I love a good analogy that lets me better understand the physical world. I’ve worked around reverse osmosis water filtering systems for years, making mineral free water to rinse cars cleanly in car washes or to brew better tasting coffee in church. Yet I’ve never had a good understanding of the RO process.

From high school chemistry I remember that osmosis happens when a filter (think window screen) separates two different types of water. If the one side has a high concentration of a mineral (say very salty) and the other side doesn’t, then osmosis occurs. The osmosis pressure helps the two types of water tend to equalize. So the very salty water and the pure water both become somewhat salty water.

The reverse osmosis process uses an external pressure to force water through the selective membrane to filter out some larger molecules. What never made sense to me, though, was the relative size of the molecules.

The Economist article,  Salty and Getting Fresh, provides a great analogy. Think water molecules blown up to the size of tennis balls. Than salt molecules are the size of softballs (about 50% bigger diameter). Viruses then become the size of trucks and bacteria the size of power plants.

Now visualizing the RO filter becomes so much simpler. I understand why filtering bacteria out of water is simple and viruses a bit more challenging but still fairly easy. Building a filter that lets tennis balls and softballs go through but not trucks or power plant sized objects wouldn’t be too challenging. And it wouldn’t clog.

I also better understand why desalinization has been such a challenge over the past 50 years. A filter that lets through tennis balls but blocks all softballs would have a tendency to get clogged with a great flow of balls passing through. So now we understand why RO for waste water treatment works well and why desalinization on a large scale has been such a challenge. Just remember tennis balls, softballs, trucks and power plants.