Have you ever wondered why the skin on your fingertips swells up with wrinkles after a long bath?
The skin cells of your fingertips swell because they’ve absorbed water from the surrounding bath. All of your cells are wrapped in a semi-permeable membrane, meaning they allow water and some other molecules through while blocking the rest. Water naturally flows across semi-permeable membranes from areas of high concentration to lower concentration, a process called osmosis. The water in your cells is less concentrated than the water in your bath — it’s full of all the biological molecules that make up living things, while the water in your tub is mostly pure.
For the same reason, water will flow out of your cells if their surroundings are full of very salty water. A high concentration of dissolved salt means a correspondingly low concentration of water, so water flows out of your cells. The same thing happens to snails and slugs if you sprinkle salt on them, to deadly effect. In fact, osmosis is a matter of life or death for all living things. It’s why saltwater fish cannot survive in freshwater and vice-versa. Osmotic pressure is what keeps plant cells rigid and why they wilt when they don’t have enough water.
Natural osmosis only works in one direction, moving water from areas of high concentration to low concentration — but sometimes, as when purifying water, we want to move water in the opposite direction, leaving contaminants behind and creating even purer water. This process is called reverse osmosis, and can be achieved using pressure from a pump or other source to force water to move in the opposite direction across a semi-permeable membrane. With enough pressure, very pure water can be created by forcing it across a membrane that rejects salt and other contaminants.
Reverse Osmosis was not practically demonstrated until the early 1960s with the invention of asymmetric membranes at University of California at Los Angeles. The membranes are composed of a thin “skin” layer supported atop a highly porous and much thicker substrate. This basic structure remains the basis of reverse osmosis membranes today however many improvements to the process have occurred.