How Does a Reverse Osmosis Membrane Work? (Detailed Explanation)

 Last updated  - November 19th, 2021

Reverse osmosis is among the many different methods of water filtration and water purification. Here we will be discussing everything there is to know about these systems, how it works, and the function of a semi-permeable membrane. The reverse osmosis membrane is an important component used in water filtration.

The contaminants in the water are removed due to the size of the pores in the RO membranes allows filtration and removes the impurities. This leaves clean water for drinking and/or for other industrial purposes.

Understanding The Process

What Is Reverse Osmosis? 


It’s a membrane-technology filtration method that removes contaminants from water by using pressure. If you have a high concentration solution, it will diffuse to a state of equilibrium.

The presence of a semi-permeable membrane makes it so that the contaminants in the water cannot pass through. This membrane has to be semi-permeable so that it will allow the water molecules to go through but not the contaminants.

As this high concentrate solution passes through the semipermeable membrane, the water molecules are the only ones that pass through. This results in a less concentrated solution, leaving TDS behind by exerting external pressure.

How Does Reverse Osmosis Work?

There are several components that make up this procedure. In RO systems you can choose from 3, 4, or 5 stages or more. Water experts agree that at least there should be 4 stages of a reverse osmosis water filter to complete the process.

These include the following: a sediment pre-filter, a carbon-block pre-filter, a membrane, and a carbon-block post filter. For those using water coming from the deep wells, or in areas with high levels of sediment, a fifth stage is optional, which is an extra pre-filtration to occur.

4 Stages of Filtration

Stage 1 – Sediment Filter

This process removes sediment and other large solid matters like dirt, silt, and rust. This will improve the taste and appearance of your water. This initial method is important to protect the carbon filter which is present in the second stage.

Stage 2 – Carbon Filter With Activated Carbon

The second stage aims to eliminate chlorine and/or chlorine compounds. It is also effective in removing other particles in the sediment filter such as volatile organic compounds (VOCs).

Carbon filters aid in removing odor in water as well. This process conditions the water before the start of the reverse osmosis and helps protect the membrane by removing chloride which corrodes the membrane.

reverse osmosis water filtration diagram

Stage 3 – Reverse Osmosis

In this stage the reverse osmosis takes place. The role of the membrane is to filter out organic and inorganic compounds such as fluoride. In this stage, impurities are known as Total Dissolved Solids (TDS) like sodium, calcium, magnesium will be reduced.

The membrane will magnify an effect to have a reduced TDS count in the water down to 1/10,000 (0.0001) of a micron. The resulting water would have been clean and considered pure by water standards. This filtered water is now available drinking water and for some, this ends the 3-stage cycle.

Stage 4 – Polishing Filter or Color Changing Resin Deionization Filter

This stage will vary as water companies offer different types of filters at this stage. The purpose of this activated carbon is to polish the water to make it crystal clear. Meanwhile, a color-changing resin deionizer filter will remove any remaining total dissolved solids (TDS) from RO membrane filtered water.

After the water passes through the membrane, there may still be a small fraction of total dissolved solids (TDS) left in the water. Another filter with the use of resin is next. The resin absorbs the TDS. Over time, the color of the resin will change from blue to amber indicating that it is time to replace the filter.

What Is The Membrane?

photograph of a cut-out cross section of an reverse osmosis membrane
a cut-out cross-section of an RO element

The heart of every apparatus is its osmosis membrane. The membrane is usually crafted from a very thin-film composite (TFC) which is a semipermeable membrane. This very important yet flimsy material is prone to corrosion by chlorine.

That is why it is very important to remove all chlorine and similar chlorine subcomponents present in the water before it reaches this membrane. You can use a carbon filter to prevent corrosion before the actual RO water treatment. For example, in Aquasana OptimH2O filter, water flows through a diluted solution using semi-permeable membranes

The membrane is usually a microfiber that has a 0.001-micron rating. This means that since the fiber pores are very fine it can filter off size ratios of atomic in radius. This composite film comprises three covers.

The innermost may be a polyester support web, with the next layer as a microporous polysulfone interlayer and the outermost layer is an ultrathin polyamide barrier that covers the top surface. The outermost layer is a cartridge.

Below is a cross-section of a membrane. It shows layers and layers of fibers around a center tubing. These are wound cartridges.

How Does The Membrane Work?

Now, the water passes through a membrane that acts as a filter. It then retains the contaminants as the unfiltered water passes through it.

Concurrent to the filter process happening on the membrane, a procedure of water flow and water pressure takes place as well. This is known as flux within water expert circles.

It may seem that the tube passes all the way through the other end. Yet, on the feeder side, that is where the raw water goes in, there is a blocker before the center of the tube. The blocker applies water pressure and pushes the water out through the membrane.

Part of the water passes through the membrane and into the orifice, creating permeated water. The contaminants which won’t pass through the membrane go down a different drain as wastewater.

Below is an illustration of the cartridge where the membrane filtration process occurs:

the cartridge in the reverse osmosis membrane water filtration process

When water passes through the layers of fibers in the membrane, the holes become smaller, this way the contaminants in the water are hindered by the membrane and at the same time are flushed out by the water that continually flows through the membrane and into the drain.

It is crucial to maintain this flux rate across the membrane. If the water flow is too much or it stops the contaminants won’t be able to flush out of the drain. They are going to aggregate onto the membrane.

Noting that on the exit side of the orifice are rubber O-rings. The function is to seal the membrane from the housing. In the case of the O-ring malfunctions, the contaminated water may slide through the O-rings and combine with the permeated water, rendering the reverse osmosis process less effective.

How Effective Is a Membrane?

The efficiency of the use of a membrane to produce clean water is shown below:

Reverse osmosis membrane that has a microfiber of 0.001-micron rating

efficiency of reverse osmosis membrane top
efficiency of reverse osmosis membrane bottom


The reverse osmosis TFC microfiber membrane is the latest technology being used in water treatment. The process engineers natural water osmosis and improves the quality of untreated water.

Understanding the different RO system stages and reverse osmosis membranes are the first step before buying a water filter system for your home.