A Fresh Solution for Desalination

All over the world there are reports of water shortages. This year, in the Middle East city of Ramallah, thousands of homes went without water for almost a week because of water shortages. A couple of years ago in China, a yearlong drought caused 12 million people to go without adequate drinking water. Here in Hawaii, farmers and ranchers have lost millions of dollars due to continuing dry conditions.

Windmill used in Dr. Liu's experiment

Some of you are probably wondering, "How can places run out of water, when there's lots of water on the Earth?"

Yes, the Earth's surface is almost 70% water, but 98% of that is salt water, which humans can't drink.

However, it is possible to convert salt water into water that is safe enough for humans to drink. Desalination is a process, which removes minerals (like salt) and contaminants from water. The problem with desalination is the amount of energy needed for its operation, which makes it very expensive.

Fortunately, Dr. Clark Liu, a professor of Civil Engineering at the University of Hawaii, has successfully developed and field-tested a wind-powered reverse osmosis desalination system at Coconut Island. Since Dr. Liu's system uses the power of the wind, an alternative energy source, it creates an inexpensive way to desalinate water.

Osmosis is a process where a semi-permeable membrane (a membrane that allows only some things, like water, to pass through it) is placed between water and a solution. The water flows through the membrane until a balance is achieved.

Reverse osmosis happens when the water flow through the membrane is stopped, or reversed by applying external pressure on the side of higher concentration. This filters out contaminants, bacteria, and salts in the water, making it one of the purest forms of water you can find.

To show how much of a difference desalination makes, we can compare the TDS or total dissolved solids of different types of water. Ocean water averages 35,000 ppm (parts per million) TDS. Brackish water is over 2000 ppm. However, the water we drink must be less than about 500 ppm. So if we desalinate ocean water to a level safe enough for us to drink, the ppm level decreases by 7000 percent.

Experiment site: Coconut Island in Kaneohe Bay, Oahu

This is not only important for humans, but for plants as well. Water for agriculture also must not have high levels of TDS, or else too many contaminants will be left in the soil. An interesting feature about Dr. Liu's desalination system is the feedback control mechanism, which allows continuous operation, even when the wind speed changes. In fact, the system is able to get fresh water with a wind speed of 5 miles per hour.

Eventually, Dr. Liu hopes his system will someday become an effective way to test new semi-permeable membranes for reverse osmosis projects in other areas besides desalination.





 

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