1959

Desalination

Samuel Yuster (1903–1958), Sidney Loeb (1917–2008), Srinivasa Sourirajan (b. 1923)

As the world’s population grows, fresh water in some areas becomes scarce. There is too little precipitation to handle the human population that has grown in the region.

Engineers look out at the ocean and ask, “Can we desalinate this water and make it drinkable?” Engineers and scientists have come up with many different ideas for industrial desalination. One technique: distillation. When the water in salt water evaporates, the vapor rises and leaves the salt behind. Condensing the vapor creates fresh water. The most common way to do this is with fossil fuels. The process is simple and straightforward but has a big carbon footprint. Another method: nuclear reactors. They leave a much smaller footprint, and this process is common in Japan. Cogeneration is also possible, where electricity generation and desalination combine.

The saltier the water gets, the more energy it takes to desalinate it. This insight led to the creation of MSF (Multistage Flash) desalination. Imagine a series of tanks, with the coldest and least salty tank at one end and the hottest and saltiest tank at the other end. Concentrated brine from the hottest tank flows through a pipe to the next tank down, and heats it via a heat exchanger. Brine from the less salty tanks flows to the saltier tanks as it concentrates. This process repeats up and down the line, trying to capture and reuse as much heat as possible and maintaining a gradient of heat and salt.

The other big technology is reverse osmosis (RO), which became viable starting in 1959, after UCLA scientist Samuel Yuster and his two students, Sidney Loeb and Srinivasa Sourirajan, were able to demonstrate its efficacy. Water at high pressure flows through an RO filter. Only fresh water makes it through. The high-pressure pumps require about as much energy as distillation, but the pumps use electricity rather than heat. A big power plant (either conventional or nuclear) can run RO desalination at night when electricity demand is low and then supply the grid during the day, making good use of otherwise idle capacity.

As more desalination plants are built, engineers pay more attention to the environment. For example, too much brine discharge can make the ocean near a desalination plant poisonous. As human population grows, engineers will need to be even more creative.

SEE ALSO Power Grid (1878), Light Water Reactor (1946), Green Revolution (1961), Bath County Pumped Storage (1985).

Diagram of a reverse-osmosis water desalination plant.