Reverse Osmosis - What is it and how it works

Before I explain what Reverse Osmosis is (RO for short), I should probably explain what osmosis is. Put simply, osmosis is when solvent molecules (eg water) pass through a semi-permeable membrane to a solution or from a thinner solution to a denser solution. Semi permeable membrane (or RO membrane) allows solvent molecules to pass through (and nothing else).

By the way, osmosis comes from the ancient greek word ὠθῶ, which translates to "push".

To better explain osmosis, let's use this picture where you can see 2 solutions seperated by a semi permeable membrane.

Let's assume that the solution (in blue color) is sugar and has a concentration C (keep in mind that sugar is dissolved in water). On the left side A ( greek: Πλευρά Α) we have pure solvent (water) with a concentration C=0. Since this membrane allows only solvent molecules to pass through, only pure water will pass through the membrane (from the left side to the right side - pure water will pass from the left side to the sugar side). Also, any pure water molecules present in the sugar will pass from the right side to the left side. So far so good? Now, since pure water can pass through both sides, eventually these concentrations will be equal. This is called Osmosis.

Now let's try to explain Reverse Osmosis. If we apply pressure to the right side (the sugar - blue color) then only the pure water in the sugar will pass to the left side. So we have one way movement, from right to left, from the sugar to the left side. This is called reverse osmosis.

In RO water treatment systems, dirty water is trying to pass from the right side to the left side. Since only pure water molecules can pass through the membrane, dirty water will be discarded and pure water will pass through to the left side (your glass or the RO tank) so that you can drink it! Got it?

Until now, RO is the best technology has to offer in water treatment and desalination. Many islands in the world use RO technology to get pure water from the sea.

Have a look at the picture below (sorry for the greek...). This is how desalination works: sea water is on the right side. We apply pressure to the sea water which is then forced to pass through the membrane to the left side. The "dirty" water in this situation in this case may be used to extract salt or left in the sea (it is sea water after all...).

You can add a mineral filter as a 6th stage that enhances the water with minerals or even a UV light. Keep in mind that this is the correct RO set up:

1st stage: Polypropylene filter

2nd stage: 5μm carbon block

3rd stage: 0.5 carbon block that can reduce heavy metals

4th stage: RO membrane

5th stage: Activated carbon filter from coconut shell

Some times we see wrong set ups. A wrong set up can be:

1st stage: Polypropylene filter

2nd stage: carbon block or granural activated carbon

3rd stage: Polypropylene filter

This is wrong!! RO membrane doesn't have a problem with sediment, since "dirty water" will be discarded. It has a problem with chemicals. Chemicals can destroy the membrane and that's the reason the correct set up needs to have 2 carbon blocks as pre filters. If you have a correct set up then you will be replacing your ro membrane every 2 years (and not sooner). If your pre filters are of good quality then you should replace them once every year (and not sooner)

If we haven't covered your RO questions, please don't hesitate to contact us. We'll be very happy to help you! Check out our available water filter systems for sale here