Double Belts, Double Suspenders: How El Paso Is Implementing Direct Potable Reuse

By Christian Bonawandt

Situated in the middle of the Chihuahua Desert, the city of El Paso, TX, has always known that water is precious. Since the 1960s, the city has been reusing water for irrigation and other purposes. More recently, its public utility, El Paso Water, piloted a direct potable reuse (DPR) plant that turns agricultural irrigation wastewater into drinking water. Now the city is ready to begin construction of a full-scale DPR facility, which aims to supply 10 million gallons per day (MGD) to the city. Reaching this phase was a challenge, requiring years of research, pilot testing, and regulatory engagement to ensure the safety and reliability of the treatment process, all of which laid the groundwork for what will soon be one of the most advanced DPR facilities in the nation.

An Early Start

Since the 1800s, the city has continuously expanded its water portfolio as demand has risen, starting with groundwater from the Wabo Zone, followed by the addition of surface water from the Rio Grande in the early 1900s. After becoming an early pioneer of water reuse in El Paso in the 1960s, the city went another step further in 1985 by applying ozone-treated wastewater to replenish aquifers. According to Gilbert Trejo, VP – Engineering, Operations and Technical Services at El Paso Water, this move was one of the biggest game changers for the city’s water supply and conservation efforts. “We can now say we’ve been creating a drinking water from a wastewater, albeit to replenish the aquifer, but nonetheless tried and true,” he said.

The new DPR plant is El Paso Water’s most ambitious step yet, sending treated irrigation wastewater directly into the drinking water network. Still, Gilbert insisted to both his team and the community, “This is just the next step and the next logical water resource strategy for us to implement to further diversify water portfolio.”

Developing The Treatment Train

While El Paso Water has been reusing wastewater for around 60 years, this is the first time it has sent it directly into the drinking water distribution system. As such, Gilbert said they took a “double belts, double suspenders” approach. El Paso Water's DPR plant utilizes a multi-barrier advanced treatment process that starts with low-pressure membranes to remove large-molecule contaminants. This is followed by the heart of the treatment train ― the reverse osmosis (RO) system. After that, the water is treated with UV and hydrogen peroxide disinfection using an advanced oxidation process (AOP). Granular activated carbon (GAC) filtration is used at the back end to remove excess peroxide and perform final polishing before distribution.

The selection of these technologies was initially heavily influenced by California’s Title 22 recommendations for potable reuse. El Paso Water adopted the sequence of low-pressure membranes, RO, and UV AOP, because this approach has a proven track record of reliability and producing high-quality drinking water in California. However, they adapted the process to suit their needs by adding GAC and eliminating post-treatment storage.

In addition, El Paso Water was able to demonstrate to the Texas Commission on Environmental Quality (TCQ) that their UV AOP system could achieve a higher log removal value than it has reached historically. This was accomplished through testing, calculations, and computational fluid dynamics modeling conducted by Carollo Engineers.

Ensuring Quality

Again, knowing that the water coming out of the plant was for drinking, Gilbert explained that the plant is heavily equipped with advanced water quality monitoring systems. During the development stages of the DPR project, EPW commissioned an independent panel through the National Water Research Institute, which included experts in water quality, public health, and communications. One of the key recommendations from this panel was to implement a critical control point strategy.

Gilbert highlighted that the DPR plant has 16 critical control points along the treatment train where water is being measured. “We see real-time data of performance of each treatment process and ultimately of the water quality from beginning to end,” he said. “That allows operators to react immediately even when they see degradation in treatment and in treatment efficiency.”

Managing Membrane Reject

While the DPR facility will produce high-quality drinking water, it will also generate a highly concentrated brine as reject from the membrane and RO systems. Gilbert mentioned that El Paso Water worked with experts and stakeholders to determine the best possible discharge options. Since the wastewater being treated is from irrigation, this wastewater will include high concentrations of nutrients that are valuable for that same purpose. As such, it was decided that the brine would be discharged to the city’s irrigation canal.

However, this solution was viable, according to Gilbert, only if the sodium absorption ratio could be kept below required parameters so as to not impact crops. After some testing, he said, “We were able to prove that we were not going to harm the water quality in their canal. Sufficient dilution was going to happen, and the sodium pollution ratio was well below the threshold.”

Discussions about building a DPR plant began in 2012, which means that, by the time it is fully operational in 2028, the project will have been more than 15 years in the making. As water scarcity continues to impact the U.S., water utilities can look to El Paso as a model for how to manage, diversify, and conserve water. As Gilbert put it, “Every gallon of water saved is a gallon of water for the future.”