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Desalination Project Worth Its Salt

By this time next year, residents in the San Antonio area will be drinking water that is currently too salty to consume. The San Antonio Water System, SAWS, says that next October it will begin operating an inland desalination plant that will eventually become the largest of its kind in the country. 

Thirty miles south of downtown San Antonio, earth movers shovel debris from around a sprawling brick building.  Inside workers are suspended 30 feet above a trench lined with water pipes.  They’re bolting a massive HVAC system into place.

“It’s still a little rough.  The sheet rock is not on, so it’s not quite finished out.” explains SAWS’s geologist Richard Donat as he leads a tour of the plant site, where an underground river of salty water will go through final cleansing and treatment before it’s blended with Edwards Aquifer water and sent to residents’ taps.

Donat says the journey begins at 12 nearby wells that will pump what’s called brackish water from 1500 feet underground.

“We’re drilling through the fresh water Carrizo into the Wilcox Aquifer,” says Donat.  In the Wilcox Aquifer the material is silts, fine sands and some clays.  It’s not as prolific as fresh water out of the Carrizo but it still produces plentiful brackish water for this treatment plant.”

The brackish water will be pumped through nine miles of pipe into the plant where pressure will push it through reverse osmosis membranes full of microscopic holes. The membranes will filter out 97 percent of the salt and solids.

“Once it goes through the reverse osmosis membranes it creates very pure water, similar to distilled water,” says Donat.

DESALINATION REDUCES RELIANCE ON EDWARDS AQUIFER 

Desalinating water from under SAWS property is part of the agency’s plan to reduce reliance on the San Antonio region’s main water supply, the Edwards Aquifer.  Right now the Edwards provides 63 percent of SAWS’s water. In two decades SAWS expects the Edwards to supply just 38 percent.  Recycled water and the underground storage of water collected when it rains, will still be important.  The future Vista Ridge pipeline will provide 12 percent of the water in 2030.  The $114 million desalination project will contribute another 8 percent.

“We’ve diversified our water portfolio to the extent we are no longer 100 percent reliant on the Edwards Aquifer,” says Gavino Ramos, Vice-President of Communications for SAWS

“We have such a diverse portfolio that it now allows companies to plan long-term.  We can go out and attract the Toyotas of the world.  We can work with the H-E-B’s of the world that are continuing to use a large amount of water because we have that resource. “

Conservation groups opposed to the Vista Ridge pipeline, which will transport water from a Burleson County aquifer, generally like the desalination project. Annalisa Peace is executive director of the Greater Edwards Aquifer Authority.

“The source of the brackish water is much closer so you don’t have the expense of building a pipeline, and then transporting and pumping from 130 miles away, as you do with the Vista Ridge water. 

saws_water_supply_pie_chart_2030.png
Credit San Antonio Water System
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San Antonio Water System
In 2035 reliance on the Edwards Aquifer will be reduced from 63 percent to 38 percent of SAWS's supply. Desalination will provide nearly 8 percent of the water.

DESALINATION RISKS AND BRINE DISPOSAL 

But desalination isn’t without some risks.

Sanjeev Kalaswad, with the Texas Water Development Board, says it’s important to ensure that adjacent fresh water aquifers aren’t polluted when brackish water is pumped  

“It is always a concern when we pump brackish water.  There may be some amount of co-mingling going on. The undesirable would be freshwater sources being contaminated by brackish water, becoming salty.”

SAWS Geologist Richard Donat doesn’t see that happening with this project, and he believes SAWS has identified the best solution for dealing with another challenge: what to do with a million pounds of salty brine that will be produced every day when the brackish water is desalinated.

“They had looked at using land application.  Could we put this on this land and property we have in South Bexar County?  But the issue was that it was too salty.  Over a period of time it would be detrimental to agriculture.  We also looked at building a pipeline and sending it to our wastewater treatment plant at Dos Rios.”

But the salty brine couldn’t be discharged with wastewater into a lake or river because it would damage the ecosystem

“So the best option we saw was doing a deep well injection,” says Donat.

Donat says the brine will be disposed of nearly a mile underground, far below the aquifers. 

He calls deep well injection a proven technology, one that allows SAWS to develop a new water supply, which will be needed to quench a growing region’s thirst.