In order to understand reverse osmosis (RO) water treatment you need to know what osmosis is, so let’s try an experiment. Place a few dried peas in a cup and fill it with purified reverse osmosis water. In another cup, pour in some brine (a thick solution of salt water) and put a few fresh peas in it. Allow the solutions to sit for a few hours, and you will see the dried peas in the reverse osmosis water swell up. The fresh peas in the salt-water solution shriveled up. What happened!? Peas are made of cells, like all living organisms, and each cell is covered with a semi-permeable membrane (in other words, some things can get through it’s skin, but some things can’t). Reverse osmosis works similar to the pea in that a semi-permeable membrane allows the water to pass through, but not any dissolved substances like salts and other contaminants.