What is a Membrane Filter and How Does It Work?

Posted by
John Woodard on April 11, 2019

Membrane filters act as a barrier to separate contaminants from water, or they remove the particles contaminating the water. Reverse osmosis, ultrafiltration, and nanofiltration all use a membrane in their different filtration processes. Our Master Water Specialist, John Woodard, explains what a membrane filter is and how it works inside different water filtration systems. 

What is membrane filtration?

Filter membranes have different configurations. There are reverse osmosis (RO) membranes, ultrafiltration (UF) membranes, and nanofiltration (NF) membranes. They all approach the membrane filtration process a little bit differently.

What is a Membrane Filter and How Does It Work?

How does a membrane filter work?

Reverse osmosis applies pressure to a semipermeable membrane that allows the water molecules to pass through while flushing the dissolved inorganic compounds to the drain. So it separates the water into two pathways.

Ultrafiltration doesn't separate the water like a reverse osmosis membrane. It actually is just an ultra-fine particulate or sediment filter. With mechanical filtration particulate down to 0.025 microns cannot pass through the ultrafiltration membrane.

Nanofiltration membrane technology works very similar to reverse osmosis, except the filtration is not quite as fine.

What's the difference between a reverse osmosis and an ultrafiltration membrane?

Reverse osmosis and ultrafiltration differ quite a bit. Reverse osmosis is able to reject dissolved minerals from the water, whereas UF only filters solids or particulate. Reverse osmosis is able to get out dissolved inorganic minerals that will pass through the UF membrane.

reverse osmosis membrane

Related articles: What is Reverse Osmosis? | How Does an Ultrafiltration Membrane Work | What is an Ultrafiltration System? | How to Select the Best RO System

What are membrane filters made of?

Membranes are made of different types of materials. With reverse osmosis, they're often referred to as thin-film composite membranes. Previously, the reverse osmosis membrane was made of cellulose triacetate or CTA. CTA membranes are no longer sold. They were the first version of the RO membrane and had a low pH tolerance. They didn't make a lot of water per square inch. Thin-film enables an RO system to make a lot of water in less space, which makes it possible to get larger membranes in smaller housings. This revolutionized the style of reverse osmosis membranes. UF uses the same kind of material with a slightly different formula.

How often should you replace a membrane filter?

Reverse osmosis membrane: Every two or three years, depending on the water quality. As the RO membrane rejects more minerals, some of those minerals start to come out of solution and clog the surface area of the membrane. If you feed the RO system with softened water, then the membrane could last five years, if you replace the filters on a regular basis.

Ultrafiltration membrane: Every other year for a point-of-use (POU) application. UF membranes do collect contaminants as they filter. 

How do you clean a membrane filter?

You don't need to clean a filter membrane in a residential POU system. To flush or clean the membrane, use chemicals to dislodge some of the scale or particles clogging up the membrane. Trying to clean or flush the membrane is not as cost-effective as replacing it every few years. You should always flush the membrane before use to make sure everything works properly.

How do you flush a filtration membrane?

A UF membrane is a flow-through, not a separation, membrane technology. To flush the ultrafiltration membrane on start-up, simply get the air out of it from manufacturing. When you startup a reverse osmosis system, flush it to help re-saturate because typically these membranes are manufactured then dried. On a POU system, fill up two or three tanks of water and let them run through the drain before using the system.

What is fouling and scaling in a membrane filter?

Scaling and fouling refer specifically to reverse osmosis membranes.

Membrane scaling

As water molecules are pulled across the membrane to allow the dissolved minerals to flow to the drain, some of those dissolved minerals eventually start coming out of solution and create scale. When you boil water and you get a white ring around the pot, inorganic minerals are coming out of solution and creating scale. You can imagine what that membrane surface starts to look like after leaving minerals behind. If we don't have enough water going to drain and too much water flowing through the membrane, scale and accumulate rapidly. Many times, callers inform us that their membrane only made water for a few months and now doesn't make anything. Troubleshooting may determine that the flow to drain is inadequate and causing the membrane to scale up.

Membrane fouling

Fouling is similar to scaling, but instead of inorganic minerals accumulating, living organisms start to slime and shut down the membrane just like scale. 

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