Offshore desalination could turn the oceans into an inexhaustible water supply.With a recent government study projecting that at least 36 states will face water shortages within the next five years, some states are looking to tap our oceans for more than a trickle of our freshwater needs. The only significant seawater desalination, or desal, facility in operation in the United States is the Tampa
Bay Seawater Desalination Plant, which after a problem-plagued start is finally
producing 25 million gallons of water a day, or about
10 percent of the region’s water supply. California, Texas, Massachusetts, and
Georgia are all cautiously considering similar saltwater desal plants. But critics
say (pdf) these plants are energy hogs that have a hugely detrimental
impact on coastal marine life.

One
potential alternative that’s getting a lot of attention these days, not just in
the United States but around the world, is the idea of offshore desalination platforms or vessels. “There are so many
obstacles and hurdles to overcome in building and running a desal plant
onshore,” says Charles “Skip” Griffin, a senior vice president with PBS&J
Engineers who has been designing water-treatment plants for 40 years,
“that going off-land is kind of a no-brainer.”

Offshore,
the water can be extracted from an optimal depth where sea life density is low
and where the water is cleaner, reducing the extensive pretreatment that
onshore plants must perform. Furthermore, the concentrated saltwater left over
after processing can be more thoroughly diluted in the deep ocean rather than
being dumped near shore, where marine life is plentiful. And the cost of
powering an offshore plant is expected to be less than for land-based plants; while land-based plants end up having to buy third-party
power, an offshore plant could produce its own without the markup.

The notion
of offshore desal platforms is not entirely new—India has built a test plant,
and a Spanish company wants to construct a wind-powered one—but most such
approaches are geared toward small productions of 5 million gallons or less per
day. Far more ambitious is a plan from Water
Standard Company, a Houston-based water-treatment outfit that
intends to build a Seawater Desalination Vessel (SDV) that could output up
to 15 times that much—up to three times the production of the Tampa Bay desal
plant. The SDV, moored a mile or more offshore, would generate its own power
with efficient gas turbines, which could use biofuels if sufficient supplies
are available. The SDV would use the same desal method the Tampa plant uses,
reverse osmosis, in which seawater is pumped at high pressure through dense
membranes to remove the salt. It’s basically the same process that cruise ships
(80,000 gallons per day) and military ships (aircraft carrier: 300,000 gallons
per day) have used to convert seawater to freshwater for decades.

“It
looks like it’s feasible,” says Mark S. Williamson, an engineer who
evaluated the Water Standard Company’s SDV proposal on behalf the Monterey
Peninsula Water Management District in California. “But it has never been
done before on this scale and so, in our assessment, we think the proponents
have probably underestimated the cost and the regulatory difficulty. And the
harm or lack of harm to the marine life has not been established in my
judgment.”

Water
Standard says it’s well aware of the costs and regulatory hurdles; to lessen
the regulatory burden, the company expects the first ship will probably be
built for Israel, Australia, China, or the Middle East—areas where there is a
great demand for water and an easier path to government approval. The company
hopes to have the first SDV up and running within two years. “There are no
untried processes here,” says spokesperson Gayle Collins. “This is
proven technology.”