Removing salt to supply a nation's water

Despite lingering doubts over the costs and environmental impacts of desalination, a National Academy of Sciences (NAS) committee predicts that desalination plants will satisfy a portion of future U.S. water demand. However, in the new report released April 22, the panel's engineers and water scientists call for more federal funding for research into water sources and energy requirements.

Desalination "will be a part of the U.S.'s water future," says Amy Zander, the head of the committee and an environmental engineer at Clarkson University. But lack of information on groundwater resources and future energy requirements is "going to remain an issue," she and the committee members emphasize. In addition to boosting federal funding, the NAS committee encourages detailed mapping of groundwater resources and their characteristics, such as salinity, underground flow, and recharge rates.

Groundwater resources in particular have gone largely unmapped in the U.S., and finding low-salt or brackish waters remains key to desalination's success inland. Reductions in energy consumption may have plateaued: reverse osmosis, the best established desalination method, is "already very close to the thermodynamic minimum needed to get salt out of water," Zander says. "There's not much further we're going to go in reducing energy in desalination." But thermal desalination, which taps waste heat from another energy source, may be worth pursuing.

Other problems are inherent to the technology, including what to do with the removed salts and the removal of other unwanted components with potential environmental or human-health impacts. For example, naturally occurring boron can be removed by more costly configurations of reverse-osmosis membranes. The World Health Organization sets boron limits in fresh drinking water at 0.5 milligrams per liter. Seawater and some groundwater sources in coastal areas contain much higher levels of boron, and the human-health effects remain unknown.

from ES&T News