What Is A Reverse Osmosis System [The Full Definition]

Here is everything you need to know about a reverse osmosis water filter system.

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What is a Reverse Osmosis Water Filtration System?

Reverse osmosis water filter system is a water purification system that uses filtration cartridges which include pre-filter, thin semi-permeable membrane, post-filter, and other accessories to remove undesirable materials and provide clean water for drinking and other purposes.

The process involves using pressure to force the water molecules to flow through the semipermeable membrane while stopping bigger molecules such as bacteria and other impurities.

The Filtration Stages of an RO System

Stages in a reverse osmosis system
Stages in a reverse osmosis system

The reverse osmosis system basically involves 3 main filtration stages, Pre-filter, RO membrane, and a Post-filter stage. Here is a closer look at what happens in each of these stages.


The first stage in the reverse osmosis water filter system is pre-filter and its main purpose is to protect the RO membrane by filtering chlorine and fluoride and removing larger sediment as well as some dissolved solids.

This stage can have several different types of pre-filters ranging from sediment filter, granular activated carbon filter (GAC) and CTO Carbon block filters.

Sediment filters which are often made of Polypropylene are primarily the first line of defense in the RO method. They reduce or completely remove suspended particles such as dirt, rust, organic materials and sediments from the water down to 5 microns.

The granular activated carbon filter is commonly used to remove natural organic compounds, chemical contaminants, taste, and odor. It also reduces a range of other dissolved contaminants particularly chlorine.  

CTO Carbon block filters on the other hand basically have a filtration capability of about 0.5 to 10 micron which makes them quite helpful for removing chlorine, fluorides, taste, and odor, as well as for particulate filtration and insoluble lead reduction.

These pre-filters are essential because they help conserve the RO membrane which can get damaged due to exposure to too much chlorine and fluoride which are often found in municipal water or get clogged by excess sediment.

RO Membrane

RO membrane
RO Membrane

The reverse osmosis membrane is a Thin-film composite membrane (TFC) which is typically a semi-permeable membrane made of two or more layered materials. It’s considered as a molecular sieve with pores sized from 0.001 to 0.0001 microns.

The RO membrane is essentially the critical point of the RO system as its extremely tiny pores have a high rejection rate of around 95 to 98%.  

It’s very effective and aids in removing microscopic contaminants such as bacteria, and viruses from the water, as well as dissolved solids like sodium and calcium.

Post Filter(s)

Before the water is ready for use, it goes through a final stage which consists of one or more post-filters that help remove any remaining contaminant that may have slip past the membrane.

This stage can consist of several other stages including carbon filter, UV light filtration, and PH & Alkaline remineralization, all serving different purposes.

The carbon filter gives the water one last polish by extracting any remaining chlorine, contaminants, odors or taste residues missed by the membrane and the other pre-filters.

Many RO systems incorporate a further stage of filtration that involves an alkaline or PH filter. The purpose of this stage is to add or restore healthy minerals to the water which may have been removed as a result of the RO filtration process. The filters also help raise the PH of the water to more alkaline levels hence reducing the acidity of the water.  

An optional stage in some RO systems is Ultra Violet light filtration which involves the use of UV rays to destroy germs and microbes after the RO stage. This stage helps kill harmful contaminants and bacteria giving you pure and healthy water to drink and use.

Other Basic Components of an RO System

Most RO systems have the same basic components and basically, work the same way. The only difference comes in the quality of the components used and how they look. With that said, the following are the main basic components that make up an RO system and their functions.

Inlet Line and Valve

The inlet line and valve is the first point of the RO process. The inlet line supplies water to the RO system from the main water source or tank.

The inlet, valve, on the other hand, is positioned between the RO system and the main water supply. Its work is to let water into the RO system via the supply lines only when the water moves out of the tank. It’s a simple but crucial component of the system.

The Auto Shut Off Valve (SOV)

To help conserve water, the RO system is equipped with an automatic shut off valve which closes immediately the storage tank is full thereby stopping any more water supply through the membrane.

The valve activates when the pressure in the tank is around 2/3 of the feed pressure which helps prevent an over-spill of the water tank.

Once the water is drawn from the faucet, the tank pressure drops and the valve opens to allow the drinking water to pass through the membrane while diverting the contaminated wastewater down the drain.

Check Valve

Also known as a backflow prevention valve, the check valve is positioned just in the RO membrane outlet end and its work is to solely let water out so as to prevent any backflow of the already treated water from the storage tank.

Due to the pressurized RO system, a backward flow could ultimately rupture the RO membrane. Therefore, the check valve ensures water doesn’t flow backward from the system.

Flow Restrictor

The flow restrictor (or PLV – pressure limit valve) is often located at the RO drain line just after the membrane. Since the RO process operates at very high pressure, the flow restrictor helps maintain the water flow at the correct level required to ensure the system runs smoothly to deliver the highest quality drinking water.

By regulating the water flow, it also helps with the system efficiency as excess water doesn’t get washed.

Storage Tank

The storage tank holds the purified water once the RO process is complete. There are two bladders inside the tank that keep the water pressurized which enables it to exit when the faucet is opened.

The pressure also enables more water to enter the tank only until it is 2/3 of the water inlet pressure. Most standard RO storage tanks have a capacity of 2-4 gallons although the size can range from 2 to 9 gallons.

Faucet: Normal / Air Gap

At the end of the RO system is the faucet which is normally installed on the kitchen sink. It’s what you use to dispense the purified water. It can be an air gap faucet or normal faucet, but the most common between the two is the non-air gap models.

Drain Line

The drain line connects the outlet end of the RO membrane housing to a waste pipe and its purpose is to dispose of the wastewater containing all the impurities filtered out by the RO system.

RO Booster pump

The RO system is a pressure-driven process and as such its efficiency often relies on the incoming water pressure of the feeding system.

Low inlet pressure will ultimately affect the performance of the system and even compromised the water quality produced if it’s below 45 psi.

A typical RO system operates better at a water pressure of at least 50 psi. Therefore, an RO booster pump is used to raise the water pressure going into the system when it’s low.

It’s an optional component as the typical city water pressure is usually 60 psi and RO units can run well under this pressure. However, they can are even much better when using the boost pump as it can increase the water pressure to 80 psi or even higher.

How Does a Reverse Osmosis System Work?

The RO system is basically reverse osmosis as it uses an external pressure to reverse the natural osmosis process.

By applying external pressure, the system overcomes the natural osmotic pressure thereby forcing the water containing different kinds of dissolved contaminants through a semipermeable membrane.

The membrane only allows the water molecules as well as some other smaller organic molecules to pass through while blocking unwanted molecules and larger particles.

These filtered contaminants are then flushed away thereby leaving clean, fresh drinking water. Typically, the efficiency of any RO system will rely on the water pressure feeding into the system.

Here are 4 basic steps of the water flow in an RO system:

  1. When the pressurized water first enters the system, it passes through the prefiltration stage which consists of several filters including sediment filter, granular activated carbon filter (GAC) and CTO Carbon block filters. Here, contaminants like sediments, chlorine, dirt, salt, and odor that may damage or clog the RO membrane are removed. 
  2. After pre-filtration, the water is then forced through the RO membrane where the smallest impurities and dissolved particles down to 0.0001 microns are trapped. Only water passes through the membrane at this stage as well as some other very tiny molecules.
  3. After the water exits the membrane, the clean drinking water flows to the RO storage tank where it’s stored until needed while the wastewater containing the removed contaminants get flushed away in the drain line.
  4. Finally, once you open the faucet, the treated water goes through a final postfiltration stage which effectively polishes the water and removes any remaining contaminants that may have slip past the membrane before it enters the faucet for consumption.

This video demonstrates the water flow in a reverse osmosis system.

A reverse osmosis system provides a simple and effective solution for purifying your water and making it safe and clean both for consumption and other uses.

Below are several distinct benefits of the water that come with using an RO system.

The Benefits of using an RO System

Reverse osmosis water benefits

Eliminates Sodium

The first huge benefit of using an RO system is that it gets rid of virtually all the sodium in your water. This is a great benefit because sodium is known to cause high blood pressure, kidney disease as well as liver problems especially when consumed at high levels.

Many water softeners can’t achieve this since they are only capable of removing hard water minerals such as iron.

Removes Lead and Other Contaminants

Lead is another very common contaminant found in water.

Like sodium, it has extremely detrimental health effects when consumed at high levels. It can increase high blood pressure, cause kidney issues, nerve damage and has even been linked to infertility.

It can also cause serious developmental problems in infants or even cause anemia in children.

Reverse osmosis systems effectively remove all traces of lead from the water making it safe for consumption.  

Apart from lead and sodium. RO rids your water of many other particles, chemicals, and contaminants that can adversely affect your health, this includes cryptosporidium parasites, iron, manganese, arsenic, bacteria, and viruses.

Better Tasting Water

Water from the tap often contains chemicals such as chlorine and fluoride which have been intentionally added to it. Together with other contaminants and impurities, these substances end up influencing water’s natural taste making it unpleasant.

By removing all these additives, chemicals and particulate matter, RO filters not only enhance the quality of your drinking water but can also drastically improve the taste and odor.

It produces water that’s pure, fresh and tastes much better than regular tap water.

No Operating Costs

Most reverse osmosis systems (except the ones with a booster pump) solely rely on natural flow and water pressure to filter out contaminants hence add nothing to your electric bill. Therefore, for those that want to utilize as little energy as possible to purifier their tap water, an RO system can be of great benefit. 

Saves space

Reverse osmosis systems are often compact and much smaller in size compared to other typical home water filtration systems. They occupy very little space, about the size of a small traveling suitcase. 

They can fit right under a kitchen sink or even in tiny corners.

Moreover, you can personalize some RO units to fit your home, so if your house falls on the small side, then an RO system can certainly be very convenient.

Easy to Maintain

Maintaining and taking care of a reverse osmosis system is a breeze for the most part as the system is fully automated.

It usually requires maintenance every 6 months or so, and typically the only you will need to do once installed is changing out the filters to keep the system running at its peak capabilities.

You can either do it on your own or hire a professional water contractor or plumber to do the job. The rest of the unit runs automatically, without any assistance while the semipermeable membranes can go for up to 3 years before requiring replacement.

Money Saver

Finally, another best benefit of using a reverse osmosis system is that it can save you a lot of money especially if you frequently purchase bottled water for drinking purposes.

Even many bottled water manufacturers utilize an RO system to produce pure water.

Therefore, by having your own RO system, you can generate bottled-quality water at the comfort of your home whenever you want for no additional cost whatsoever.

You can even buy reusable water bottles, fill them with your RO filtered water and take it with you to the office or any other place saving you the cost of having to purchase bottled water.

The Disadvantages

Although reverse osmosis systems offer substantial benefits, they do have some downsides too that are considering when purchasing a unit.

They include:

Removal of Essential Minerals

One of the major drawbacks of a reverse osmosis system is that due to its excellent filtration system, it also ends up getting rid of healthy minerals like iron, calcium, and magnesium which are naturally found in water.

These minerals are essential to the body as they aid in overall muscle and heart health and without them in the RO water can lead to mineral deficiency in the body. However, using a re-mineralization filter alongside your RO system can help replace these beneficial minerals.


The small pore of a reverse osmosis membrane can become clogged especially when the pre-filters replacement does not meet the system guidelines or the maintenance schedule is not followed.

Ensuring that you invest in an RO system that has a sediment or carbon pre-filter and doing routine maintenance can help reduce the chances of clogging.  

Slow Process

While reverse osmosis provides high-quality water, it’s a relatively slow process compared to other water filtration methods.

This is mainly due to the fact that the system relies on water pressure. Therefore, if you need a large amount of water within a short time, then you will have to be a little patient and allow the system to refill the RO storage tank.

May not Kill all Bacteria

There’s a high chance that a fraction of microorganisms and bacteria can pass through the RO membrane. It’s quite rare for this to happen but using a complete RO system with additional stage containing UV light filtration can help prevent the probability of microbiological contamination.

The uses of a reverse osmosis system

Reverse osmosis system use is growing rapidly but it finds frequent use in the following areas:

Filtering Drinking Water at Home

Many households around the world use reverse osmosis systems alongside water softeners to purify and soften water entering the house for drinking, cooking and other purposes.

Production of Bottled Water

Reverse osmosis systems are used in bottled water factories in the production of bottled water.

Water is passed through an RO water processor where pollutants and microorganisms are removed.

Food and Beverage Industry

Reverse osmosis system is extensively used in food and beverage processing plants as a more economical method for concentrating food liquids like fruit juices.

It’s also widely used in the dairy industry purposely for the production of whey protein powders.


The pharmaceutical drugs industry requires highly-purified water that is free of bacteria, dissolved solids and other contaminants for production purposes. Therefore, many companies utilize RO systems with UV sterilizers to treat and disinfect the feed water in order to produce the high-purity water needed for production processes.


Freshwater aquarists also use reverse osmosis systems for filtering tap water that enters their aquariums to remove excess chlorine and other contaminants which can lead to excessive or unwanted algae growth.

Boiler Feed-Water Treatment

 In industry, reverse osmosis systems are extensively used to remove minerals and impurities from boiler feed water as they can cause corrosion or leave deposits inside or outside boiler tubes resulting in reduced efficiency and poor steam production.

What can an RO System remove?

The RO process can remove a wide range of contaminants from dissolved to suspend ones both organic and inorganic including bacteria and viruses. The RO membrane consists of tiny pores ranging from 0.0001 to 0.001 micron.

Therefore, only water molecules get through while all other contaminants with larger size are collected and flushed away. This includes dirt, dissolved salt, rust, metallic ions, fluoride, chlorine, nitrates and nitrites, cryptosporidium (cyst), bacteria, virus and more.

Below is the list of the contaminants that removed by a reverse osmosis system with the rejection rate estimation.

ContaminantRejection RateContaminantRejection Rate
Aluminum 97-98%Manganese96-98%
Amoebic Cysts 99%Mercury97-99%
Bromide93-96% Magnesium 96-98%
Chromate90-98% Radioactivity95-98%
Chromium90-98% Radium99%
Ferro cyanide98-99%Sodium92-98%
Fluoride94-96%Strontium 90-95%
Giardia 99%Sulfur/Sulfate97-98%
Herbicides97%THMs 90+%

What may not be removed?

In theory, some dissolved gases such as carbon dioxide, hydrogen sulfide, and ammonia may permeate the RO membrane without being removed. Other molecules and dissolved organic matter like formaldehyde and methanol may also pass through the membrane.

According to experts some bacteria and viruses may as well get through the RO membrane although it’s quite rare. An RO system fitted with UV light filtration can help remove all the living organisms and viruses.


In general, reverse osmosis systems are one of the best and most effective water purification systems available for households.

When you install an RO system, you can be guaranteed pure, great tasting, healthy drinking water right from your kitchen faucet.

It’s a smart investment that will not only help keep you and your family healthy but will also save you a lot of money in the long run as you will be spending less on bottled water.