By John Balliew
Seated at the western tip of Texas and located at the northern extreme of the Chihuahuan desert, El Paso is a large Southwestern city of nearly 700,000 residents. This mountainous city is part of a larger metropolitan area that includes Ft. Bliss, Texas; Ciudad Juarez, Mexico; and several smaller communities. With its arid climate, the city thirsts for water resources, which are extremely important for the viability and sustainability of the community.
For much of its history, El Paso was highly dependent on groundwater from one aquifer. Since the early 1900s, the community relied on the Hueco Bolson to provide most of the water supply for the area. Because El Paso needed more water, another well was drilled and not much thought was given to the potential of depleting the aquifer. After pumping levels dropped and salinity increased gradually, a series of groundwater models were developed in the 1970s; the results were frightening.
The models showed a high likelihood of increasing brackish water intrusion into freshwater wells. Exhaustion of the aquifer would cause saline and brackish encroachment to become more significant—to the point that the remaining freshwater supplies would be so heavily impacted by migration of salt that without desalination, the water would be unusable. Despite serious flaws that would not be discovered until early in the new century, the results were noted and the importance of desalination was given consideration.
To begin, a comprehensive Water Resources Management Plan was developed that examined and prioritized all of the available information. A roadmap also was developed that included, in order, increased conservation, reclaimed water, surface water, local groundwater, desalination and importation of groundwater. With the completion of the plan in the 1990s, it was put in motion. Enormous strides were taken in water conservation, and there were increases in reclaimed water and surface water and better utilization of local groundwater.
By 1993, the first pilot plant that treated brackish water from the Rio Grande using reverse osmosis (RO) membranes was constructed. In parallel, efforts were made to measure the amount of brackish water available for treatment and to analyze the possibilities for concentrate disposal. Ten years later, sufficient information was available to initiate a new round of pilot testing on the Hueco Bolson brackish groundwater. This new series of tests would include extensive piloting and research on concentrate disposal.
Although it is possible to take existing water quality and—through the use of various models—select membranes suitable for meeting some treatment goal, there is no substitute for pilot plant testing. A pilot plant is key, not only for membrane selection but also for selection of the proper pretreatment system.
During the time of testing, Ft. Bliss had also identified the need to utilize brackish groundwater and area form-ations with a potential for deep well injection of concentrate. At the urging of Rep. Silvestre Reyes and Sen. Kay Bailey Hutchison, an agreement was reached between Ft. Bliss and El Paso Water Utilities (EPWU) to go forward with a joint desalination plant that would use RO and deep well injection of concentrate.
Based on a careful analysis of the movement of brackish groundwater by EPWU Water Resources Manager Dr. Bill Hutchison, a decision was made to locate the plant close to existing wells that had become brackish soon after they were drilled in the mid-1980s. A string of new wells would be constructed, creating a subterranean trough to intercept the brackish water movement and direct it to the new plant.
After the location of the plant had been decided on, the next approach was education. EPWU strongly believes in the power of education, as it is critical to achieving water conservation goals and implementing the entire water resource strategy. A key part of educating customers is to provide access to facilities so that they can see what they are investing in and where their water is coming from.
In the case of the Kay Bailey Hutchison (KBH) facility, customer access could have been problematic because the plant is located in Ft. Bliss and was required to be designed with security paramount. In order to construct the plant with the necessary security and access controls, yet still allowing a space viewable by the public, the plant was constructed with separate public and secure areas.
The appearance of a membrane treatment plant is vastly different from that of a surface water treatment plant, so from the standpoint of the public, the facility does not take nearly as much time to see in its entirety.
In comparison to a surface water treatment plant or other conventional treatment schemes, a desalination plant is uniquely suited to a high degree of automation and a minimum staffing. Operators rely on instrumentation to effectively operate the plant and are provided with the best possible tools to do their job. By doing so, the level of staffing is considerably reduced; the KBH plant operates successfully with a staff of 14 in rotating eight-hour shifts.
As a plant built to utilize existing wells as the source of raw water for the membranes, careful attention had to be given to the hydraulics of the feed wells. In the case of this existing facility, the feed wells had a considerable amount of head required to move the water into a ground storage tank. Rather than waste that head by placing a new tank in between the feed wells and the RO feed pumps, the designers decided to pump the water from the wells directly to the RO feed pumps.
When the plant is running, there are no particular limitations with this system. During shutdown, however, the water in the lines from the wells to the RO pumps needs to go somewhere. Thus, the designers chose to incorporate a large pond at the plant site. The pond functions to retain storm water runoff from the plant in accordance with the Drainage Design Manual for the city of El Paso.
The concentrate disposal method used at the KBH plant is deep well injection. The permit issued by the Texas Commission on Environmental Quality limits the seven common drinking water metals to drinking water standards, but the only one of concern is arsenic. The Underground Injection Permit limits the arsenic concentration to 10 ug/L in the concentrate stream.
Arsenic is present in some of the feed wells to the KBH plant. Its laboratory, however, is well equipped to analyze for arsenic at low levels. The only issue is time. The well arsenic content is moving up and down based on some natural mechanism. This causes a delay between sample collection and delivery to the offsite lab. Such delays lead to some arsenic concentrations detected in the concentrate stream at or slightly higher than 10 ug/L limit. What was needed was a rapid method of measuring the arsenic content at the plant.
Based on experience with operating arsenic treatment systems for other wells, KBH plant staff selected the SafeGuard analyzer by Trace Detect, a fully automated arsenic analyzer capable of low-level detection that is accurate and simple to use.
Successful implementation of a large inland desalination plant is a complicated, coordinated effort between customers, designers, operators, regulators and security professionals.