What is a membrane filter and how does membrane filtration work?

Membrane filtration is a globally applied technique for purifying and separating liquids, especially in water treatment. In this blog, our Process Product Specialist, Ruud Meinen, explains what membrane filtration is, how it works, and what the benefits are.

What is membrane filtration?

Membrane filtration is a separation technique that uses semi-permeable membranes to separate substances based on particle size and pressure difference. This technique involves passing a feed stream through a membrane system, separating the stream into two separate streams: the permeate (the filtered water) and the retentate (the concentrated stream with separated substances).

How does a membrane filter work?

The membrane filter acts as a physical barrier with special properties, allowing only selected components of the feed stream to pass through. The pores of the membrane material are very small, ranging from 0.1 micrometers to a few nanometers, and pressure is needed to push the liquid through. The driving force behind the separation process is the pressure difference on the two sides of the membrane.

Types of membrane filtration

Membrane filtration can be applied in different ways, depending on the specific requirements of the process.

Dead end filtration

Dead-end filtration involves feeding the membrane and stopping at the end. This allows water to pass through the membrane and dirt to accumulate. To remove the dirt, a backwash is periodically performed. This involves sending the liquid under pressure in the reverse direction through the membrane. This helps to remove the buildup of pollution on the membrane surface and in the pores. This also extends the lifespan of the membranes. However, RO filters are not suitable for backwash, they require chemical cleaning (CIP). And there are also membranes (or membranes including housing) that need to be replaced when they become dirty or the pressure drop is too high (single use).

Crossflow filtration

In addition, crossflow filtration is widely used, with the feed flow moving parallel to the membrane. This helps reduce pollution on the membrane surface, as the flow carries dirt to the drain. Crossflow filtration is often used in industrial processes where large amounts of liquids need to be filtered, such as in the production of drinking water or the purification of industrial wastewater.

Reverse osmosis and nanofiltration

Other advanced techniques, such as reverse osmosis (RO) and nanofiltration (NF), are used when a very high level of filtration is required. RO systems are widely used for desalination of seawater and production of ultrapure water, while NF is effective in removing salts and organic substances in applications such as water treatment in the food industry.

Where is a membrane filter applied?

Membrane filters in the form of pleated patterns are widely used in various filtration processes, including:

• Drinking water purification: for the production of high-quality drinking water, effectively removing chemical contaminants and microorganisms.
• Food and beverages: in filtration processes for producing beverages such as beer, wine, bottled water, and dairy products.
• Industrial water treatment: for the purification and reuse of industrial wastewater and the recovery of valuable substances.
• Membrane bioreactors (MBR): in combination with biological processes to purify wastewater and produce reusable water.

The benefits of a membrane filter

Membrane filtration offers several benefits that are important in the various industries where membrane filters are used.

• High purity: Membrane filtration can achieve very high purity levels, making it ideal for drinking water purification and other applications where high quality is required.
• Lower costs: Although energy consumption may be higher, a membrane filter saves costs in the long term through lower production and disposal costs and fewer required processing steps.
• Environmentally friendly: A membrane filter helps reduce CO2 emissions and other environmental impacts. This is evident in its application in sustainable drinking water purification and reductions in, for example, the disposal of filter cake as a membrane filter leaves no residues behind. However, RO systems can consume a lot of energy, which can put this into perspective.
• Sterile filtering: Membrane filtration, especially ultrafiltration and reverse osmosis, can be very effective in removing bacteria, viruses, and other microorganisms, making the water sterile.

Purification classifications of a membrane filter

Membrane filters include different techniques, each with their own specific applications and pore sizes.

Membrane filter for microfiltration (MF)

Microfiltration is used to separate particles with a size of 0.1 to 10 microns. The separation mechanism is based on sieving and the driving force is a pressure difference of 0.1 to 3 bar. MF is often used as a pretreatment step for hyperfiltration, to block colloidal particles (small particles present in the medium), and to stop microorganisms.

Membrane filter for other filtration

In addition to microfiltration, there are other types of filters that can be called membrane filters. For example, ultrafiltration (UF) uses a membrane with a pore size of 0.01 μm and is designed to effectively remove viruses and microorganisms. Nanofiltration (NF) goes a step further: in addition to viruses, divalent ions are also removed due to the finer pore size of 0.001 μm. Reverse osmosis (RO) is the most advanced filtration technique and is mainly used for desalination and water purification, with an exceptionally small micron size of only 0.0001 μm.

Characteristics of membrane filter (pleated cartridges)

Membrane filters are available in various types and sizes, suitable for your process. The most common characteristics of the membrane filter with pleated medium are as follows:

• 222 O-ring, 226 O-ring, or a spear/fin
• Double open end (DOE) or Single open end (SOE)
• With or without a twist lock

More information or advice on membrane filters?

Membrane filtration is a powerful and flexible technology used in many industries for the purification and separation of liquids. If you would like to learn more about using membrane filters in your processes or request a quote without obligation, please contact me via my contact details at the top of the page or via info@interfilter.nl.