had RO filters in their homes.

The U.S. military uses RO technology to turn salt water into drinking water, and other countries use RO units during floods and other natural disasters. 

No matter who you are or where you live, RO water is the creme-de-la-agua of hydration.

Finding the Best Reverse Osmosis System for Your Household

So, you’ve decided to put an RO system in your home... 

Congrats! Your family will soon be drinking the finest filtered water on the planet. 

Make sure that whichever system you choose it uses ion exchange resins to remove hard-to-trap pollutants and remineralization to balance pH and support nutrition.

Aside from that, look for accreditations from the following institutions:

Clean Drinking Water
Still have questions about putting an RO filtration system in your home? Take a look at this article that outlines 10 things to consider when buying a water filter.

As water contamination rates soar, so do new innovations in RO filters. 

Cloud features a state-of-the-art alkaline water technology that remineralizes and balances pH. Want to constantly monitor your water quality? With Cloud, you can do that. It interfaces with a mobile app for your paranoid pleasure. 

Cloud also provides full-service, white-glove installation (hey, not everyone is great with “righty-tighty, lefty-loosey''). Actually, we always recommend professional installation because it can be a pretty tricky process. 

Depending on your family’s usage, replacement water filters will automatically ship to your door every 6 to 18 months, so you never have to worry about dirty water ever again. 

And Cloud’s filter cartridges are so easy to replace that even a monkey, or a Millennial, could do it. 
Click here to join the waitlist for the world’s most innovative RO system.


Do you know what’s in your tap water?

Have you thought about what's in your tap water? You should check.