Direct Potable Reuse: The Engineering Behind Cities Learning to Drink Recycled Water

On February 27, 2025, El Paso Water held a groundbreaking ceremony for the Pure Water Center, a $295 million facility that will do something no full-scale public water system in the United States has done before: take treated wastewater and send it directly into the city's drinking water distribution system without any intermediate environmental step.

The concept has a name, direct potable reuse, and it has an unfortunate unofficial nickname, toilet to tap, that has been a persistent barrier to public acceptance. But the engineering behind it is rigorous, the treatment process is thorough, and the water that comes out the other end meets every drinking water standard the EPA sets, often exceeding them. The water that goes into El Paso's distribution system from the Pure Water Center will be cleaner by many measures than the source water that enters the city's conventional treatment plant from the Rio Grande.

Why does this matter right now? Because the Colorado River, which supplies water to seven western states and roughly 40 million people, is in its 26th consecutive year of drought. Lake Powell and Lake Mead are at historic lows. The states that depend on that system have been fighting over increasingly scarce allocations for years. El Paso is at the front end of a wave that is going to reach a lot of American cities that have been comfortable with conventional water supply for generations. This post explains what direct potable reuse is, how the treatment process works, where it's being built, and what it means for the future of urban water supply.

1. The Difference Between Indirect and Direct Potable Reuse

Water reuse isn't new. Orange County Water District in California has been purifying treated wastewater and injecting it into underground aquifers since 2008, where it blends with natural groundwater and is later recovered by drinking water wells. That's indirect potable reuse: the purified water goes into an environmental buffer, groundwater or a surface reservoir, before it's drawn back into the drinking water system. The environmental buffer provides additional treatment time, psychological distance from the source, and a layer of natural blending.

Direct potable reuse removes that buffer. Purified water goes directly from the advanced treatment facility into the potable distribution system. Conceptually, this sounds like a bigger step than indirect reuse. From an engineering and water quality standpoint, it's actually a smaller one: the purified water often achieves better quality before it reaches the distribution system under direct reuse than it does after passing through a natural aquifer under indirect reuse, because natural systems can introduce their own contaminants, biological and chemical, that advanced treatment removes.

Big Spring, Texas, has been operating a direct potable reuse system since 2013, making it the first such system in the country. What El Paso is building is something slightly different: the first direct-to-distribution system, meaning there's no intermediate storage step before the water enters the pipes that carry it to customers. It's the most direct form of potable reuse, and getting regulatory approval for it required nearly a decade of pilot testing, monitoring, and engagement with the Texas Commission on Environmental Quality.

$295 million

The cost of El Paso's Pure Water Center, the first direct-to-distribution potable reuse facility in the United States. At 10 million gallons per day by 2028, it will supply drought-resistant water to El Paso customers from a source that never runs dry.

2. The Treatment Process: How Wastewater Becomes Drinking Water

The treatment process in advanced water purification for potable reuse is a multi-barrier system, meaning it uses several independent treatment steps in sequence, each of which removes different categories of contaminants. The redundancy is intentional: if one step performs below expectation, the others compensate. The process El Paso is using and that most DPR facilities employ follows a similar sequence.

Microfiltration:  The first treatment step uses membranes with very small pores, typically 0.1 to 0.4 microns, to remove suspended solids, bacteria, protozoa, and larger particles. The feed water for this step is secondary treated effluent from the wastewater plant, meaning it's already been through biological treatment to remove organic matter and pathogens. Microfiltration takes it further, producing a clear, particle-free stream that can proceed to the next step.

Reverse osmosis:  RO membranes push water through a semi-permeable membrane at high pressure, removing dissolved salts, pharmaceutical compounds, personal care products, and virtually all organic contaminants at the molecular level. The process rejects about 20 to 30 percent of the water as concentrate, which has to be managed separately, typically returned to the wastewater treatment stream or in coastal locations discharged to the ocean after further treatment.

Advanced oxidation:  UV light combined with hydrogen peroxide destroys trace organic compounds that managed to pass through the RO membrane. This step addresses the pharmaceutical and emerging contaminant removal that RO alone doesn't fully achieve. It also produces disinfection byproducts that are managed by the subsequent step.

Remineralization and stabilization:  The RO permeate is very low in minerals, which makes it corrosive to pipe systems. Remineralization, adding calcium and other minerals back to the water, stabilizes it for distribution and improves its taste. pH adjustment and final disinfection with chlorine prepare the water for entry into the distribution system.

The monitoring system for a DPR facility is as intensive as the treatment itself. Real-time sensors monitor each treatment step. Online analyzers check water quality at each stage. If any sensor goes out of range, automated systems divert the water away from distribution until the issue is resolved. The regulatory frameworks that have been adopted in California, Colorado, Arizona, and Texas for DPR include extensive monitoring requirements and upset response protocols specifically to address the risk of treatment failures.

3. Where DPR Is Going Next

El Paso broke ground in February 2025, and the facility is expected to be online by 2028 at 10 million gallons per day. Phoenix is planning to add direct potable reuse at its 91st Avenue Wastewater Treatment Plant, targeting 60 million gallons per day by 2030, which would make it one of the largest DPR systems in the world. Communities across Texas, from Amarillo to Dripping Springs to Liberty Hill north of Austin, have submitted plans to the Texas Water Development Board that include DPR in their regional water supply strategies.

Arizona and California both adopted DPR regulations in 2023 and 2024, establishing the permitting framework, treatment requirements, and monitoring standards for facilities in those states. Colorado finalized its DPR rules in 2024. HR 4099, which passed a House subcommittee, would create a new federal funding pool for 17 western states to develop wastewater recycling capacity. The regulatory and funding infrastructure for DPR expansion is coming together at the same time that water scarcity pressure is making the case for it more urgent.

4. The Perception Problem

The technology works. The water quality data is compelling. The regulatory frameworks are in place. The obstacle that every DPR project faces isn't engineering, it's perception.

The 'toilet to tap' framing has derailed politically viable DPR projects in multiple cities over the past two decades. San Diego voters rejected an indirect potable reuse project in 1998 largely on perception grounds; the city eventually got there through a more gradual public education and engagement process that culminated in the Pure Water San Diego program launched in 2022. Most cities that have successfully advanced potable reuse projects have done so by investing heavily in public communication, transparent monitoring data, and facility tours that let skeptical community members see the treatment process for themselves.

The engineering community has a role to play in this. Water engineers who can explain clearly and honestly what the treatment process removes, how the monitoring works, and what the regulatory oversight looks like, in terms that a non-engineer can follow, are more effective advocates for DPR than anyone else in the conversation. The technology deserves to be understood accurately.

The Water Quality Reality

The water produced by advanced purification in a well-designed DPR system typically meets or exceeds drinking water standards for every contaminant the EPA regulates, plus many emerging contaminants that aren't yet regulated. Orange County Water District has been monitoring its indirect potable reuse system's output continuously for nearly 20 years. The data is publicly available. The water quality is consistently excellent. The engineering question has been answered. What remains is the public communication work of helping communities understand what their data actually shows.

5. What This Means for Water Utilities and Cities

Direct potable reuse doesn't work for every city or every water supply situation. It's most compelling where conventional water supplies are scarce, expensive, or climatically vulnerable, which describes the entire American West and an increasing number of communities elsewhere. It requires a city to have both a wastewater treatment plant producing secondary effluent and a distribution system that can accept the purified water, which is almost universally the case.

The capital cost is significant, $295 million for El Paso's 10 MGD facility, though the per-gallon cost of water produced is competitive with desalination and significantly lower than major water import projects. The operating cost is dominated by energy for the RO and UV systems, which can be offset partially through energy recovery from the high-pressure RO concentrate stream.

For cities that are still relying entirely on surface water and groundwater from sources that are stressed or declining, the question isn't whether to consider potable reuse. It's when. El Paso spent nine years getting to groundbreaking. The cities that start that process now, with regulatory engagement, pilot testing, and public communication, will have options available to them when supply crises arrive that cities that wait will not.

Conclusion

El Paso's Pure Water Center is a landmark project because it removes the last conceptual barrier between advanced water treatment and drinking water distribution. The technology has been proven. The water quality data is compelling. The regulatory frameworks in the states where scarcity is most acute are in place. What happened on February 27, 2025, in El Paso wasn't just a groundbreaking ceremony. It was a demonstration that the most demanding form of potable water reuse is achievable, affordable, and safe.

The seven states that depend on the Colorado River are in a race against a physics problem: less water than the system was designed to deliver, serving more people who need it. Direct potable reuse won't solve that alone. But it's one of the very few tools that can genuinely expand the amount of water available to communities independent of precipitation, snowpack, or river flow. That makes it infrastructure that the American West needs to build.