What Is a Reverse Osmosis (RO) Membrane and How Does It Work?
Written by: Gene Fitzgerald // Last Updated: Mar 24, 2023
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Reverse osmosis filtration is one of the most popular methods for purifying water, both among households as well as in the industrial sector.
The reverse osmosis membrane is at the heart of the process. It’s the point where most of the water contaminants and impurities are being removed.
If you’re planning to buy a reverse osmosis system for your household, it’s a good idea to put some effort into understanding how the RO membrane works.
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Key Takeaways
- A reverse osmosis membrane is used in reverse osmosis water purification, which is a very effective method to remove unwanted impurities from a water supply.
- The membrane uses extremely fine pores to allow water molecules to pass through while rejecting contaminants.
- The flat membrane sheet is tightly wound in a spiral to accomplish the cylindrical shape.
- During the filtration process, water is separated into 2 streams: The clean water for you to use and the reject water carrying the contaminants down the drain.
What Is a Reverse Osmosis Membrane?
An RO membrane is an elongated cylinder which is used to purify water in a process called reverse osmosis.
There are different types of RO membranes, thin-film composite (TFC) being the most commonly used for home water treatment nowadays. Cellulose acetate used to be popular in the past, but it’s mostly been pushed out by TFC.
A TFC reverse osmosis membrane sheet consists of three layers (one supporting layer, one interlayer, and the actual barrier layer rejecting contaminants) and is packed in a spiral configuration rolled around a perforated pipe.
What makes an RO membrane special is that its surface is covered with microscopic pores which are tiny enough to (mostly) only allow water molecules through. How tiny? About 0.0001 microns.
How Does a Reverse Osmosis Membrane Work?
The basic principle behind reverse osmosis water purification is pretty simple mechanics:
Water enters the membrane housing tube and is forced through the membrane element at high pressure. Due to the restrictive nature of the pores on the membrane, only water molecules can pass through (with a few small exceptions). As a result, water is split into two separate streams, one containing clean water and the other carrying the contaminated portion. Unfortunately, the latter part of the water is mostly wasted. That’s also considered to be the main disadvantage of reverse osmosis by most.
Understanding Osmosis
To further understand how reverse osmosis membranes work, it makes sense to first understand osmosis.
Well, osmosis is a natural process. When two bodies of water with different salt concentrations are separated by a semipermeable membrane, there’s a natural tendency for water molecules to start flowing over to the side where there are fewer salts. No external energy is required to make that happen – it’s a natural result of the unbalanced concentration of salts on both sides of the membrane.
Understanding Reverse Osmosis
With reverse osmosis, the same principle is applied in the opposite direction. However, this requires some energy to work – which is why reverse osmosis relies on external water pressure. As we described above, natural osmosis will cause water molecules to flow over from the more concentrated to the less concentrated side.
If we want to accomplish the opposite, water must be pushed through the membrane from the more contaminated side. Remember, only water molecules are small enough to diffuse through the pores. Everything else – which is more or less all contaminants in the water supply – can’t make it through and is left behind.
How Water Flows Through an RO Membrane
Water enters the RO membrane housing or vessel through the feed line and starts to flow across the surface of the membrane.
Due to high pressure, some of the water diffuses through the membrane’s surface continuing its way into the permeate pipe in the middle. From there, the clean (permeate) water flows out of the membrane housing – and with a few extra steps directly into your glass.
The rest of the water continues to flow forward without penetrating the membrane. The only difference is that it’s now carrying a higher concentration of contaminants. This concentrate water simply leaves the RO membrane housing on the other end and flows down the drain.
RO Membrane Operating Conditions
Reverse osmosis requires specific conditions to work properly. You need to be able to maintain a certain level of pressure first and foremost. You must also ensure that the turbidity of your water is below a certain level, and you must also control for the presence of certain elements like iron and manganese. And there’s more:
TFC RO Membrane – Required Operating Conditions
| Parameter | Requirement |
|---|---|
| Feed Water Pressure | 40-85 psi |
| Water Temperature | 40-100°F (5-35°C) |
| Water pH | 2.0-11.0 |
| TDS | < 2000 ppm |
| Turbidity | < 1 NTU |
| Calcium Carbonate Level | < 350 ppm |
| Iron Level | < 0.05 ppm |
| Manganese Level | 0 ppm |
| Hydrogen Sulfide Level | 0 ppm |
| Chlorine Level | < 0.1 ppm |
Which Water Contaminants Does an RO Membrane Remove?
Reverse osmosis membranes remove a very wide range of water contaminants and impurities, including:
- Aluminum – ~98%
- Ammonium – ~98%
- Arsenic – ~96%
- Barium – ~96%
- Cadmium – ~96%
- Calcium – ~96%
- Chloride – ~88%
- Chromium 6 – ~94%
- Copper – ~97%
- Cyanide – ~88%
- Fluoride – ~88%
- Iron – ~96%
- Lead – ~96%
- Manganese – ~96%
- Magnesium – ~96%
- Mercury – ~97%
- Nickel – ~97%
- Nitrate – ~68%
- Phosphate – ~97%
- Potassium – ~90%
- Radium – ~80%
- Selenium – ~95%
- Sodium – ~90%
- Sulfate – ~97%
- Sulfite – ~97%
- Zinc – ~97%
In addition, an RO membrane can remove colloids, asbestos, pesticides, sediments, VOCs, microorganisms, proteins, and more.
What Is a Reverse Osmosis System and How Does It Work?
A reverse osmosis membrane for home use is usually packaged in a complete system that can be installed under your sink or in another part of the household. Filtering directly at the point of use is the most popular approach among domestic users right now. However, whole house reverse osmosis is possible, too.
A complete RO system usually combines reverse osmosis filtration with some additional pre-filtration steps to ensure that water is as free of damaging contaminants as possible before it reaches the RO membrane. This extends the lifespan of the membrane and improves its performance.
Filtered water can be stored for later use. The size of the storage tank varies from one model to another. Tankless is also an option.
Post-filtration is frequently used, too, especially in systems with a larger storage tank. It gives you some additional reassurance about the purity of your water once it’s been sitting in the tank for a while.
Components
A reverse osmosis system consists of several main components. The exact configuration may vary from model to model, but the general idea is always the same.
Feed Water Valve
The feed water valve is the main valve connecting the RO system to your household water supply. Before water enters the first filter, it must pass through this valve.
Pre-Filtration
Pre-filtration involves using several filters to remove various contaminants from the water supply before it reaches the RO membrane. You will frequently see sediment and carbon filters used at this stage. Proper pre-filtration is important to ensure that the RO membrane doesn’t get clogged or foul too fast and that it can operate properly.
RO Membrane
Here’s where the actual reverse osmosis filtration takes place.
Storage Tank
Once water has passed through the RO membrane, it’s fed to a storage tank for convenient access. Since reverse osmosis is a bit slow, this is a useful feature that can provide you with immediate access to filtered water instead of having to wait for it to get purified on demand.
Post-Filtration
Before water is dispensed from the storage tank, it might pass through some post-filtration stages to remove unpleasant taste/odor that might have accumulated in the tank. Usually, this stage doesn’t need to be as aggressive as the pre-filtration, and these filters are mostly there to fine-tune.
Additional Features
Your reverse osmosis system usually includes some additional features as well. These include, but are not limited to:
- Automatic shut-off valve to prevent water wastage once the storage tank is full
- A check valve to prevent backflow into the membrane from the storage tank
- A flow restrictor to assist with maintaining adequate pressure in the system
- A drain line to drain wastewater directly
Further Reading:
Water Contaminants Removed by RO Systems
It’s important to understand that a reverse osmosis system removes a wider range of contaminants than reverse osmosis alone. That’s because a complete system includes all these additional filter stages.
As a result, the average reverse osmosis system can remove a wide variety of contaminants, including:
- Heavy metals like lead and mercury
- Certain microorganisms including bacteria and cysts
- Chlorine and other chemicals used in water treatment
- Pesticides and herbicides
- Minerals and salt
- Sediment
- Nitrates and nitrite
- VOCs
- Radiologicals
Benefits: Why Choose Reverse Osmosis Water Purification?
Here are some reasons to choose a reverse osmosis system for filtering your home’s water.
Thorough Filtration
A reverse osmosis system provides you with one of the most thorough approaches to water filtration available on the consumer market. You can remove various contaminants from your water supply which would normally be problematic for other types of filters or may require advanced, expensive setups.
Improved Water Quality
You will get to enjoy clean, fresh drinking water with no impurities and an improved taste and smell. Of course, this depends on the kinds of contaminants you have in your water in the first place, but many people report a noticeable improvement in their water quality after starting to use an RO system.
Simple Installation and Maintenance
Of all the types of water filters on the market, reverse osmosis systems are among the simplest to install and maintain. You don’t have to replace any components of the system too frequently, and as long as you’ve invested in a high-end model, it will take a while for any part to fail.
Flexibility
The modular design of RO systems makes them perfect for customization. You can easily fine-tune the quality of your drinking water by adding specific stages. For example, you can add a remineralization stage if you’re concerned about losing some of the minerals contained in your water.
Reverse Osmosis Disadvantages
Every type of water filtration has some disadvantages, and reverse osmosis is no exception. Here are some of the main points you need to consider when planning to install an RO system.
Mineral Loss
Reverse osmosis is very effective at purifying water. For some people, it may be a little too effective. Along with all the contaminants that it removes, it also gets rid of most minerals that are naturally present in your water supply. These minerals have an important presence in the average person’s diet, and you should carefully monitor your intake if you’re concerned about your health. On the bright side, it’s easy to compensate for that loss with the help of supplements or by adding a remineralization stage.
Slow Filtration Speed
The whole process takes some time to work, meaning that a storage tank or strong pump is pretty much mandatory if you want to always have easy access to drinking water. This can make reverse osmosis less than ideal in large households. In this case, it can be worth investing in a larger system or adding additional RO systems around the house.
Wastewater
As explained before, reverse osmosis wastes some water as part of the filtration process. The ratio can be quite high too – it could be as high as 1:5 in some cases, meaning that you waste 5 gallons of water for every single gallon that you’ve purified.
Good news is, an up-to-date system can easily reduce the amount of wastewater produced by 90%.
How Can You Reduce Water Wastage in Your RO System?
Most convenient is using a permeate pump or pressure pump. These can be added to most standardized RO systems without hassle.
If you have any thoughts about the questions, what is an RO membrane and how does an RO membrane work, please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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