Converting Wastewater to Tapwater

Phoenix utility aims to turn wastewater into high-quality drinking water

Converting Wastewater to Tapwater

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For years, proposals have been floated for what some might consider the Holy Grail of the water industry, especially in drought-stricken regions: converting wastewater into high-quality drinking water.

But if everything goes according to plan — albeit on a small scale initially — that long-standing dream will become a reality in Phoenix by 2028, says Nazario Prieto, assistant water services director at the Phoenix Water Services Department.

To accomplish this, the city plans to rehabilitate its Cave Creek Water Reclamation Plant, located on the city’s far north side. The facility was built in 2000, then mothballed in 2009 due to lower-than-expected population growth, he says.

If the Phoenix City Council approves the proposed $300 million project, the facility will initially supply recycled water to existing reuse customers and for groundwater recharge. If it then becomes the city’s first facility to use an advanced water-purification system to convert wastewater into high-quality drinking water, it could initially produce 5 to 6 million gallons per day.

But that can’t happen until state officials finish drafting rules governing this process.

“We always thought that someday, the Cave Creek facility could house Phoenix’s first advanced water-purification system,” Prieto says. “Now it’s finally close to becoming a reality.

“We believe it’s a great first step toward developing a more sustainable source of water supply,” he says.

“This absolutely represents the future of water,” Prieto adds, noting that Phoenix already treats around 132 million gallons of wastewater per day, so there’s plenty of wastewater available for recycling.


Phoenix obtains about 60% of its water from the Salt and Verde rivers, which are fed by snowmelt from mountains north of Phoenix. The Colorado River contributes roughly another 40% of the city’s water supply. Underground aquifers also supply a small amount.

Two primary factors have pushed adoption of advanced water purification technology, which has been available for years, to the forefront. The first factor is the prolonged mega-drought in the Southwest, which over the years has resulted in lower and lower water allocations from the Colorado River, even as the state’s population continues to surge, Prieto explains.

“For us, it’s a matter of diminishing water resources,” he says. “We need to supplement water shortages stemming from reductions in Colorado River allocations.

“We know further reductions are coming, so we’re trying to look ahead to ensure we have the right water portfolio for now and in the future,” he continues. “And advanced water purification is one piece of the puzzle.”

Furthermore, prior to 2018, the state didn’t allow cities to use recycled water for direct potable reuse; it only allowed indirect potable reuse, Prieto points out. 


The advanced water purification process involves taking treated effluent and running it through additional treatment processes to remove everything from bacteria, pathogens and viruses to chemicals and heavy metals. The end product is high-quality drinking water that meets or exceeds all state and federal regulatory standards and that draws taste comparisons to bottled water.

The first step in the purification process is microfiltration, which uses membrane bioreactors to remove solids and create effluent. This system will be housed inside the Cave Creek facility. The equipment for the other processes will be located in a roughly 44,000-square-foot addition to the facility; construction is expected to start in early 2024.

The series of purification steps will include reverse osmosis, ultrafiltration, an ultraviolet advanced oxidation process (which destroys chemicals and pathogens) and chlorination. A granular activated carbon filtration system could also be installed, Prieto says.

“We will use a combination of these technologies, depending on what state regulations will require,” he says. “We may even need additional treatment after the advanced purification.”

The Arizona Department of Environmental Quality is developing those water quality regulations, which should be finalized by the end of 2024, he says.

“What we’re planning to do so far is in line with the roadmap under development by the ADEQ. We’re basically making an educated guess about what processes will get us there.

“If our processes are too good, we’ll live with it,” Prieto adds. “And if they’re not good enough, we’ll add processes.

“In the long run, we think it will save ratepayers money by installing the processes now. We’ve seen what’s happened during the last two years with inflation and high project costs, so it makes sense to implement all of these processes now.”

The Cave Creek facility could be operational by 2026 and could start supplying some Phoenix residents with drinking water in 2028, following a two-year pilot program in which the utility will fine-tune the facility’s operations, train operators, collect data and demonstrate to regulators and customers that it can produce safe and healthy water.


Phoenix is not the only city in the United States either considering or moving forward with so-called “direct potable reuse” projects. They offer the promise of more sustainable water supplies for large cities because the amount of wastewater they produce typically represents 50 to 60% of the total water supplied, according to the U.S. Environmental Protection Agency.

California regulators approved new rules in December 2023 that allow water utilities to proceed with such projects. According to published reports, several large California water agencies in the state subsequently have announced plans for direct potable reuse projects, including the Metropolitan Water District of Southern California, which serves 19 million people; the San Jose-based Santa Clara Valley Water District; the San Diego Public Utilities Department; and the Los Angeles Department of Water and Power.

Similar efforts also are underway in cities in Arizona, Colorado, Florida and Texas, including El Paso.

Furthermore, other water agencies nationwide — such as the Orange County Water District — already use recycled wastewater to replenish aquifers that supply drinking water, a practice known as indirect potable reuse.

Prieto says implementation of a direct potable reuse program is not a new concept in Phoenix. 

“We’ve been thinking about doing this for a very long time — ever since the early 2000s, or maybe even earlier than that.

“Throughout its history, Phoenix Water has been a pioneer,” he continues. “It’s just part of the culture here. We’re always looking at ways to expand our portfolio of water resources and operate in a responsible way.”

As another example of that innovative mindset, Prieto notes that the city also is planning for a proposed multibillion-dollar regional advanced water-purification project that could serve Phoenix and five other local municipalities. The proposed facility would be built adjacent to the utility’s existing 91st Avenue Wastewater Treatment Plant, co-owned by Phoenix and the cities of Mesa, Scottsdale, Glendale and Tempe and operated by Phoenix Water Services.

Treated effluent from the 91st Avenue plant currently is used to cool the reactors in a nearby nuclear power plant and for agricultural irrigation. The utility would maintain those commitments, which would leave an average of approximately 60 mgd a day for direct potable reuse, Prieto says.

“It’s very exciting to think about how advanced water purification will help Phoenix create a sustainable water source in the years ahead.”


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