During reverse osmosis, saltwater is forced through a semipermeable membrane that allows water molecules to pass through while all other impurities, including salt, are held back. Here's a look at the step-by-step process of reverse osmosis desalination:
- To set up a reverse osmosis desalinator, you first need an intake pump at the source of the seawater.
- Next, you need to create flow through the membrane. This will cause water to pass through the salted side of the membrane to the unsalted side. Pressure comes from a water column on the salted side of the membrane. This will both remove the natural osmotic pressure and create additional pressure on the water column, which will push the water through the membrane. Generally, to desalinate saltwater, you need to get the pressure up to about 50 to 60 bars [source: Lentech].
- Feed water is then pumped into a closed container. As the water passes through the membrane, the remaining feed water and salt solution become more concentrated. To reduce the concentration of the remaining dissolved salts, some of the feed water and salt solution is taken out of the container because the dissolved salts in the feed water would continue to increase and thus require more energy to overwhelm the natural osmotic pressure.
- Once water is flowing through the membrane, and the pressure is equal on both sides, the desalination process begins. After reverse osmosis has occurred, the water level will be higher on the side where salt was added. The difference in water level is caused by the addition of the salt and is called osmotic pressure; generally, the osmotic pressure of seawater is 26 bars. The quality of water is determined by the pressure, the concentration of salts in the feed water, and the salt permeation constant of the semi-permeable membrane. To improve the quality of the water, you can do a second pass of membrane.
Once the freshwater and saltwater are separated, the freshwater should be stabilized; that is, the pH should be tested to make sure it's fit for consumption.
On the next page, we'll take a look at how reverse osmosis desalinators are used.