What Is A Reverse Osmosis System [The Full Definition]

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

Select the questions that you want to see the detailed answers or just scroll down to read all the information.

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.

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.

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.

Details of How an RO System Work

  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.


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.

Reverse Osmosis Membrane & Replacement Filters

Here you will find everything you need to know about reverse osmosis membrane and its filter cartridges.

Choose the questions that you want to see the answers or just scroll down to read all.

The reverse osmosis membrane and filter cartridges are perhaps the most important parts of any RO system. They determine the overall performance of the reverse osmosis process when it comes to filtering water.

In this article, we will primarily focus on RO membrane and filter cartridges plus steps to replacing them.

Read moreReverse Osmosis Membrane & Replacement Filters

Reverse Osmosis Tank Replacement [2-Gallon | 5-Gallon]

Here you will find everything you need to know about the reverse osmosis storage tank.

Choose the questions that you want to see the answers.

The reverse osmosis storage tank is a very essential and necessary component in any typical reverse osmosis system. Without it, it would probably take 5 to 10 minutes just to get a glass of purified water because the filtering process of most RO systems takes quite a bit of time.

An RO storage tank, thereby, allows the system to maintain stable and constant water flow throughout. It makes water supply quick, easier and more controllable.

In this article, we are going to give you an insight on everything to do with reverse osmosis storage tanks including how they work and our recommendation for some of the best RO tanks available in the market today.

Therefore, read on and find out more.

Read moreReverse Osmosis Tank Replacement [2-Gallon | 5-Gallon]

Reverse Osmosis Booster Pump [RO Permeate Pump]

Here you will find everything you need to know about the reverse osmosis booster pump.

Choose the questions to see the answers or just scroll down to see all the information.

If you are struggling with a low feed water pressure and you are thinking of getting a booster pump for your reverse osmosis system, then this post will come in handy.

Read on and find out more.

In this post, you are going to learn everything about the reverse osmosis booster pump, how it works, how to integrate it to your RO system and much more.

We’ve as well shared our top 8 recommendations of the best RO booster pumps you can find on the market today.

Read moreReverse Osmosis Booster Pump [RO Permeate Pump]

Alkaline & pH of RO Water [How To Remineralize RO Water]

In this post, we will take an in-depth look at everything to do with reverse osmosis water remineralization and give you a brief review of the top 8 mineral filters for RO systems that are worth considering.

Here you will find:

Once you are done going through this article, you will have a great insight on how to remineralize your RO water and what type of mineral filter to get.

So, without further ado, let’s get started.

Read moreAlkaline & pH of RO Water [How To Remineralize RO Water]

Reverse Osmosis Installation & Maintenance [A How To Guide]

This post is an ultimate guide to all the information you need when it comes to reverse osmosis (RO) system installation.

Choose the sections that you want to jump in.

We cover all the three types of RO systems and give you a trouble-free way of how you can set-up each one of them.

Therefore, whether you’ve got a countertop, under the sink, or whole house RO system, or you are planning to get one, this guide will certainly come in handy in ensuring your system is installed properly for optimum performance.

Read moreReverse Osmosis Installation & Maintenance [A How To Guide]

Reverse Osmosis System Diagrams [The Complete List]

Here you will find a collection of reverse osmosis diagrams and diagrams of all the elements on a reverse osmosis system.

Reverse osmosis system diagrams will show you the detailed images of an RO system, the diagrams describe the elements in the system, how it works, how to set it up in a clear and visual way.

(These images are under the copyright of the listed sources, please contact the owners for commercial uses.)

Read moreReverse Osmosis System Diagrams [The Complete List]

10 Best Reverse Osmosis Systems Reviews [Alkaline | pH+ | UV]

This is the ultimate review of 10 Best Reverse Osmosis Systems in the U.S. market this year 2019.

Your best reverse osmosis system could be one of the following options.

Pick the one you are considering to read the reviews.

Or keep going to read all the reviews.

Without further ado, let’s get started with the comparison table.

Read more10 Best Reverse Osmosis Systems Reviews [Alkaline | pH+ | UV]

APEC Water Filter System Reviews [Reverse Osmosis | Filters]

This is the list of 15 best water filter systems from the well-known brand APEC Water in 4 groups:

Reverse Osmosis | Whole House | Undersink | Countertop

(The water filters are also top sellers of this brand in the U.S. market this year 2019)

APEC Water is a leading manufacturer of reverse osmosis systems and water filters in the United States.

Their water filter systems remove up to 99 percent of contaminants found in well and tap water. Their reverse osmosis (RO) membrane is the best in the industry for removing contaminants.

APEC supplies whole house and commercial reverse osmosis filters, car wash systems and shower and bath filters.

Read moreAPEC Water Filter System Reviews [Reverse Osmosis | Filters]