Drought proof: Desalination plant ensures water supply

·5 min read

Jun. 11—When the Rio Grande's flow and accompanying water levels at Amistad and Falcon lakes became alarmingly low for a span of years in the late 1990s and early 2000s, the Brownsville Public Utility Board decided to do something.

The result was the Southmost Regional Water Authority brackish water desalination plant, completed in 2004 and expanded twice, the most recent expansion — the addition of a microfiltration system on top of the original reverse osmosis (RO) system — completed in 2015.

The SRWA desal plant, which has capacity of 10 million gallons per day (MGD) but is designed to double that capacity, makes Brownsville and the surrounding area essentially drought proof, according to BPUB General Manager and CEO John Bruciak.

Brackish water is a combination of saltwater and freshwater. In this case it's pumped from 20 wells drilled into the Rio Grande Alluvium, part of the vast Gulf Coast Aquifer. The wells, between roughly 250 and 300 feet deep, all are located west of Rancho Viejo since the water in that part of the aquifer is less salty, and as a result much less expensive to desalinate, than the aquifer water closer to the SRWA plant.

SRWA was formed by the state legislature in 1981 as a water reclamation and conservation district but remained dormant until 2000, when BPUB revived it for construction of the desal plant, which is a partnership. BPUB owns 92.9 percent, Valley Municipal Utility District No. 2 (Rancho Viejo) 2.5 percent, the city of Los Fresnos 2.28 percent, the Brownsville Navigation District 2.1 percent, and the Town of Indian Lake 0.20 percent.

"We were going to do it ourselves, really, because of the (low river) conditions," Bruciak said. "We said, well, we've got this authority, let's see if there's interest from the other entities, and there was a lot of interest. So it's worked out real well."

It would have been more expensive for the other partners to build their own desal plants rather than piggybacking on BPUB's large-scale facility, he said. Each partner buys its share of desalinated water, which is the quality of bottled water, and then resells it to customers.

"It's a regional approach," Bruciak said. "The state likes regional plants. They don't like five plants. They like one plant."

SRWA applied for funding through the Texas Water Development Board for the microfiltration pre-treatment system that was completed in 2015. There are currently seven desal plants in the Valley, though not with the capacity of BPUB's, he said.

The drought that precipitated construction of the SRWA plant was bad enough that the Rio Grande stopped flowing past the Brownsville weir BPUB uses to draw water for its No. 1 and No. 2 surface treatment plants. Matamoros was left high and dry as well, and upstream Mexico was holding onto its water rather than releasing it into the Rio Grande/Rio Bravo as required by international treaty.

Judy K. Adams, BPUB area manager, said that in the late 1990s conservation capacity for the U.S. share of Amistad and Falcon reservoirs together fell below 20 percent, and pointed to a PowerPoint graph showing current lake levels.

"You can see that we're starting to see a similar trend," she said. "Right now we're at 29.9 percent."

Brownsville's water conservation and drought contingency plan calls for voluntary water conservation measure when reservoir levels drop below 51 percent, while restrictions on landscape irrigation kick in at 25 percent, Adams said.

"If we drop to below 15 percent ... we have the ability to apply surcharges to people who use over a certain allocated amount," she said.

The fourth and final stage, dubbed a "water shortage emergency," triggers water rationing. The desal plant and BPUB's two surface water treatment plants have a combined capacity of 49.3 MGD, with an average flow of 20.4 MGD, with the desal plant accounts for 31 percent of the total, Adams said.

Bruciak said that if the river ran dry again and there wasn't enough water to supply all of Brownsville, rationing would be triggered but the city would still have a supply of water thanks to the desal plant.

Also, the aquifer isn't expected to run dry anytime soon. According to a 2018 geological estimate, the Rio Grande Alluvium contains more than 132 million acre-feet of groundwater and by 2068 will have only 0.04 percent less than that. The entire state of Texas uses only 16 million acre-feet of water per year, and the state's total lake storage capacity is less than 31.3 million acre-feet. An acre-foot is the volume of water it takes to cover one acre to a depth of 12 inches.

"I'd feel a little more comfortable if we were at 20 MGD at this plant," Bruciak said. "I think that's where we're heading pretty soon."

Still, based on current capacity and demand, doubling the plant's capacity can wait a bit longer, though SRWA has dedicated space when the time comes, he said.

"We can do it quick too," Bruciak said. "Just put some wells in and add the (filtration) membranes and maybe another ground storage tank. ... Two years from now, if there's been no more rain in Falcon and Amistad, I imagine it'll be built. We could at that point be shipping water up the Valley."

BPUB still has pipeline right-of-way in four counties originally intended for natural gas pipeline to feed the Brownsville Tenaska power plant, which was never built, he noted.

"That right-of-way was acquired and we can also put water (pipeline) in there," Bruciak said. "We already have that, so you could hit Raymondville, McAllen and Edinburg with water from here."