Reverse Osmosis System Definition, Principle, 11 Components, Benefits

A reverse osmosis system is a water purification system designed to operate based on a mechanism of repetitive ultrafiltration, and which is capable of removing most contaminants in water.

This article discusses reverse osmosis system as follows;

Components of a Reverse Osmosis System

How does a Reverse Osmosis System Work?

What does a Reverse Osmosis System remove? Major Contaminants Removed by a Reverse Osmosis System

Reverse Osmosis and Water Wastage

Reverse Osmosis and Hard Water

Advantages of Softening Hard Water

Installing a Water Softener and a Reverse Osmosis System

Reverse Osmosis System and the Environment

Benefits of using a Reverse Osmosis System

Durability of Reverse Osmosis Systems: How long does a Reverse Osmosis (RO) System Work?

How to Maintain a Reverse Osmosis System

Best Places to Use a Reverse Osmosis System

Cost of Reverse Osmosis System


RO System
An elaborate Reverse Osmosis System for Water Treatment (Credit: Barna 2016 .CC BY-SA 4.0.)


Reverse Osmosis is a process of water purification which utilizes a semi-permeable membrane to remove particles and molecular (or ionic) contaminants from a fluid [5]. The contaminants that can be removed through reverse osmosis include bacteria, viruses, heavy metals, and chlorine.

Because of the efficiency and effectiveness of reverse osmosis, it is usually applied to purify and to treat drinking water.


Components of a Reverse Osmosis System

1). Storage Tank

Also referred to as the ‘pressure tank,’ the water storage tank in a Reverse Osmosis system, serves to hold water which has been purified by the pre-filter and the semi-permeable membrane [7].

In order to produce pressure, the tank usually contains an expandable bladder into which the water flows. This bladder separates the water itself from the air-filled interior of the storage tank.

As it becomes filled with water, the bladder expands, pushing against the air within the rank vessel and thereby creating pressure. This pressure is what forces the water out of the tank when the faucet is opened.

The storage tank is usually made of hard plastic or metal, and has an inner, non-corrosive coating to preserve it from damage. Most tanks have a standard size of about 15 inches by 12 inches [2]. Pressure is the driving force that helps to push the water out of the tank.

This pressure is proportional to the volume of water in the tank, and it determines the flowrate of water from the tank.

2). Water Supply Connector

The function of the water supply connector is to provide a source of water supply to the reverse osmosis system.

It serves as a link between the water supply of the house and the system, thereby ensuring that water is purified before use. There are different sizes and types of fittings which come along with the water supply connector, to make it possible to match any size or configuration of plumbing.

3). Pressure Valve

In order to regulate the pressure which is supplied to the reverse osmosis system, a pressure valve is often needed.

The pressure valve helps to prevent a pressure surge that could damage or deform the pre-filter or the membrane. It also ensures that the maximum allowable pressure of the system is not exceeded during operation. In most cases, this pressure is about 75 PSI.

4). Pre-filter

The role of the pre-filter is to remove insoluble contaminants and sediments from the water.

Among the main materials that a pre-filter removes, are; iron oxides, silt, and mineralogical precipitates. By removing these materials, the pre-filter helps to preserve the membrane from damage which they could cause.  It also prevents such materials from clogging the drain flow restrictor.

Ideally, it is necessary to replace a pre-filter once in every three months. Also, the most common size range for drinking water filtration using a pre-filter, is between 5 and 20 microns.

5). Carbon Filter

Carbon filters serve the main purpose of removing chlorine from water. This protects the semi-permeable membrane, especially for Thin-Film Material (TFM) and Thin-Film Composite (TFC) which could be damaged by chlorine.

Aside chlorine, the carbon filter eliminates organic materials in the water, and helps to remove unpleasant taste and odor.

In cases where the membrane is composed of Cellulose Tri-Acetate (CTA), the carbon filter may not be required, since this material is fairly resistant to chlorine.

Carbon filters also remove chloramine, which is equally used as a water disinfectant. like chlorine. Chloramine is generally not as hazardous as chlorine in terms of its capability of damaging the membrane.  

6). Automatic Shut-off Valve

Abbreviated as the ‘ASO Valve’ or ‘SO Valve,’ the auto shut-off valve helps to control the supply rate of water to the semi-permeable membrane.

In order to do this, the shut-off valve stops the flow of water to the membrane when up to 2/3 of the feed pressure of the storage tank has been filled. This flow is restored when the feed pressure declines to about 1/3 of its maximum value.

The ASO Valve protects the membrane from damage by excessive pressure, while preventing water from flowing to the drain.

When water has been drawn from the storage tank and the feed pressure drops, the shut-off valve usually re-opens, allowing water to pass through the membrane and into the storage tank.

This mechanism ensures that there is a sustainable supply of purified water through the faucet.

Also, by regulating the flow rate to the membrane, the shut-off valve helps to maintain the flow rate at a safe and optimal level to ensure quality filtration.

7). The Semi-Permeable Membrane

The reverse osmosis (RO) membrane can be considered to be the most important part of the entire reverse osmosis system. This is because it performs the major part of the purification process.

The RO membrane is believed to remove up to 96% of all dissolved solids like mineral, metals and salts in the water. These are usually the most hazardous contaminants that affect the quality of the water.

The RO membrane also helps to separate the water into the filtered or purified product, and the waste. Filtered water is that which flows through the membrane, and is channeled to the storage tank. The unpurified water or wastewater is that which does not pass through the membrane, and is channeled to the drain.

Common types of semi-permeable membrane used in reverse osmosis systems include the TFM/TFC (Thin-Film Material/Thin-Film Composite polyamide); and the CTA (Cellulose Tri-Acetate). The TFM/TFC membrane is vulnerable to damage by chlorine, whereas the CTA is not.

8). Check Valve

The design of the check valve enables it to prevent filtered water to flow back to the RO membrane from the storage tank. This is essential, since the water in the storage tank is highly pressurized and can flow back, causing potential damage to the RO membrane.

The check valve ensures that the filtered water has a one-way flow, restricting its backflow. It is usually located close to the outlet of the RO membrane cartridge or section.

9). Post-filter

The post-filter performs the final polishing of the already purified water, which is in the storage tank, before it is used.

Post-filters are usually located at the top of reverse osmosis membrane section. ‘Polishing’ of the filtered water usually involves removing any contaminants that may have been missed by the pre-filter and the RO membrane.

The post-filter is usually composed of activated carbon, which filters and deodorizes the water, while also improving its taste. After passing through the post-filter, the purified water is channeled through the faucet, for use.

 10). Water Faucet

The RO faucet is simply the outlet through which purified water leaves the reverse osmosis system.

In most RO system, the faucet is a non-air gap faucet which prevents reverse suction. The faucet is also provided with an inner lining of synthetic material which helps to prevent corrosion.

11). Drain Line

The drain line is used to dispose waste water, containing the impurities which have been removed by the reverse osmosis system. It is usually connected to the membrane cartridge in an opposite direction to the post-filter and faucet.


Reverse Osmosis System
A Reverse Osmosis System showing main Components and Installation Configuration


How does a Reverse Osmosis System Work?

To understand the importance of reverse osmosis systems, it is important to discuss the mechanism by which the process of reverse osmosis itself, works.

As the term implies, reverse osmosis is a variant of the natural process of osmosis. It makes use of a semi-permeable membrane as a filter, to remove impurities from a fluid. In order to achieve this, pressure is applied to the fluid, to push it through the membrane. In most reverse osmosis systems, a valve is used to control the supply of this pressure, as well as the flow of water through the system [4].

The semi-permeable membrane is usually designed in such a manner as to remove particles, ions and biological contaminants from the fluid which passes through it.  

Asides the membrane, a reverse osmosis system also comprises of a pre-filter and post-filter.

The pre-filter helps to purify the water before it reaches the membrane. This is important as it helps to eliminate some of the larger contaminants that could damage the membrane.

The post-filter purifies the water after it has passed through the semi-permeable membrane. Usually, the purified water which passes through the membrane is stored in a tank, from which it is released when needed.

The post-filter plays the role of polishing this water which leaves the membrane, thereby further improving its purity, taste and smell, before it is released through the faucet, for use.

The main steps in reverse osmosis include;

1). Pre-Filtration

This stage involves the use of the pre-filter to remove insoluble impurities like silt, as well as chemical substances like chlorine.

The importance of pre-filtration is mainly to protect the semi-permeable membrane from damage by these relatively heavy pollutants

2). Semi-Permeable Membrane Passage

After the pre-filtration stage, the water is passed through the semi-permeable membrane.

As earlier stated, this passage is facilitated by the use of pressure which is supplied by a pump, in most cases.

The importance of a semi-permeable membrane is to eliminate inorganic contaminants, and resilient, microscopic organic contaminants.

3). Water Storage

The water which passes through the semi-permeable membrane is subsequently stored for later use. For this purpose, a storage tank is often utilized.

4). Post-Filtration

Post-Filtration is carried out for the purpose of removing all contaminants that have not been eliminated by the semi-permeable membrane. It usually involves the use of a carbon post-filter which removes organic and gaseous contaminants.

The end result of reverse osmosis is the production of pure permeate which passes through the semi-permeable membrane. This permeate is usually free of most of its ionic and particulate content, and is therefore safe for use, while meeting standards for purity.


What does a Reverse Osmosis System remove? Major Contaminants Removed by a Reverse Osmosis System

As we have mentioned, a RO system eliminates almost all forms of contaminants in water.

The main contaminants which you should expect to remove from drinking water, using a reverse osmosis system, include the following;

1). Arsenic

2). Lead

3). Fluoride

4). Sodium Chloride

5). Sediments (silts, debris, particulate matter)

6). Volatile Organic Compounds (VOCs)

7). Copper

8). Chlorine

9). Chloramine

The RO system may also reduce some other contaminants like;

1). Calcium

2). Phosphorus

3). Sodium

4). Potassium

5). Nitrogen

6). Phosphorus

7). Sulfates


Reverse Osmosis and Water Wastage

Compared to other methods of water purification, reverse osmosis is water-intensive.

This means that is consumes more water in the process of purification. In general, the rate of production of purified water by a reverse osmosis system is about 4:1. Therefore, for every four (4) gallons that are fed into the system, only about one (1) will be produced as pure water.

The remaining 3/4 of the water usually released as waste. This water fails to pass through the semi-permeable membrane, and usually contains all the contaminants that have been removed from the purified water which has passed through the membrane.

Some products like pump upgrades can help to increase the efficiency of the system, and the amount of water that is purified.


Reverse Osmosis and Hard Water

The main cause of hardness in water is the presence of mineral constituents. These include magnesium and calcium ionic contaminants. Such contaminants may precipitate and cause the formation of scale, which can affect the performance of the system.

Also, these precipitates can damage the membrane over a period of time. As a result, the output of the membrane and the quality of the water which it filters, will both reduce.

Although it is possible to replace the membrane, this usually incurs significant cost, if done frequently. Backwashing can help to clean the membrane as well, but this is neither an optimal option.

The best approach will be to treat hard water before it undergoes reverse osmosis. When installing a reverse osmosis system, it is necessary to be aware of the contaminants that are present in the water source which needs to be purified.

Ion exchange by the use of resins is usually a good way to soften hard water before it passes through the RO membrane.


Advantages of Softening Hard Water

-Improves Health

While the consumption of hard water is not directly related to any adverse health problems, it is known to have some effects on the human body, including dryness of the skin and hair [3].

It is also not recommendable to consume extremely hard water (which has very high concentrations of magnesium and calcium) over a long period of time. This is because it may have negative effects on the kidneys, which could lead to ailment.

-Energy and Cost Saving

Hard water produces precipitates known as scales, which can negatively affect the efficiency of water filters, Reverse osmosis systems, and heaters.

Scales place much stress on purification systems, heater and pumps. These devices consume energy, therefore the energy demand and cost will increase.

Other appliances can also be damaged by hard water. These include laundry machines, dishwashers and coffee machines. When scales build up in these machines, they become less efficient and do not last long.

-Increases Cleaning Efficiency

One of the most well-known properties of hard water is its tendency to waste cleaning agents like soap and detergent. By doing so, it decreases the efficiency with which cleaning can be done. Softening hard water helps to solve these problems.


Installing a Water Softener and a Reverse Osmosis System

Some contaminants like calcium and magnesium in hard water, can be removed by the use of a water softener. However, there are several other contaminants which the water softener alone cannot remove.

What this implies is that, for hard water purification, it is advisable to make use of both a water softener and a reverse osmosis system. By installing both devices, you will ensure that the water is well treated for safe and economical use.

In addition to improving the quality, taste and odor of hard water, installing a water softener and a reverse osmosis system will help to extend the lifespan of the reverse osmosis system, by preventing calcium and magnesium precipitates from coming in contact with the membrane.


Reverse Osmosis System and the Environment

For home-based reverse osmosis systems, the main environmental issues involve the production and handling of waste water by the system. To prevent any adverse effects, it is recommendable to send waste water to a treatment plant before releasing it into the environment.

Generally, however, the waste water or brine which is produced by a reverse osmosis system, is not very hazardous, especially since it has gone through the pre-filtration stage already.

For large-scale reverse osmosis systems, like Sea Water Reverse Osmosis (SWRO) systems; the potential effects on the environment include intake of water, outfall of waste or brine, and energy consumption [6].

Knowing these potential effects makes it possible to design SWRO systems that are optimal and have minimal environmental impacts. In conclusion, reverse osmosis systems are eco-friendly, such that the environmental impacts are almost entirely negligible.


Benefits of using a Reverse Osmosis System

1). Cost-Effective Option

A reverse osmosis system provides an economic means to treat and purify water.

Because of its effectiveness and relatively low energy demand, RO systems can treat large amounts of water with efficiency and affordability.

2). Metallic Contaminant Removal

As shown earlier, Reverse Osmosis helps to eliminate metallic contaminants in water like copper, lead and arsenic. Alkaline metallic elements that cause hardness, like magnesium and calcium, can also be removed through reverse osmosis. However, these elements can damage the system and are better removed using a water softener.

3). Eco-Friendly Operation

We have mentioned that reverse osmosis has minimal effects on the environment.

Compared to other elaborate water treatment methods like distillation, reverse osmosis does not consume much energy. It can also perform large-scale treatment operations without much environmental impact.


Durability of Reverse Osmosis Systems: How long does a Reverse Osmosis (RO) System Work?

Provided the system is properly maintained and serviced, a Reverse Osmosis device or apparatus is capable of working effectively for 10 to 15 years. The durability and quality of the system also depend on factors like the brand, regularity of use, and size.


How to Maintain a Reverse Osmosis System

1). Cleaning and Sanitization

It is necessary to clean and sanitize a reverse osmosis system at least once every year.

The steps involved in cleaning and sanitizing a reverse osmosis system are as follows;

-Shut off the main water supply valve to the RO system

-Drain all water from the storage tank

-Remove the pre-filter and post filter, and their housings or cartridges

-Remove the Reverse Osmosis membrane

-Having removed the main filters and the membrane, reconnect the empty post-filter housing to the system

-Pour the sanitizer into the pre-filter housing. In most cases, your sanitizer will be composed of bleach

-Reconnect the pre-filter housing to the system and open the main water supply valve

-Allow the system to fill with water, then to set for a period of 60 seconds to 30 minutes

-Turn on the water faucet and let the system drain completely

-Repeat the process at least once, and check for any leaks in the storage tank or flow lines


2). Filter Replacement

The post-filter and pre-filter in a reverse osmosis system require periodic replacement. In most cases, the period for replacement is between 6 months and 1 year.

Replacement of the reverse osmosis membrane is dependent on the composition and quality of the water which is being purified. Usually, it is replaced at intervals between 2 to 4 years.


Best Places to Use a Reverse Osmosis System

The ideal locations where reverse osmosis systems can be installed include;

i). Aquariums

ii). Water Wells

iii). Home (Under a sink)

iv). Businesses


Cost of Reverse Osmosis System

Factors that determine the cost of a reverse osmosis system include the size, quality, lifespan, brand, efficiency, number of filters, add-ons and certifications.

The following table [1] shows the range of cost of reverse osmosis systems of different categories;

Category Price Range
Under-sink (no storage tank) $300-$600
Countertop $300-$500
Under-sink (with storage tank) $200-$600
Point-of-entry (for entire home or building) $1,500-$10,000



As this article has shown, a reverse osmosis system is one which is capable of filtering water through a semi-permeable membrane, under high pressure. Reverse osmosis has some benefits that include;

-Cost effectiveness



-Effective removal of various contaminants

-Low energy demand

-Ease of maintenance



1). Campbell, B. (2021). “How Much Does a Reverse Osmosis System Cost?” Available at: (Accessed 23 December 2021).

2). Cannon, D. (2019). “WHAT ARE THE IMPORTANT PARTS OF A REVERSE OSMOSIS EQUIPMENT?” Available at: (Accessed 23 December 2021).

3). Sengupta, P. (2013). “Potential Health Impacts of Hard Water.” Int J Prev Med. (8): 866–875. Available at: (Accessed 23 December 2021).

4). Tameson (2021). “Reverse osmosis.” Available at: (Accessed 23 December 2021).

5). Theobald, D. (2015). “What is reverse osmosis and how is it best used?” Available at: (Accessed 23 December 2021).

6). Thomas, M.; Burton, J.; & Robert, M. (2015). Intakes and outfalls for seawater reverse-osmosis desalination facilities: Innovations and environmental impacts. DOI: 10.1007/978-3-319-13203-7. Available at: (Accessed 23 December 2021).

7). Woodard, J. (2019). “What is a Reverse Osmosis Tank and How Does it Work?” Available at: (Accessed 23 December 2021).

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