One of the most common questions in the realm of water security is, what are the main barriers to dramatically expanding desalination? It seems to offer an essentially unlimited supply of water, and it's practiced on a large scale in other parts of the world, so why not here? Scientific American and YubaNet have two timely pieces laying out the main challenges. The biggest one, not surprisingly, is cost: It usually costs between $1 and $2 to extract a cubic meter (264 gallons) of freshwater from the ocean, about ten times what river water generally costs, depending on location.

Another related problem is energy. Moving and processing water in general is very energy-intensive--19 percent of the electricity consumed in California is used to pump water, purify or treat it, and deliver it to customers--and desalination is particularly intensive. Indeed, one of the main reasons why it's been successful in the Persian Gulf (in addition to there being very little freshwater there) is that Gulf States have a huge energy-generating capacity, particularly in the winter, when air conditioners aren't running and excess electricity can be used for desalination. It's cost-effective to do this in Dubai, but California, while dry, isn't quite dry enough for large-scale desalination to be worthwhile--yet. A half-century of a warming climte might change that. Finally, there's the often-overlooked question of environmental impact, on both ends of the process (intake of water and release of waste). Fish and other wildlife can easily get sucked into the pumps that take water from the ocean, and when you extract freshwater from seawater, you're left with a waste product of hypersaline brine, which can wreak havoc on local ecosystems at the point where it's discharged back into the ocean.

None of these problems seems insurmountable, though, and since desalination might be needed as a last-resort freshwater source in California and elsewhere, it's encouraging that plans are moving forward for what would be the western hemisphere's largest desalination plant, north of San Diego in Carlsbad, California. Poseidon Resources, a Connecticut-based water infrastructure company, has spent tens of millions of dollars over a decade laying the groundwork for the plant (metaphorically speaking--physical ground has yet to be broken), which was granted tentative approval by the powerful California Coastal Commission in November and could be granted final approval at a hearing on Wednesday. In order to alleviate the environmental challenges discussed above, the Carlsbad plant would be built on the grounds of an existing power generation station, which uses and discharges a huge amount of seawater for cooling purposes. The desalination plant would take a fraction of it, remove freshwater, and use the rest of the wastewater to dilute the hypersaline brine. The plant could be online by 2011, and would replace 8 percent of the water imported last year by San Diego County, which is unusually dry even by California standards. Quite a few hurdles remain, but the Los Angeles Times is right that the project merits approval and would be an important step forward for desalination.

--Josh Patashnik 

Photo: Desalinated water at a plant in Carboneras, in southern Spain (Getty Images)