How Is Reverse Osmosis Different from Filtration


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Reverse Osmosis

There are many different methods to filter water and remove its contaminants. Reverse osmosis is probably the most widely known method of all. Utilizing it is also the most common way to process water and make it suitable for human consumption.

But how is reverse osmosis different from other filtration methods and why is it the most appropriate technology to process drinking water?

We will answer those 2 questions in the following blog post. You will also learn how reverse osmosis, ultrafiltration and nanofiltration work and how they are different from each other.

What is Osmosis?

Osmosis is a natural process which describes the diffusion of water molecules through a semipermeable membrane from a lower concentrated solution to a higher concentrated solution. The simplest definition of a semipermeable membrane is one that lets the solvent (water) but not the solute (e.g. salt) pass through. This way the water is able to balance both solutions to the point where they have the same concentration of dissolved substances.

Osmosis Illustration

What is Reverse Osmosis?

Reverse Osmosis (RO) is the exact opposite of osmosis. As stated above, water naturally flows from the lower to the higher concentrated solution. By applying external pressure to the higher concentrated solution, water molecules are now pushed back through the semipermeable membrane and diffuse to the lower concentrated side. Again no dissolved substances can pass through the membrane resulting in purified water on the other side.

Reverse Osmosis Illustration

A negative aspect of ro is that the process does not only remove harmful substances that are dissolved in the water, but also essential minerals like magnesium and calcium, which are beneficial for your health. This aspect is called demineralization and it’s the reason why many ro filter systems feature an alkalinization stage to balance pH-levels and add calcium and magnesium back into the water.


Filtration describes many different physical, mechanical or biological methods which utilize a medium through which only a fluid can pass and thereby separating it from solids. A microfiltration filter has a pore size of around 0.1 micrometer. This means that all dissolved solids that are smaller than 0.1 micrometer such as viruses can pass the filter and remain in the water.

Ultrafiltration and Nanofiltration

Compared to a microfiltration filter, a ultrafiltration filter has much smaller pores of around 0.01 micrometer and can remove larger viruses.

A nanofiltration filter has a pore size of 0.001 micrometer and is therefore able to remove most organic substances, almost all viruses and other organic molecules and a range of salts.

Microfiltration Filter

Microfiltration Filter

Ultrafiltration filter

Ultrafiltration Filter

Nanofiltration filter

Nanofiltration Filter

Future Challenges

The biggest challenge with membrane filtration is to produce filters with a small enough pore size to remove all viruses that contaminate the water and are harmful to our health.

The Difference between Reverse Osmosis and Filtration

The predominant removal mechanism in membrane filtration is the exclusion by particle size. Theoretically speaking membrane filtration can achieve perfect efficiency regardless of other parameters such as (external) pressure and concentration of dissolved solids. In contrast, due to the diffusion of water through the membrane reverse osmosis is dependent on pressure, water flow rate and other conditions, which is the difference between the two of them.

The Most Appropriate Technology to Filter Drinking Water

Reverse osmosis is the most appropriate technology to filter drinking water. Why is that?

RO removes up to 99% of ions, molecules and larger particles such as bacteria and viruses, which is more than any other technology can remove. It provides exceptional pure water and is also a very cost effective technology, if used properly. Although it also removes minerals that benefit your health, it is the best method to process drinking water.

PureWaterInc has produced a YouTube-video comparing water purification by filtration, reverse osmosis and distillation. If you still have questions or are unsure, if you’ve understood everything correctly, take a look at the brief video. It might help you out:

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About the Author

My name is Derek, 28, and I am an advocate for the protection of nature and the environment due to a very selfish reason: I want my family, friends and myself to stay healthy. Quote: "There are no compromises when it comes to drinking water quality."

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