In this blog post, we’ll examine the principles behind converting seawater into fresh water and how reverse osmosis membranes work, and explore how chemical and biological engineering is applied to these technologies.
Today, just like every other day, my day begins with buying a bottle of bottled water on my way to school. And I pay my water bill faithfully every month. Still, I’m grateful that I can breathe the air for free. It’s hard to imagine having to pay even for air. Perhaps for people in the Joseon Dynasty, paying to buy water must have felt much the same—even though water is everywhere. That’s right. More than 70 percent of our bodies is water, and about two-thirds of the Earth’s surface is covered by water. However, about 97.5 percent of the water on Earth is seawater, while freshwater accounts for only about 2.5 percent. Furthermore, since a significant portion of that freshwater exists in the form of glaciers and perpetual snow, the amount of water actually available for human use is more limited than one might think. Israel, a water-scarce country, has long been actively utilizing seawater desalination technology to address this water shortage. However, contrary to popular belief, it does not involve large-scale desalination of water from the Dead Sea—which has the highest salinity on Earth—for drinking purposes. Instead, Israel’s large-scale desalination plants are primarily located along the Mediterranean coast, where they convert seawater into freshwater. So, how can seawater be turned into freshwater?
The answer is simple: you just need to extract the water from the saltwater. There are two main methods for doing this. The first involves evaporating the saltwater to collect the water, and the second uses reverse osmosis membranes. A membrane can also be referred to as a “separation membrane” in Korean, but since the term “membrane” is more commonly used in engineering, I will use that term in this article. First, the principle behind the first method is to apply heat to the saltwater so that its temperature reaches the boiling point of water (100°C), taking advantage of the property that water turns into steam at this temperature. Under normal atmospheric pressure, water boils at 100°C and turns into steam, but since the salt dissolved in the water does not vaporize as easily as water does, it is possible to separate the water from the salt. However, since what we want is water—not water vapor—we can achieve the desired result by completing the process of cooling the water vapor back down to liquid water. Since this method is relatively well-known, this article will focus more on the second method, which is a newer approach.
To understand what the second method—reverse osmosis membranes—is, we must first understand the phenomenon of osmosis. Osmosis is similar to the process of straining a broth containing solids through a sieve to separate the solids from the liquid. Looking at this process in more detail, solids larger than the holes in the sieve cannot pass through, while only the liquid passes through. In this way, the sieve acts similarly to a semipermeable membrane by selectively allowing only the liquid to pass through. Membranes used in actual osmosis also possess this selective permeability, allowing certain substances to pass through while blocking others. For example, a membrane used to separate water from salt in a saltwater solution allows water to pass through but blocks most solutes, such as salt. Therefore, if two saltwater solutions with different concentrations are placed on either side of this membrane, it can be described as if a bridge had been built between them, allowing only water to move between the two solutions. Here, the two saltwater solutions with different concentrations refer to solutions where the concentration of salt dissolved in water differs. These two solutions naturally change in a way that reduces the concentration difference and reaches equilibrium. When two brine solutions are placed on either side of a membrane—since salt cannot easily pass through the membrane but water can—water moves from the relatively dilute side to the more concentrated side. This phenomenon, in which water moves through the membrane due to a concentration difference, is called osmosis, and the pressure required to prevent this movement of water is called osmotic pressure. In other words, osmotic pressure can be understood as the pressure required to halt the naturally occurring phenomenon of osmosis.
Based on the explanation so far, under natural conditions where no external force is applied, if seawater and a very dilute saltwater solution—which is nearly fresh water—are placed on either side of a membrane, water from the very dilute saltwater solution will pass through the membrane and move toward the seawater. As a result, the very dilute saltwater solution becomes more concentrated, while the seawater becomes diluted, thereby reducing the concentration difference between the two solutions. However, to produce fresh water from seawater, the opposite process must occur: in a system with a membrane separating the two, water must be moved from the seawater toward the very dilute saltwater solution to increase the volume of fresh water. Ultimately, the direction of water flow through the membrane must be reversed—from the high-concentration seawater side to the low-concentration side—which is the opposite of the natural flow of water from the low-concentration, i.e., very dilute brine, to the high-concentration seawater. This is achieved by applying a pressure higher than the osmotic pressure in a direction that counteracts the natural osmosis phenomenon described earlier. In osmosis, water tends to move from the dilute brine toward the seawater; therefore, by applying a pressure greater than the osmotic pressure to the seawater side, water can be forced to move from the seawater through the membrane to the opposite side. As a result, water from the seawater moves toward the very dilute saltwater solution, causing the solution on the opposite side to become even more dilute—approaching freshwater—and its volume to increase. This is the reverse osmosis membrane method, the second method for converting seawater into freshwater. In actual reverse osmosis desalination, high pressure is used to force seawater through a semipermeable membrane, a process that allows water to pass through while removing most of the salts.
This concludes our explanation of this magical technology that transforms seawater into freshwater. But that’s not all. By utilizing the selective permeability of membranes, as discussed earlier, this technology can also be applied to purify water in water treatment fields such as wastewater treatment. Chemical and biological engineering can be used for such magical feats, and its range of applications is limitless.
