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It gets ugly quick, when the water runs out

August 01, 2014
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From Los Angeles to San Francisco Bay, at least 15 desalination-plant proposals are current along the California coast. No wonder all eyes are on the $1-billion Carlsbad desalination plant. Post 2016, it’ll knock out about 50 million gallons of potable water a day and be the largest such plant in the Western hemisphere.

Seven oceans of water make it seem like a no-brainer. You need water, there it is. Of course, it’s not that simple.

Desalination by reverse osmosis removes salt by filtration using membrane technology. As is well known, osmosis is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration.

At Carlsbad, seawater will be membrane-blasted at six times the pressure of a fire hose. Since it takes two gallons of seawater to produce one gallon of drinking water, it should take about 38 megawatts of power a day to force 100 million gallons of seawater through those filters. 

Before flushing it back into the ocean, the plant must blend its brine with other seawater at a 5:1 ratio.    

Here’s the deal

San Diego County will pay from $2,014 to $2,257 an acre foot for water, and signed a 30-year contract for at least 48,000 acre feet per year. The builder, Poseidon Water of Boston, Massachusetts, has guarantees of a 9 to 13 percent return.

Desalinated water typically costs around $2,000 an acre foot, double that of water obtained by building a new reservoir or recycling wastewater, according to a 2013 study from the state Dept. of Water Resources.  

Moreover, Pacific Ocean-produced drinking water costs about four times that of “new water” obtained by conservation methods, says the Oakland, California-based Pacific Institute, such as paying farmers to instill drip irrigation, rebates to home owners for ripping out lawns or by water-efficient toilets.

Yet judging San Diego’s decision to desalinate based solely on cost is not entirely on point. Water drawn from the oceans will still be there even when all other sources dwindle. Again in California, the Santa Barbara city council recently voted to spend near $1 million to restart its shuttered desalination plant. One councilwoman, according the Los Angeles Daily News, referencing a former drought period, noted that it’s really ugly when the water runs out.

Future footprint

Seawater accounts for about 60 percent of desalinated water today, according to David Sedlak, University of California-Berkeley professor of civil and environmental engineering.

Besides its use along the ocean’s shores, the technologies used in seawater desalination cleanse brackish water, industrial effluent and other forms of liquid waste. Compared to seawater, says Sedlak, it’s more practical and sustainable to desalinate less-salty brackish water and recycle wastewater.

A large volume of brackish groundwater, agrees an article by Cheryl Katz on the website E-360, is a much cheaper source for drinking water than is desalinating seawater.

Many people in California oppose desalination due to its energy intensity, possible aquatic eco-system damage, harsh chemicals used for membrane cleaning and large volumes of extremely salty brine. Given the market opportunity, research & development to refine the technologies involved proceeds at quick step.

Trevi Systems of Petaluma, California, is a developer of forward-osmosis systems, which use thermal heat rather than high-pressure pumps to separate the draw solute from the recovered water. Lawrence Livermore National Laboratory and Stanford are developing a method that uses porous carbon aerogel electrodes. Flow-through electrode capacitive desalination, or FTE-CD, removes salt electrically.

Today about 1 percent of freshwater consumed daily is derived from desalination, based on a global capacity of nearly 80 million cubic meters per day. Annual global growth of about 19 percent is anticipated according to Global Water Intelligence.

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