How Do Reverse Osmosis Membranes Work?


RO membranes play an important role in the reverse osmosis process, and their task is to be responsible for filtering out contaminants from the water. So how should we perform their function of purifying water and how do we maintain them?

Whether you are a homeowner trying to improve the quality of your tap water or a water treatment specialist, this article will give you a better understanding of reverse osmosis membranes and their function in water purification, and the many factors that affect the function of these membranes.

Reverse Osmosis Membranes

Reverse Osmosis Membranes

Reverse osmosis systems technically depend on thin film composite (TFC) polyamide membranes to purify water. They are made to block pollutants and toxins while letting water molecules pass through. Reverse osmosis membranes are quite good at getting rid of many different kinds of pollutants, including bacteria, viruses, dissolved solids, and many more. They are an important component of many water treatment systems, including those used in homes, businesses, and industrial settings.

Reverse Osmosis RO Membrane

Types of Reverse Osmosis Membranes

The kind of membrane that is employed determines how well your reverse osmosis unit purifies water. RO membranes come in a variety of varieties, each with particular properties and use.

Types of Reverse Osmosis Membranes

1. Cellulose Triacetate (CTA) Membranes

These are made from cellulose acetate, a plastic material that is derived from wood pulp. CTA membranes are relatively inexpensive and have a good rejection rate for small molecules such as dissolved salts and sugars. However, they are prone to fouling and are not very durable.

Cellulose Triacetate (CTA) Membranes

2. Thin Film Composite (TFC) Membranes

These are made from a combination of polymers such as polyamide, polysulfone, or polyacrylonitrile. TFC membranes outlast CTA membranes and have a higher rejection rate for impurities. However, they are sensitive to chlorine and are prone to fouling by organic matter.

Thin Film Composite (TFC) Membranes

3. Polyvinylidene Fluoride (PVDF) Membranes

These are made from a synthetic fluoropolymer and are known for their high chemical resistance and durability. PVDF membranes have a high rejection rate for contaminants, but they are more expensive than other types of membranes.

Polyvinylidene Fluoride (PVDF) Membranes

4. Polyethylene Terephthalate (PET) Membranes

These are made from a synthetic polymer that is derived from petroleum. PET membranes are relatively inexpensive and have a good rejection rate for contaminants. However, they are not as durable as other types of membranes and are prone to fouling.

Polyethylene Terephthalate (PET) Membranes

5. Ceramic Membranes

These are made from a variety of materials such as aluminum oxide, silicon carbide, or zirconium oxide. Ceramic membranes are highly durable and resistant to fouling, but they are also expensive and have a lower rejection rate for contaminants compared to other types of membranes.

Ceramic Membranes

In conclusion, the type of reverse osmosis membrane to use relies on the application at hand as well as the impurities that must be eliminated. CTA and PET membranes are suitable for basic water treatment, while TFC, PVDF, and ceramic membranes are suitable for more demanding applications.

How Do Reverse Osmosis Membranes Work?

In the reverse osmosis (RO) method of water filtration, reverse osmosis membranes are used to remove ions, molecules, and larger particles from drinking water. Because it uses pressure to force water molecules through the membrane, as opposed to the way water molecules naturally sieve through membranes during natural osmosis. Hence the process is called “reverse” osmosis.

To understand how a reverse osmosis system works, follow these simple steps:

  • Water from a tap or other source is put into the system to start the process. After that, the water is sent through a sediment filter to get rid of any bigger debris like rust, dirt, and sand.
  • After that, it passes through a carbon filter that effectively removes chlorine and other impurities from it that can ruin the taste and smell of the water.
  • Next, the water passes through a membrane material with very small pore sizes, known as a reverse osmosis membrane. This membrane allows only water molecules to pass through, and its high removal rate blocks almost all contaminants.
  • As the water is forced through the membrane under pressure, the contaminants are left behind and are flushed down the drain.
  • The purified water is collected on the other side of the membrane and is stored in a holding tank.
  • The water that cannot pass through the membrane often referred to as the reject water is thrown away.
  • The RO system may also include a post-filter to further remove any remaining contaminants and improve the taste of the purified water.

That’s a general overview of how reverse osmosis membranes work. The specific details may vary depending on the make and model of the membranes.

Reverse Osmosis Systems work flow

Features of Reverse Osmosis Membranes

To purify water of impurities and toxins, reverse osmosis membranes are used in a variety of water treatment applications, including the purification of drinking water, the production of ultrapure water for industrial and laboratory use, and the desalination of seawater.

· High Selectivity

One of the main features of RO membranes is their high selectivity, which means that they are able to effectively remove a wide range of contaminants from water while allowing pure water to pass through. This is achieved through a process called “size exclusion,” which involves the membrane physically blocking the passage of larger contaminants while allowing smaller molecules such as water to pass through.

· High Rejection Rate

Another feature of RO membranes is their high rejection rate, which refers to the percentage of contaminants that are removed from the water during the filtration process. RO membranes typically have a rejection rate of more than 95% for most contaminants, making them highly effective at purifying water.

High Rejection Rate

· Resistance to Fouling

RO membranes are also resistant to fouling, which is the build-up of contaminants on the surface of the membrane that can reduce its performance over time. To prevent fouling, RO membranes are often coated with a layer of a hydrophilic material that helps to keep the surface of the membrane clean and free of contaminants.

Resistance to Fouling

· Durability

Another important feature of RO membranes is their durability. They are able to withstand a wide range of operating conditions, including high pressures and temperatures, and have a long lifespan when properly maintained. Ordinarily, they are capable of performing optimally for 2 to 3 years.

· Ease of Maintenance

In addition to their high selectivity, high rejection rate, resistance to fouling, and durability, RO membranes are also relatively easy to maintain. They can be cleaned and sanitized regularly to ensure optimal performance, and replacement membranes are readily available when needed.

These features of RO membranes make them an effective and reliable option for purifying water in a variety of applications, including residential and commercial water treatment, industrial processing, and desalination.

Figure 4 RO membrane element.

Factors That May Affect the Efficiency of Reverse Osmosis Membranes

RO membrane efficiency can be impacted by a variety of elements, such as the following:

· Membrane Material

The efficacy of a membrane can be affected by the material from which it is produced, as different types of membranes are made from various materials. For instance, although some membranes are built of polyamides or other polymers, others are made of cellulose acetate. The choice of membrane material can affect the rate at which impurities are removed, as well as the overall efficiency of the membrane.

· Operating Pressure

The pressure at which a reverse osmosis membrane operates can also affect its efficiency. Higher pressures can result in a higher flux rate, which means that more water can be treated in a given amount of time. However, higher pressures can also lead to increased fouling, which can reduce the efficiency of the membrane.

· Temperature

The effectiveness of a reverse osmosis membrane can also be impacted by the temperature of the water being treated. In general, greater temperatures can cause more fouling, which can reduce the membrane’s effectiveness.

· Feed Water Quality

The effectiveness of a reverse osmosis membrane can also be impacted by the quality of the feed water being treated. If the feed water is heavily contaminated, it may be more difficult to purify and may require a more efficient or specialized membrane.

Feed Water Quality

· Membrane Age

As a reverse osmosis membrane ages, it can become less efficient. This is because the membrane may become fouled or damaged over time, which can reduce its ability to remove impurities.

· Pretreatment

The feed water may occasionally need to be pretreated before being fed through a reverse osmosis membrane. This can help to remove any large particles or contaminants that could potentially foul the membrane, which can increase its efficiency.

How to Maintain Your Reverse Osmosis Membranes?

Maintain Your Reverse Osmosis Membranes

Reverse osmosis membranes are an important part of a reverse osmosis (RO) system, and it is important to maintain them to ensure that the RO system is working efficiently. Here are some tips for maintaining your RO membrane:

  • Flush the membrane regularly: It is recommended to flush the membrane with high-quality feed water every 3 to 6 months to remove any accumulated contaminants.
  • Replace the membrane when needed: Over time, the membrane will become less effective at removing contaminants from the water. It is recommended to replace the membrane every 2 to 3 years, or sooner if the water quality becomes poor.
  • Store the membrane properly: If the membrane needs to be removed for any reason, be sure to store it in a clean, dry place.


Reverse osmosis membranes are a critical component in the process of reverse osmosis, which is a widely used technique for purifying water. They are effective at removing a wide range of contaminants, including bacteria, viruses, and dissolved solids, making them an essential tool in the treatment of drinking water and the purification of water for industrial and commercial purposes.

We hope this article has boosted your understanding of reverse osmosis membranes, their working principles, and essential care practices.

Scroll to Top