Tucson plans to build toilet-to-tap-water plant with US funds

Tucson Water appears likely to build the region's first plant to treat wastewater for drinking by the early 2030s, as part of a pending deal with the federal government to pay $86 million for plant construction in return for the city's willingness to further reduce its take of Colorado River water.

The plant, slated for the metro area’s northwest side, would provide only a small portion of the city’s total drinking water supply but it could lead to more and bigger such projects in the future if successful, utility officials say.

The Tucson City Council will vote Tuesday night on whether to accept the agreement negotiated by the water utility and the U.S. Bureau of Reclamation, under which the bureau would pay the city up to $86.7 million to build the treatment plant on the northwest side before 2032.

In return, the city would agree to leave about 56,000 acre-feet of its Central Arizona Project water supply in Lake Mead over a decade.

The item is on the City Council’s consent agenda, which is reserved for routine items likely to pass. The meeting begins at 5:30 p.m. Jan. 7 in the mayor and council’s chambers at City Hall, 255 W. Alameda St.

The treatment plant would convert about 2.5 million gallons a day of treated wastewater from the Pima County-run Tres Rios wastewater plant into drinkable water. That 2.5 million gallons represents between 2% and 5% of the city’s total drinking water supply, said Tucson Water Director John Kmiec.

The northwest side area was chosen for the plant location in part because those residents have lost a major portion of their local supply due to the shutdown of a number of drinking wells in that area due to contamination by the “forever chemicals” known as PFAS compounds, Kmiec said.

“We want to be able to still create local water there,” Kmiec said Monday. “Now, most water there is pushed north from the rest of the city.”

Recycled wastewater from the Tres Rios plant is now dumped into the Santa Cruz River, where it eventually leaves the Tucson area by flowing downstream into Pinal County, he said.

“There’s lost water to the community that is not being fully taken advantage of,” Kmiec said. “By keeping it local, it allows us to use a drop of water more than once.”

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At the same time, the diversion of river water for this treatment plant won’t appreciably diminish the river’s existing flows that nourish a major riparian area along the river banks, Kmiec said. The potential loss of river water for riparian uses such as cottonwoods and willows has long been a major concern of many environmentalists about the prospect of large-scale wastewater treatment for drinking.

The CAP water savings that the city would accept in return for federal money to build the wastewater plant would be on top of 110,000 acre-feet of CAP water Tucson has already agreed to leave in the lake for calendar years 2023, 2024 and 2025 in return for additional compensation.

Another part of the agreement to be voted on Tuesday night would put the city on the hook for leaving another 50,000-acre feet of CAP water in the lake in return for another $20 million in federal funding.

Until these cuts were made, the city was using about 100,000 acre-feet of CAP water a year.

This project, if proven successful, could be the first of more wastewater-for-drinking treatment plants in the future, city officials say.

The city’s One Water long-range water plan, adopted about a year ago, called for the utility to build small, pilot wastewater-for-drinking projects as a possible preview for building a larger plant to serve water for more people.

Around the West, many cities including Phoenix and Scottsdale now are actively studying or planning to build wastewater-to-drinking plants as a way of replacing other supplies such as groundwater and Colorado River water that have diminished in recent years due to drought and over-pumping.

The Arizona Department of Environmental Quality adopted regulations last year authorizing local entities to treat wastewater for drinking via a method known as “direct potable reuse.”

“Our stakeholders want to make sure we are optimizing the use of our effluent resources,” Kmiec said. “It is definitely the first step. It’s a small project. That’s why we were willing to work out a deal with the bureau.

“One hundred years from now, there could be much larger projects. It’s diversifying our community’s water resource portfolio.”