Fresh water is scarce in Arizona, and around much of the globe. Ocean water, however, is plentiful. Why can’t we make ocean water into fresh water?
The short answer is economics. There are two primary methods of removing salt from water. One is to heat the water, evaporating it into steam, which doesn’t contain salt. This is very energy-intensive. Wikipedia says that a typical nuclear-powered aircraft carrier in the US Navy uses a form of this method to distill 400,000 gallons of water every day.
The other method is to filter the water through a series of semipermeable membranes – often reverse osmosis. This method uses far less energy, but the membranes are still expensive, and often require lots of maintenance.
In much of Arizona, the soil contains lots of salt. This means that even rainfall that runs across the soil might absorb enough salt to make it unsuitable for many uses. Desalination efforts in Arizona have focus on runoff or groundwater that is too salty to use.
The Yuma Desalting Plant, or YDP, was completed in 1992 at a cost of $250 million to deliver fresh water down the Colorado River to Mexico. It was intended to use “saline agricultural return flows” (runoff from the area’s irrigated farms) to fulfill the US’ legal obligation to Mexico (more on international water rights in the future). Due to high flows on the Colorado, it was only operated twice before 2010, when a Pilot Run was undertaken by the Bureau of Reclamation and several water districts and non-governmental organizations.
The Pilot Run, which wrapped up in July 2012, was an operational success – the aging plant apparently runs more efficiently than anyone had anticipated, at least with reduced energy and chemical costs due to the recession. This does not make it an economic success, however. Water resource managers (Central Arizona Project,the Metropolitan Water District of Southern California, and Southern Nevada Water Authority) in the Lower Colorado River Basin who are currently releasing enough water from Lake Mead to Mexico must decide. Would they rather pay millions of dollars to keep water in Lake Mead for their purposes and treat brackish runoff to send to Mexico? As flows in the Colorado River continue to fluctuate, this question will only become more urgent.
Alternatives include purchasing water rights from Lower Colorado farmers, effectively retiring farms. Look for a future post on this topic.