Desalination-One Way the Lone Star State Will Beef Up Its Water Supply
It should come as no surprise that certain areas of Texas are looking seriously at desalination to meet demand. One location is even moving toward its first pilot for a large-scale seawater desalination plant.
The old expression “big like Texas” easily applies when it comes to diversity in water needs. Texas has regions with typical southeastern US rainfall totals in the eastern part of the state, as well as places where the rainfall is a scant 8 inches—more in line with those of Las Vegas or Tucson.
Recent Changes in the Way Texas Does Water Planning
In 1997 the State of Texas enacted a law, SB-1, which revamped the way water planning is conducted in the state. Charged with seeing through the implementation of SB-1, the Texas Water Development Board divided the state into 16 regional water planning groups operating on a five-year cycle. The second of these cycles has just finished.
The regions are designed, more or less, to conform to river basins or boundaries of underground water sources. The law also specifies the various stakeholders that must be represented on each of the 11 regional planning groups. These groups include agriculture, small businesses, large businesses, steam-electric generation, and river authorities, among others.
The groups get state funds to pay for consultants to develop water demand projections, available supplies, needs, and strategies for filling those needs. The law also specifies that any water projects that come to the state for financing through the water development board or for permits through the Texas Commission on Environmental Protection must be consistent with the regional plan. “There are some teeth in there to make sure, if you have a project you are thinking about, that it’s in the regional water plan,” says Linda Fernandez, principal with Fernandez Group Inc., which specializes in public outreach and public relations on water issues.
“The second round of planning has just been completed and the 16 groups submitted their regional water plans to the board; the board has reviewed them and now they are combining them into a statewide plan.
“Based on recommendations or concerns from the regional plans,” Fernandez continues, “the Water Development Board will compose a list of suggested legislative actions for the next session of the Texas Legislature starting in January. This whole planning process has really brought water to the forefront in terms of public awareness and the fact that the different stakeholders need to work together to coordinate what they’re doing; the time is right for all of these things to be happening.”
 |
Photo: Fernandez Group Inc. |
| A panel at the SRWA brackish groundwater desalination facility |
Sources of water previously considered unusable are now receiving further scrutiny, according to Fernandez. Those sources of water range from the Gulf of Mexico and brackish groundwater to water high in total dissolved solids or other substances. “Because we have a booming population with increasing demands, and because our climates and rainfall vary greatly—from 60 inches per year in the eastern section to 8 inches in the western parts—there is much activity going on in this area,” says Fernandez.
“There’s definitely a push in this direction in south Texas, which seems to be the big growth area of the state for trying out new and innovative technology and new approaches for extending water supplies, not just for municipal uses but also for agricultural uses. One area for greater scrutiny in water efficiency is in the area of water for agricultural uses. Perhaps greater than 75% of water use now goes for agricultural uses, but due to population growth and because the value of water is increasing, that proportion should decline to less than 50%.
“This is one big area where you can see a tremendous return on investment, greater than you would see putting similar funds into the municipal side.”
Fernandez says south Texas seems to be a leader in innovative technology because it’s in a situation in which the water supply is finite. The surface water from the Rio Grande has been the source of water for all uses. But it is also shared with Mexico.
“Mexico accrued a tremendous debt, though, and did not allow inflows into the Rio Grande,” says Fernandez. “That caused a lot of agricultural suffering for several years. As a result of the recent drought, the Mexican water debt issue and the rapid pace of urbanization in south Texas have led to the question of how we will extend our water supply.”
 |
Photo: Fernandez Group Inc. |
| Fernandez desal plant site |
Using Ocean Water
Several years ago Texas Governor Rick Perry announced an initiative to explore using the Gulf as an uninterruptible drought-proof supply. The Texas Water Development Board, the state agency charged with developing water supply, was tasked with putting out a request for proposal. Three sites were selected for a feasibility analysis: Brownsville, Corpus Christi, and Freeport. After a two-year period, results were turned in to the Texas Water Development Board.
Then the three sites were all invited to submit proposals for pilot studies. Based on the feasibility analyses, Brownsville clearly was the site ready to go forward.
In early 2006 Brownsville Public Utility Board was awarded $1.34 million from State of Texas funding for the pilot desalination plant. The city is also contributing $500,000 in cash and another $600,000 in in-kind services. NRS Consulting Engineers, a Texas-based firm, is the lead engineer and consultant on the pilot study. The goal of the pilot study is to have results by August 2006.
NRS is doing two seawater desalination pilot plants, one at the Port of Brownsville, taking water from the port channel, and the other on South Padre Island. “These are scaled-down versions—one-hundredth the size—of the plants that will contain all the elements of the larger facilities,” says Bill Norris, an engineer with NRS.
During the course of testing at the pilot plant, NRS will look at four different membranes, finally narrowing it down to two. “To decide which membrane to go with we will look at efficiencies, pressures, and how well they perform in general,” says Norris.
 |
| An illustration of the El Paso facility |
The membranes themselves are 8 inches in diameter and are simply slid into the vessels. The membranes last an average of five years before they must be replaced. “Probably after five years, when you go to replace them, the technology is going to improve enough that maybe the pressure will be reduced,” says Norris. “This is what we’ve seen in the past.”
Typically the membranes will cost several hundred dollars for a series of membranes within the vessels. But by the time you multiply how many membranes are being used in the system, for the 7.5 million gallons being processed daily, that can add up to a half-million dollars’ worth of membranes being used.
“But perhaps the biggest challenge is environmental,” says Norris. “The salt content of seawater just makes it more corrosive in nature; plus you have barnacles to contend with. We don’t have the ground to filter things out like we would on a brackish water project. Here it is just the seawater coming in from an ocean intake. This adds a lot of costs to the project as well.”
Making Brackish Groundwater Work
Brackish groundwater has recently turned into another big solution. Brackish water is defined as water that is 3,000 parts per million (ppm) for sodium, which exceeds drinking-water standards and means that it must be treated. (Seawater, by contrast, is 30,000 ppm for sodium, though seawater’s benefit is that it’s an uninterruptible source of water during drought.)
Since the salinity levels for brackish groundwater are so much lower than those of seawater, there are not the same kinds of costs involved. “The biggest expense with seawater comes with the energy required to force the water through the membranes,” says Fernandez. “Dubai now has the largest desalination plant in the world going up and other places such as Spain are very involved with this type of seawater desalination membrane technology.”
Because groundwater in the south Texas region was brackish and unsuitable for any use, it remained untapped. Much of this drive to use brackish groundwater has come from the rise in the cost of using surface water. Ten years ago, surface-water rights went for approximately $400 per acre foot; they now go for $2,000 per acre-foot.
“The technology for making brackish groundwater usable was simply too expensive; it was therefore not a viable option,” says Fernandez. “What’s happened now is that the cost of the membrane technology has gone down, just like the cost of computers dropped over the years. Also, since the cost of surface rights has soared, the two lines of the graph of economic viability have finally intersected.”
Brownsville and Vicinity Boosts Supplies With Brackish Water Desalination
Brownsville PUB is a partner in the Southmost Regional Water Authority (SRWA) in south Texas. Two years ago the SRWA built the largest brackish groundwater desalination facility in Texas. That facility has an output of 7.5 million gallons per day. NRS was also involved in the design and construction of that facility. “It just made more economical sense to have a regional facility so that everyone wasn’t building their own little plant,” says Fernandez. “Things are working out great with this facility; the plant is also easily expandable. As time goes by and water demand increases, that’s an important consideration.”
 |
Photo: Fernandez Group Inc. |
| SRWA pump...Texas-sized |
NRS is now at work on its sixth brackish water desalination plant in south Texas. Its third such plant should be up and running soon, and two others are out for bids and, at the time of this writing, were expected to have construction start in the latter part of 2006. “NRS started looking at brackish water desalination about 20 years ago,” says Norris.
“Our first plant was not a brackish plant but a wastewater plant. We took the technology and applied it to wastewater at a Fruit of the Loom plant to create bottle-quality water. This was at the time one of the largest water reuse plants of its kind in the world. The plant has since shut down due to ‘off-shoring.’”
In 1995 NRS designed and constructed a small brackish water desalination plant, 125,000 gallons per day, in Laredo, TX. Rancho Viejo or Valley Municipal Utility District was then the first plant in the valley that at 250,000 gallons per day provided 30% to 40% of its water needs for a resort in the area. That started up in 1999.
 |
Photo: Fernandez Group Inc. |
| The SRWA desalination facility has an output of 7.5 million gallons per day. |
 |
Photo: Fernandez Group Inc. |
| Outside the SRWA desalination facility |
“These projects spurred much interest locally,” says Norris, “the thinking being, ‘We can treat this water pretty reasonably.’ In 2000 and 2001 interest in the SRWA became strong and the feasibility of doing a regional plan was studied; in 2004 the largest brackish water desalination plant in Texas was constructed in Brownsville, treating 7.5 million gallons per day. Until El Paso is built we will remain the largest.”
The plant operates 24/7 and now provides approximately 40% of the water supply for Brownsville, a city with a population between 150,000 and 200,000. It is also one of the fastest growing areas of the US. “What has driven this whole push toward brackish water desalination is the cost of buying the rights for the water from out of the river,” says Norris. “At $2,000 per acre-foot I can build a plant for just the cost of the raw water alone. That makes the whole thing feasible along with the fact that I can build a groundwater treatment plant cheaper than I can build a surface-water treatment plant, though the operation costs for a groundwater treatment plant are somewhat higher.”
The wells draw brackish water up with 12-inch-diameter pipes from depths of anywhere from 300 to 1,000 feet. In certain areas closer to the coast, the water is not as useful, as the water becomes too saline.
“We now know enough about this process that we can account for the degree of corrosiveness of the water when it comes to our piping,” says Norris. “We install a large amount of fiberglass or high-grade stainless steel piping; even our well casings are fiberglass.”
Brownsville has 20 brackish water wells in a distribution network similar to a natural gas pipeline network. A 30-inch pipeline comes into the plant and goes directly into a cartridge filter. This filters out any of the few suspended solids that might be contained in the water. The next step is the high-pressure pump, which pumps at 160 pounds per square inch through the membrane treatment system.
From that point the water goes into a storage tank where the pH is adjusted back to being a bit more acidic so some of the particles do not precipitate out onto the membranes. The water is eventually neutralized back before being distributed into a concrete storage tank and pump station.
Brownsville Public Utility District provides all of its own power for its brackish water desalination plant. This is competitive with rates in other cities in the state.
“Our biggest challenge with the plants is making sure that we are optimizing the performance all the time,” says Norris. “We have good operators that run the Southmost plant. We really don’t have many challenges, but then again, maintenance is an ongoing challenge.”
The challenge in getting these plants built has a lot to do with the discharge of the concentrate. Being close to the Gulf Coast makes it less of a problem when it comes to discharging the sodium that is generated.
“Challenges with the seawater plant are much greater,” says Norris. “The water has a much higher salt concentration there, and there are much more details to deal with from that as well as the higher pressure generated by the higher salt content. For brackish water the pressure used in pumping must have 160 pounds per square inch, while seawater takes a pressure closer to 900 pounds per square inch for pumping.”
El Paso’s Desalination Solution
El Paso is now hard at work building a brackish water desalination facility as well, which will be the largest desalination facility in Texas as well as one of the largest in the United States. It will supply between 25 million and 30 million gallons per day when it comes online, according to Fernandez.
El Paso is totally dependent on groundwater supplies. “Their freshwater aquifer is sinking rapidly,” says Fernandez. “That and the fact that they receive 8 inches or less in rainfall each year makes a second look at brackish groundwater supplies feasible. There are plenty of supplies of such water in the area.”
El Paso has one plant completed and one that is under construction. The completed plant makes use of several brackish groundwater wells in the Mission Valley section of the city. The wells went brackish a number of years ago, so since they are already hooked up to the distribution system the solution identified to get those wells back online was to construct new well heads.
“Those wells now produce 8 million gallons per day for the City of El Paso,” says Bill Hutchinson, El Paso water resources manager. “In addition to those we are constructing a big plant that is approximately one year away from completion. It is a 27.5-million-gallon-per-day plant that will take brackish water from the Hueco Bolson, the region’s aquifer. Our plant will then be the biggest in the state, and from what I’ve heard, the largest inland brackish water treatment plant in the world.”
The water from the plant will be treated through reverse osmosis before it’s delivered into the system. But at the same time it will generate approximately 3 million gallons per day of concentrate that will be disposed of in deep injection wells roughly 22 miles from the plant.
“Currently we’re pumping about 40,000 acre-feet per year out of the Hueco Bolson aquifer, which is the groundwater basin on the east side,” says Hutchinson. “We have three sources of water in El Paso, the Hueco Bolson, the Mesilla Bolson, and the Rio Grande.”
The Hueco produces one-third or less of El Paso’s total water supply in a normal year, when there is water in the river. “If this new plant were to be run at basically 100% it would represent slightly over 30,000 acre-feet of pumping; depending upon the year, 65% to 75% of groundwater pumped in the Hueco will be run through our new desal plant.
“This won’t be our one and only source,” says Hutchinson. “This is just part of our diverse portfolio of water sources, which is what we are trying to manage: resources that are somewhat redundant so we can take care of things during a drought under a conjunctive use management plan, as well as diversify things as far as brackish water, fresh water, reuse, and all the other various components.”
By locating and pumping the wells in this new manner, El Paso will have the opportunity to isolate the two sources, brackish and fresh water. For years the city has struggled with the problem of having a well start drawing brackish water after awhile.
“The idea here is to create a barrier,” says Hutchinson. “Once that’s done we can intersect the brackish water, pump it, treat it, and then preserve the fresh water for use during drought years when the rivers aren’t running very well. We are already facing that scenario this year, and certainly for next year it looks like it’s going to be tough as well.”
The biggest contrasts in the desalination projects of El Paso and those in the Brownsville area come in the area of concentrate disposal. When it comes to the concentrate, the cities near the coast have a large body of water they’re able to deposit that into. “That is the biggest challenge from our point of view,” says Hutchinson. “For an inland plant the big question is, ‘What do you do with the byproduct of the treatment process, the concentrate?’ We recently had a two-day technical conference in our area devoted primarily to how to manage concentrate from desal processes.”
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Membrane Technology May Hold the Key
With the more stringent water-quality rules enacted by the EPA, such as treating for viruses, cryptosporidiosis, giardiasis, and others, Linda Fernandez feels that this makes the membrane technology the most effective to use. “The entire membrane filtration business has been really booming, resulting in the formation of the South Central Desalting Association [SCDA],” says Fernandez.
As growing numbers of membrane plants come online and are so different from conventional treatment plants, using chlorine and other additives, there is an acute need for training geared to operators running such plants. “Though the costs for the membranes have dropped and they are somewhat competitive, it’s still fairly expensive technology and it’s easy to ‘kill’ them if you are not careful,” says Fernandez. “SCDA has been active in training membrane operators and doing technology transfer. I think membrane technology is an important link in meeting the region’s growing water needs.”
Author's Bio: Peter Hildebrandt specializes in science and engineering topics.
January-February 2007
Desalination-One Way the Lone Star State Will Beef Up Its Water Supply
It should come as no surprise that certain areas of Texas are looking seriously at desalination to meet demand. One location is even moving toward its first pilot for a large-scale seawater desalination plant.
The old expression “big like Texas” easily applies when it comes to diversity in water needs. Texas has regions with typical southeastern US rainfall totals in the eastern part of the state, as well as places where the rainfall is a scant 8 inches—more in line with those of Las Vegas or Tucson.
Recent Changes in the Way Texas Does Water Planning
In 1997 the State of Texas enacted a law, SB-1, which revamped the way water planning is conducted in the state. Charged with seeing through the implementation of SB-1, the Texas Water Development Board divided the state into 16 regional water planning groups operating on a five-year cycle. The second of these cycles has just finished.
The regions are designed, more or less, to conform to river basins or boundaries of underground water sources. The law also specifies the various stakeholders that must be represented on each of the 11 regional planning groups. These groups include agriculture, small businesses, large businesses, steam-electric generation, and river authorities, among others.
The groups get state funds to pay for consultants to develop water demand projections, available supplies, needs, and strategies for filling those needs. The law also specifies that any water projects that come to the state for financing through the water development board or for permits through the Texas Commission on Environmental Protection must be consistent with the regional plan. “There are some teeth in there to make sure, if you have a project you are thinking about, that it’s in the regional water plan,” says Linda Fernandez, principal with Fernandez Group Inc., which specializes in public outreach and public relations on water issues.
“The second round of planning has just been completed and the 16 groups submitted their regional water plans to the board; the board has reviewed them and now they are combining them into a statewide plan.
“Based on recommendations or concerns from the regional plans,” Fernandez continues, “the Water Development Board will compose a list of suggested legislative actions for the next session of the Texas Legislature starting in January. This whole planning process has really brought water to the forefront in terms of public awareness and the fact that the different stakeholders need to work together to coordinate what they’re doing; the time is right for all of these things to be happening.”
|
Photo: Fernandez Group Inc. |
| A panel at the SRWA brackish groundwater desalination facility |
Sources of water previously considered unusable are now receiving further scrutiny, according to Fernandez. Those sources of water range from the Gulf of Mexico and brackish groundwater to water high in total dissolved solids or other substances. “Because we have a booming population with increasing demands, and because our climates and rainfall vary greatly—from 60 inches per year in the eastern section to 8 inches in the western parts—there is much activity going on in this area,” says Fernandez.
“There’s definitely a push in this direction in south Texas, which seems to be the big growth area of the state for trying out new and innovative technology and new approaches for extending water supplies, not just for municipal uses but also for agricultural uses. One area for greater scrutiny in water efficiency is in the area of water for agricultural uses. Perhaps greater than 75% of water use now goes for agricultural uses, but due to population growth and because the value of water is increasing, that proportion should decline to less than 50%.
“This is one big area where you can see a tremendous return on investment, greater than you would see putting similar funds into the municipal side.”
Fernandez says south Texas seems to be a leader in innovative technology because it’s in a situation in which the water supply is finite. The surface water from the Rio Grande has been the source of water for all uses. But it is also shared with Mexico.
“Mexico accrued a tremendous debt, though, and did not allow inflows into the Rio Grande,” says Fernandez. “That caused a lot of agricultural suffering for several years. As a result of the recent drought, the Mexican water debt issue and the rapid pace of urbanization in south Texas have led to the question of how we will extend our water supply.”
|
Photo: Fernandez Group Inc. |
| Fernandez desal plant site |
Using Ocean Water
Several years ago Texas Governor Rick Perry announced an initiative to explore using the Gulf as an uninterruptible drought-proof supply. The Texas Water Development Board, the state agency charged with developing water supply, was tasked with putting out a request for proposal. Three sites were selected for a feasibility analysis: Brownsville, Corpus Christi, and Freeport. After a two-year period, results were turned in to the Texas Water Development Board.
Then the three sites were all invited to submit proposals for pilot studies. Based on the feasibility analyses, Brownsville clearly was the site ready to go forward.
In early 2006 Brownsville Public Utility Board was awarded $1.34 million from State of Texas funding for the pilot desalination plant. The city is also contributing $500,000 in cash and another $600,000 in in-kind services. NRS Consulting Engineers, a Texas-based firm, is the lead engineer and consultant on the pilot study. The goal of the pilot study is to have results by August 2006.
NRS is doing two seawater desalination pilot plants, one at the Port of Brownsville, taking water from the port channel, and the other on South Padre Island. “These are scaled-down versions—one-hundredth the size—of the plants that will contain all the elements of the larger facilities,” says Bill Norris, an engineer with NRS.
During the course of testing at the pilot plant, NRS will look at four different membranes, finally narrowing it down to two. “To decide which membrane to go with we will look at efficiencies, pressures, and how well they perform in general,” says Norris.
|
| An illustration of the El Paso facility |
The membranes themselves are 8 inches in diameter and are simply slid into the vessels. The membranes last an average of five years before they must be replaced. “Probably after five years, when you go to replace them, the technology is going to improve enough that maybe the pressure will be reduced,” says Norris. “This is what we’ve seen in the past.”
Typically the membranes will cost several hundred dollars for a series of membranes within the vessels. But by the time you multiply how many membranes are being used in the system, for the 7.5 million gallons being processed daily, that can add up to a half-million dollars’ worth of membranes being used.
“But perhaps the biggest challenge is environmental,” says Norris. “The salt content of seawater just makes it more corrosive in nature; plus you have barnacles to contend with. We don’t have the ground to filter things out like we would on a brackish water project. Here it is just the seawater coming in from an ocean intake. This adds a lot of costs to the project as well.”
Making Brackish Groundwater Work
Brackish groundwater has recently turned into another big solution. Brackish water is defined as water that is 3,000 parts per million (ppm) for sodium, which exceeds drinking-water standards and means that it must be treated. (Seawater, by contrast, is 30,000 ppm for sodium, though seawater’s benefit is that it’s an uninterruptible source of water during drought.)
Since the salinity levels for brackish groundwater are so much lower than those of seawater, there are not the same kinds of costs involved. “The biggest expense with seawater comes with the energy required to force the water through the membranes,” says Fernandez. “Dubai now has the largest desalination plant in the world going up and other places such as Spain are very involved with this type of seawater desalination membrane technology.”
Because groundwater in the south Texas region was brackish and unsuitable for any use, it remained untapped. Much of this drive to use brackish groundwater has come from the rise in the cost of using surface water. Ten years ago, surface-water rights went for approximately $400 per acre foot; they now go for $2,000 per acre-foot.
“The technology for making brackish groundwater usable was simply too expensive; it was therefore not a viable option,” says Fernandez. “What’s happened now is that the cost of the membrane technology has gone down, just like the cost of computers dropped over the years. Also, since the cost of surface rights has soared, the two lines of the graph of economic viability have finally intersected.”
Brownsville and Vicinity Boosts Supplies With Brackish Water Desalination
Brownsville PUB is a partner in the Southmost Regional Water Authority (SRWA) in south Texas. Two years ago the SRWA built the largest brackish groundwater desalination facility in Texas. That facility has an output of 7.5 million gallons per day. NRS was also involved in the design and construction of that facility. “It just made more economical sense to have a regional facility so that everyone wasn’t building their own little plant,” says Fernandez. “Things are working out great with this facility; the plant is also easily expandable. As time goes by and water demand increases, that’s an important consideration.”
|
Photo: Fernandez Group Inc. |
| SRWA pump...Texas-sized |
NRS is now at work on its sixth brackish water desalination plant in south Texas. Its third such plant should be up and running soon, and two others are out for bids and, at the time of this writing, were expected to have construction start in the latter part of 2006. “NRS started looking at brackish water desalination about 20 years ago,” says Norris.
“Our first plant was not a brackish plant but a wastewater plant. We took the technology and applied it to wastewater at a Fruit of the Loom plant to create bottle-quality water. This was at the time one of the largest water reuse plants of its kind in the world. The plant has since shut down due to ‘off-shoring.’”
In 1995 NRS designed and constructed a small brackish water desalination plant, 125,000 gallons per day, in Laredo, TX. Rancho Viejo or Valley Municipal Utility District was then the first plant in the valley that at 250,000 gallons per day provided 30% to 40% of its water needs for a resort in the area. That started up in 1999.
|
Photo: Fernandez Group Inc. |
| The SRWA desalination facility has an output of 7.5 million gallons per day. |
|
Photo: Fernandez Group Inc. |
| Outside the SRWA desalination facility |
“These projects spurred much interest locally,” says Norris, “the thinking being, ‘We can treat this water pretty reasonably.’ In 2000 and 2001 interest in the SRWA became strong and the feasibility of doing a regional plan was studied; in 2004 the largest brackish water desalination plant in Texas was constructed in Brownsville, treating 7.5 million gallons per day. Until El Paso is built we will remain the largest.”
The plant operates 24/7 and now provides approximately 40% of the water supply for Brownsville, a city with a population between 150,000 and 200,000. It is also one of the fastest growing areas of the US. “What has driven this whole push toward brackish water desalination is the cost of buying the rights for the water from out of the river,” says Norris. “At $2,000 per acre-foot I can build a plant for just the cost of the raw water alone. That makes the whole thing feasible along with the fact that I can build a groundwater treatment plant cheaper than I can build a surface-water treatment plant, though the operation costs for a groundwater treatment plant are somewhat higher.”
The wells draw brackish water up with 12-inch-diameter pipes from depths of anywhere from 300 to 1,000 feet. In certain areas closer to the coast, the water is not as useful, as the water becomes too saline.
“We now know enough about this process that we can account for the degree of corrosiveness of the water when it comes to our piping,” says Norris. “We install a large amount of fiberglass or high-grade stainless steel piping; even our well casings are fiberglass.”
Brownsville has 20 brackish water wells in a distribution network similar to a natural gas pipeline network. A 30-inch pipeline comes into the plant and goes directly into a cartridge filter. This filters out any of the few suspended solids that might be contained in the water. The next step is the high-pressure pump, which pumps at 160 pounds per square inch through the membrane treatment system.
From that point the water goes into a storage tank where the pH is adjusted back to being a bit more acidic so some of the particles do not precipitate out onto the membranes. The water is eventually neutralized back before being distributed into a concrete storage tank and pump station.
Brownsville Public Utility District provides all of its own power for its brackish water desalination plant. This is competitive with rates in other cities in the state.
“Our biggest challenge with the plants is making sure that we are optimizing the performance all the time,” says Norris. “We have good operators that run the Southmost plant. We really don’t have many challenges, but then again, maintenance is an ongoing challenge.”
The challenge in getting these plants built has a lot to do with the discharge of the concentrate. Being close to the Gulf Coast makes it less of a problem when it comes to discharging the sodium that is generated.
“Challenges with the seawater plant are much greater,” says Norris. “The water has a much higher salt concentration there, and there are much more details to deal with from that as well as the higher pressure generated by the higher salt content. For brackish water the pressure used in pumping must have 160 pounds per square inch, while seawater takes a pressure closer to 900 pounds per square inch for pumping.”
El Paso’s Desalination Solution
El Paso is now hard at work building a brackish water desalination facility as well, which will be the largest desalination facility in Texas as well as one of the largest in the United States. It will supply between 25 million and 30 million gallons per day when it comes online, according to Fernandez.
El Paso is totally dependent on groundwater supplies. “Their freshwater aquifer is sinking rapidly,” says Fernandez. “That and the fact that they receive 8 inches or less in rainfall each year makes a second look at brackish groundwater supplies feasible. There are plenty of supplies of such water in the area.”
El Paso has one plant completed and one that is under construction. The completed plant makes use of several brackish groundwater wells in the Mission Valley section of the city. The wells went brackish a number of years ago, so since they are already hooked up to the distribution system the solution identified to get those wells back online was to construct new well heads.
“Those wells now produce 8 million gallons per day for the City of El Paso,” says Bill Hutchinson, El Paso water resources manager. “In addition to those we are constructing a big plant that is approximately one year away from completion. It is a 27.5-million-gallon-per-day plant that will take brackish water from the Hueco Bolson, the region’s aquifer. Our plant will then be the biggest in the state, and from what I’ve heard, the largest inland brackish water treatment plant in the world.”
The water from the plant will be treated through reverse osmosis before it’s delivered into the system. But at the same time it will generate approximately 3 million gallons per day of concentrate that will be disposed of in deep injection wells roughly 22 miles from the plant.
“Currently we’re pumping about 40,000 acre-feet per year out of the Hueco Bolson aquifer, which is the groundwater basin on the east side,” says Hutchinson. “We have three sources of water in El Paso, the Hueco Bolson, the Mesilla Bolson, and the Rio Grande.”
The Hueco produces one-third or less of El Paso’s total water supply in a normal year, when there is water in the river. “If this new plant were to be run at basically 100% it would represent slightly over 30,000 acre-feet of pumping; depending upon the year, 65% to 75% of groundwater pumped in the Hueco will be run through our new desal plant.
“This won’t be our one and only source,” says Hutchinson. “This is just part of our diverse portfolio of water sources, which is what we are trying to manage: resources that are somewhat redundant so we can take care of things during a drought under a conjunctive use management plan, as well as diversify things as far as brackish water, fresh water, reuse, and all the other various components.”
By locating and pumping the wells in this new manner, El Paso will have the opportunity to isolate the two sources, brackish and fresh water. For years the city has struggled with the problem of having a well start drawing brackish water after awhile.
“The idea here is to create a barrier,” says Hutchinson. “Once that’s done we can intersect the brackish water, pump it, treat it, and then preserve the fresh water for use during drought years when the rivers aren’t running very well. We are already facing that scenario this year, and certainly for next year it looks like it’s going to be tough as well.”
The biggest contrasts in the desalination projects of El Paso and those in the Brownsville area come in the area of concentrate disposal. When it comes to the concentrate, the cities near the coast have a large body of water they’re able to deposit that into. “That is the biggest challenge from our point of view,” says Hutchinson. “For an inland plant the big question is, ‘What do you do with the byproduct of the treatment process, the concentrate?’ We recently had a two-day technical conference in our area devoted primarily to how to manage concentrate from desal processes.”
Membrane Technology May Hold the Key
With the more stringent water-quality rules enacted by the EPA, such as treating for viruses, cryptosporidiosis, giardiasis, and others, Linda Fernandez feels that this makes the membrane technology the most effective to use. “The entire membrane filtration business has been really booming, resulting in the formation of the South Central Desalting Association [SCDA],” says Fernandez.
As growing numbers of membrane plants come online and are so different from conventional treatment plants, using chlorine and other additives, there is an acute need for training geared to operators running such plants. “Though the costs for the membranes have dropped and they are somewhat competitive, it’s still fairly expensive technology and it’s easy to ‘kill’ them if you are not careful,” says Fernandez. “SCDA has been active in training membrane operators and doing technology transfer. I think membrane technology is an important link in meeting the region’s growing water needs.”