How Reverse Osmosis Works

Osmosis Down, Flip it and Reverse it

Freddie Mercury and David Bowie both recognized that being under pressure can burn a building down, split a family in two, put people on the street and also create a seriously catchy tune. One thing they left out? That pressure also makes reverse osmosis work.

So we learned that in osmosis, a lower-concentrate solution will filter its solvent to the higher concentrate solution. In reverse osmosis, we are (literally) just reversing the process, by making our solvent filter out of our high concentrate into the lower concentrate solution. So instead of creating a more equal balance of solvent and solute in both solutions, it is separating out solute from solvent.

But as we've explored, that isn't something that solutions really want to do. How do we make reverse osmosis occur? Just like Bowie and Freddie, we put the solution under pressure. Let's take saltwater as an example:

In reverse osmosis, we'd have ourselves a saltwater solution on one side of a tank and pure water on the other side, separated by a semi-permeable membrane. We would apply pressure to the saltwater side of the tank--enough to counteract the natural osmotic pressure from the pure water side, and then to push the saltwater through the filter. (This takes about 50-60 bars of pressure [source: Lenntech]. But because of the size of the salt molecules, only the smaller water molecules would make it to the other side, thus adding fresh water to the water side, and leaving the salt on the other.

And voila, you've seen reverse osmosis. To distill it (ha!): reverse osmosis takes place when pressure applied to a highly concentrated solute solution causes the solvent to pass through a membrane to the lower concentrated solution, leaving a higher concentration of solute on one side, and only solvent on the other.

It's great to be able to define reverse osmosis at dinner parties, but there are surprisingly interesting uses for reverse osmosis that might make more compelling conversation. Let's push our way through to the next page to learn more about what we can do with reverse osmosis.