What is Reverse Osmosis: Explained in Simple Terms

Reverse osmosis offers to provide the cleanest water available. Are there anReverse osmosis water filters provide the cleanest drinking water available. So what is reverse osmosis and how does it work? down sides to the RO process?

By: Rachel Carollo

Reverse Osmosis (RO) is a popular water filtration process that can remove up to 99.99% of specific contaminants. The semi-permeable membrane captures the smallest particles in your drinking water. RO is utilized in recycling and wastewater treatment, as well as consumer water filtration systems.

This filtration technology not only impacts health positively — it’s implemented worldwide. From residential homes to large-scale plants filtering millions of gallons per day, RO is at the forefront of clean water. Let’s take a look at what reverse osmosis water is and how reverse osmosis works.

What is reverse osmosis water?

Reverse Osmosis water is purified water that goes through the reverse osmosis process and is free from most harmful particles. The United States Environmental Protection Agency (EPA) describes reverse osmosis as a process that “forces water through a semi-permeable membrane under pressure, leaving contaminants behind.” The process involves more than simply forcing water through a “screen”. RO requires a considerable amount of water pressure to overcome natural osmotic pressure and effectively remove impurities.

The history of reverse osmosis water

It was back in the 1950s that scientists first considered the use of RO to remove salt from ocean water, and although it worked, it was not practical due to the small volume produced. This changed when two UCLA scientists created hand-cast membranes made from cellular acetate, allowing larger quantities of water to move through the RO process more efficiently. In 1965, the first commercial RO desalination plant started a small-scale operation in Coalinga, California.

How does reverse osmosis work?

To understand how reverse osmosis works, you need to first understand how osmosis works. Osmosis, without the reversal part of the equation, is when a low-concentrated solution moves through a semi-permeable membrane to get to the higher-concentrated solution, thus weakening it. 

Reverse Osmosis is the opposite. By applying pressure, the highly concentrated water (high contaminants) is pushed through a filter to a lower concentration (pure water). Most contaminants get trapped by the membrane, leaving you with filtered water.

For a visual explanation of how reverse osmosis works, you can check out this video by the Government of South Australia, which uses RO to desalinate water in the city of Adelaide. In the video, a water tank is displayed with a saltwater solution on one side and pure water on the other, separated by a semi-permeable membrane. The RO process is as follows:

  • Pressure is applied to the saltwater side of the tank, counteracting the natural osmotic pressure from the pure water side.
  • This pushes the saltwater through the filter. 
  • Due to the size of the salt molecules, only the smaller water molecules make it to the other side.
  • As a result, fresh water is added to the water side, leaving the salt on the other side.

Most RO systems work by using three to four stages of filtration in addition to the reverse osmosis membrane to remove contaminants. For example, a sediment filter can help filter debris, while a carbon filter can help remove organic compounds.

What does reverse osmosis remove?

Now that you know how reverse osmosis works, you may be wondering what contaminants reverse osmosis removes. The effectiveness of each RO system varies, meaning some may actually remove more contaminants than others. The differences will likely come down to which contaminants they remove or reduce, and how effective they are at doing so. But generally, a good RO system is highly effective at removing most:

  • Lead, arsenic, and fluoride
  • Bacteria, viruses, and cysts
  • Pesticides, herbicides, and fungicides 
  • Chlorine and chloramine 
  • Microplastics, PFOA/PFOS, VOCs
  • Minerals 
  • Sediment

Reverse osmosis is considered the best available technology by the EPA when it comes to removing uranium, radium, and other radionuclides. It is the only type of filter capable of removing small contaminants like fluoride. The EPA states that RO filters “are effective in eliminating all disease-causing organisms and most chemical contaminants.”

"The EPA states that RO filters, 'are effective in eliminating all disease-causing organisms and most chemical contaminants.'"

Aquasana’s SmartFlow™ Reverse Osmosis reduces the presence of 90 harmful contaminants including more than 99% of asbestos and cysts, 99% of lead and microplastics, 96% of chlorine and arsenic, and 90% of fluoride in addition to several other tap water hazards. It’s also important to note that many RO systems strip away too much — including removing healthy minerals that positively impact the taste and benefits of the water you drink. To ensure you’re getting the most out of your water, Aquasana’s Reverse Osmosis system uses remineralization technology to restore a good pH balance and retain healthy minerals.

What does reverse osmosis not remove from water?

Reverse osmosis filtration can remove up to 99% of contaminants but not 100% of them. This is simply because some contaminants are moleculary smaller than water so they can slip through the RO membrane. Contaminants a RO system may have trouble completely removing from water include:

  • Dissolved gasses like hydrogen sulfide
  • Some pesticides, herbicides, and fungicides
  • Some chlorine and chloramine
  • Some fluoride and arsenic

What is the difference between reverse osmosis and a regular water filter?

If you’re debating between an RO system and a regular under sink or countertop water filter, there are a few major differences you’ll want to consider:

Contaminant reduction

ROs are the only proven method of water filtration on the market today that removes fluoride. Because fluoride is such a microscopic contaminant, it takes a powerful filtration technology to remove it, which is where the semi-permeable membrane technology comes in. ROs also remove contaminants that most water filters cannot such as arsenic, total dissolved solids (TDS), and nitrate, a tasteless compound that is harmful for babies if ingested. To ensure your RO removes the tougher contaminants, look for a certification for NSF/ANSI Standard 58, which is the contaminant testing standard specifically created for ROs.


Reverse osmosis systems take a little more time to install and you should be familiar with your under sink plumbing. Unlike direct connect or countertop filters, ROs have permanent fixtures. Most ROs require a dedicated faucet to be installed, which means drilling a small hole in your countertop if you don’t already have one.

Water flow

The purpose of a water tank in an RO setup is to store filtered water so that when you need filtered water, it’s ready. Depending on how much water you use in one use, you may experience low water pressure from the dedicated faucet while the water tank refills. This is because the water tank has been emptied and needs to refill with filtered water. Tank capacities will vary from system to system, so if you have a larger family or plan to fill large water bottles, you may want to consider a system that offers a larger water tank.

Reverse osmosis facts

A few interesting reverse osmosis facts and ways the process is used:

  • RO water filters are gaining popularity in the foodservice industry – Many restaurants are investing in RO systems to help improve the taste of their food using the best quality water for cooking.
  • Reverse osmosis systems help car washes achieve a spotless rinse – Water that hasn’t been filtered can cause scaling and leave spots after a carwash, so some car wash facilities are utilizing RO systems to remove soluble salts that cause these issues so they can provide a “spotless rinse” for customers.
  • In the production of maple syrup, RO is used to separate the sugary concentrate from water in the sap.
  • The dairy industry uses RO filtration to concentrate whey and milk.
  • Wastewater goes through the RO process to create something drinkable, thereby earning the nickname, toilet to tap which may be unappealing but provides developing nations with the ability to produce drinkable water.

As stated above, RO is also used to desalinate seawater. In Dubai, where fresh water is limited, large-scale reverse osmosis filters convert about 416 million gallons of seawater to fresh water every day. Dubai’s groundwater supplies only 0.5% of the city’s water – that means the other 99.5% has to come from reverse osmosis. In order to produce 416 million gallons of fresh water, the system has to pump about 2.8 billion gallons of water through it each day.


SmartFlow™ Reverse Osmosis

High-efficiency reverse osmosis system removes up to 99.99% of 90 contaminants, including fluoride, arsenic, chlorine, and lead.

Reverse osmosis benefits

Many people wonder if reverse osmosis water is good or bad, but RO filtering has some seriously positive health benefits.

Helps reduce sodium intake

In homes that use water softeners to remove minerals from hard water, the ion exchange process that softeners use will leave you with water that’s fine for cleaning, bathing, and laundry — but not great for drinking. Most water softeners replace hard minerals with sodium, thus leading to a salty water taste. Using a reverse osmosis water filter enables you to enjoy the benefits of softer water without the added sodium and accompanying salty taste.

Helps prevent dementia

A classic example is this study which found that dialysis patients could prevent dementia (a comorbidity that occurred in 18 out of 258 patients) by simply using an RO filter. With no other treatment, scientists were able to improve the condition in 7 out of 9 previously exposed patients and prevent dementia in those whose water was treated from the start of the study.

Helps prevent gastrointestinal illness

Another study tracked the gastrointestinal health of 1400 families and found 14% more gastrointestinal illness in families drinking tap water than in those who were drinking water purified with reverse osmosis. The study noted that “14-40% of gastrointestinal illnesses are attributable to tap water meeting current standards and that the water distribution system appears to be partly responsible for these illnesses.”

To learn more about the benefits of reverse osmosis filtration, check out the pros and cons of reverse osmosis filtration.

How to choose the best reverse osmosis water filter

The best RO filters will have carefully engineered membranes that stand up to daily use. They filter for even the smallest particles and include additional steps to ensure that the water they provide is healthier and safer.

There’s a lot of misinformation out there so it’s important to understand how many contaminants a RO filter truly removes. Look for certifications from organizations like WQA, NSF, and IAPMO. A reverse osmosis system should pass NSF/ANSI Standards, including Standards 42, 53, 58, and 401. Read more about NSF standards here.

RO systems generally do a good job of stripping out harmful contaminants,, but in the process, many also strip essential minerals that naturally occur in water. There is some disagreement on this issue, with some health experts stating that the amount of minerals in water is negligible anyway, so RO systems are a great option. However, the venerable World Health Organization and many other health experts explain that water is an important source of minerals such as calcium, magnesium, and iron, especially for people in developing countries.

That’s why the best water filter system is one that removes harmful contaminants while remineralizing the water. That assures that it not only tastes great but also has the health benefits that essential minerals provide. Unlike other RO systems, Aquasana’s SmartFlow™ Reverse Osmosis includes a remineralizer to restore beneficial minerals for healthier drinking water. This filter effectively removes 90 harmful contaminants and is WQA tested and certified to meet NSF/ANSI standards 42, 53 (include P473), 58, 401, and CSA B483.1.