I was sitting quietly in class after lunch, waiting for my teacher to begin a video. Taking a sip from my water bottle, I reread the label taped around the plastic. Most of the statements were familiar, from the purified statements to the zero calorie claims. However, something else also caught my eye.
The words, “filtered by reverse osmosis” were printed neatly in blue font. I had read various water labels before out of pure boredom, but I had never seen the mention of reverse osmosis. It made me wonder about how if the companies claimed that they bottled their spring water at the source, what was the point in undergoing reverse osmosis?
Previously in AP Biology, the class had covered a unit regarding water’s unique properties as well as the movement of solutions across membranes. Osmosis, which is the process by which solvents move through a semipermeable membrane from a high concentration to lower concentration, was among the concepts discussed. Solvents continue to shift and move down their concentration gradient until equilibrium is reached. At the same time, water is moving as well from areas of high concentration to low concentration.
Would reverse osmosis simply be water moving against their concentration gradient?
The simple is answer is yes. In reverse osmosis, the solvent moves against their concentration gradient, from areas of low concentration to high concentration. On the other hand, the water moves against its concentration gradient as well, from low concentration of free water to high concentration. However, because this process goes against the concentration gradient of solutions, solutes and water do not move freely. Instead, pressure must be applied to the process in order for the molecules to move against their concentration gradient.
Explained on this website, a demonstration includes a saltwater solution on one side of a tank and a solution of pure water on the other. Enough pressure is applied to the side of a tank containing saltwater to counteract the natural osmotic pressure of the pure water. The salt water is then pushed through the semipermeable membrane that separates the solution. However, because the pores of the membrane are small, it causes only the water to move through the membrane. The pressure helps prevent the water from moving back into the side that held saltwater. This allows for the salt to be left on one side of the tank and purified water on the other.
For purposes like the one printed on my water bottle, reverse osmosis is used to help filter minerals that are dangerous to human health. In water sources of high fluoride content, drinking it could lead to harmful effects to one’s enamel of their teeth or even bending in the bones. Reverse osmosis is used to filter out the large amounts of fluoride that filters in homes are unable to. This is how the clean water used in everyday life is produced.
Ocean water can also be desalinated for further uses in the world. Currently, over one hundred countries are desalinating ocean water in large plants for drinking or providing fresh water to citizens. On a smaller scale, this is also the process used in water filters campers use when in the wilderness.
Surprisingly, reverse osmosis is useful in other processes as well. Chemicals are used when metals are recycled and often creates wastewater. With the process of reverse osmosis, this wastewater can be filtered to produce clean water that could even be potentially drinkable. In the production of maple syrup and milk, reverse osmosis is used to separate the concentrate and water by a similar process described previously.
Though reverse osmosis is not completely self-sustaining, as water often needs to be pretreated with chemicals in order to not damage the semipermeable membrane, it is a crucial processes deeply embedded in our everyday lives.