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A Summary of Membrane Separation

Membrane separation is a process that allows us to separate pollutants from a liquid, or even separate different liquids and gases from each other. While membrane separations were originally considered to be expensive, niche applications when they were first introduced in the 1960’s, uptake of membrane technologies has exploded since the early 2000’s due to new market drivers such as increased waste treatment standards and growing awareness of resource scarcity. Thanks to advancements in membrane research, membrane technology is now more energy efficient, uses fewer chemicals, requires a smaller physical footprint and is more economical than previous conventional technologies. Today, membrane separation plays critical role industries ranging from medical engineering, to energy production, to water purification, providing better quality than prior technologies at a competitive price.

Membrane Filtration & Filtration by Size

In many ways, membrane separations are similar to the sieves and pasta strainers we use in our kitchens every day– but on a much smaller scale. We can easily see how a pasta strainer drains water from our pasta by retaining chunky pasta pieces and letting water flow through. Because the holes in the strainer have been designed to be too small for pasta and too large for water molecules, the strainer is able to separate a water-pasta mixture based on their significantly different sizes. In a similar way, membrane separation can separate mixtures at a molecular level based on the physical and chemical properties of the components in the mixture, by passing the mixture through a molecular-scale “pasta strainer” known as a membrane.


 Selectivity of different membrane filtration processes towards the removal of common pollutants.

What is a “semi-permeable” membrane?

A membrane is often technically defined as a “semi-permeable, selective barrier;” meaning that it is barrier that allows certain things to pass through while stopping others. The holes in the membrane that are largely responsible for determining what passes through and what is blocked are called pores, and the size of these pores can be adjusted to target different pollutants. It is this property that makes membranes so valuable in the field of water purification, as they can be specifically designed to retain different types of pollution while letting water flow through them easily.


Selectivity of different membrane filtration processes towards the removal of common pollutants.

There are four main categories of membranes used in water purification, distinguished primarily by the types and sizes of pollutants they are best at removing. These categories are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. MF membranes remove pollutants in the 100-10,000 nm size range like dust, asbestos and bacteria. UF membranes are nearly identical to MF membranes in make-up and operation, but are better suited to remove pollutants in the 10-100 nm size range such as viruses for sterilization. NF membranes have much smaller pores than either MF or UF membranes and are used to remove pollutants in the 1-10 nm size range like sugars and other large molecules. RO membranes are the most complex membrane technology used today – they are effectively non-porous and rely on more advanced principles to function. They are also the most selective membranes available and are typically used to target pollutants in the <1 nm size range such as dissolved salts and metal ions.

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