Article (science for the public) - desalination technology explainer (~500 words)
This
is one of a series of "explainers" on scientific and technological topics, written for the general public.
Water, Water, Everywhere, but Not a Drop to Drink…Yet
How can salty water be made drinkable for thousands of people?
The most common processes used today are reverse osmosis and distillation. In reverse osmosis, seawater moves under pressure through fine membranes that filter out dissolved salts. The distillation process turns water into vapor, leaving salts behind. Some distillation plants boil seawater, while others pump it into low-pressure chambers, taking advantage of the fact that as atmospheric pressure drops, so does the boiling point of water. In both types of distillation, salts remain in the chamber while the vapor moves to a condenser, where it cools and forms pure drinkable water.
On a scale large enough to be useful to an entire community or region, desalination can be expensive. Facilities need power for pumps, heaters, and vacuums, and they require materials for structures that can withstand extreme temperature and pressure changes. These huge costs leave the field of desalination wide open for innovation.
James Klausner and his group of engineers at the University of Florida have developed a new process called diffusion-driven desalination (DDD), which is based on a simple principle. Air, as long as it is dry, will take up water vapor at temperatures below boiling. This happens all the time in standing bodies of water, and it will occur more quickly if the water is heated.
According to a report published in the Journal of Energy Resources Technology, a DDD plant would work like this: Surface seawater is pumped into a water heater and warmed to a temperature as low as 50°C (122°F). The heated water is sprayed into a diffusion tower where it mixes with very dry air and evaporates, leaving salts behind. The humid air is then pumped to a condenser, where fresh water accumulates. The fresh water is chilled, and the dry air is reused.
The researchers claim their process can be cheaper than other industrial desalination methods. First, the incoming seawater is heated to only 50°C, far lower than the boiling temperatures created in some distillation plants. Second, it could be warmed by industrial waste heat normally dumped into the environment. Ideally, this heat source would be steam from a power plant, but could also be from a combustion engine, a refinery, solar energy or a fossil fuel furnace. Additionally, DDD does not require an expensive structure able to tolerate high temperatures and pressures, as is the case with other desalination processes.
Finding sites for DDD plants near heat-producing industrial neighbors should be relatively easy. Electric power plants are usually located near concentrations of people and adjacent to bodies of water used for getting rid of waste heat. A desalination plant built in conjunction with such a power plant near the ocean would have a ready source of heat, seawater and potential customers.
The scientists calculate that a diffusion-driven desalination plant using waste heat from a typical 100 megawatt (MW) power plant could produce 1.51 million gallons of fresh water a day, or enough for about 100,000 people Though the efficiency of fresh water production using DDD is low, the electricity required is about half that needed for reverse osmosis desalination.
