Wastewater, more than 14 million gallons a day from sinks, toilets, showers and washing machines, enters the treatment plant in a filthy condition. It leaves the plant, about a mile north, and flows into the Santa Cruz River so clean that fish and aquatic insects will thrive and children can wade in it.

What made this possible is the $64.6 million upgrade project at the Nogales International Wastewater Treatment Plant, which treats wastewater generated by Nogales, Sonora; Nogales, Ariz.; and areas of Rio Rico.

In the past, there was too much nitrogen and noxious ammonia left in the water, and the water coming from the plant was silty. That’s no longer the case. Water is clear and unpleasant odors have disappeared.

Recently, a group of volunteers with River Watch, affiliated with Friends of the Santa Cruz River, was given a guided tour of the improvements. Friends President Sherry Sass was excited by what she saw.

“I can’t even believe the fish are back already. That’s just astounding,” she said. She’s anticipating that the river “will repopulate itself” with fish that used to thrive here and that there will be “a real healthy aquatic environment, from dragonflies, to ducks, to everything.”

With less nitrogen in the water, the cottonwoods and willows that line the river “will get back to the normal rate of growth rather than the abnormal super-fertilization the trees were under. “Certainly the recreational potential can be realized. For people to go into this lush environment where it’s quiet, for birding, and to have really nice smells instead of ammonia smells,” is positive. Sass said she’s happy that wading is again safe.

The City of Nogales and the U.S. Section of the International Boundary and Water Commission (IBWC) are co-owners and co-permit holders of the plant, with the day-to-day operation of the plant overseen by the IBWC’s U.S. section.

Other project participants are the Environmental Protection Agency (EPA), the Arizona Department of Environmental Quality, the Border Environment Cooperation Commission, the North American Development Bank and the University of Arizona.

Funding was from a $60 million EPA grant administered by the North American Development Bank, a $3 million contribution from the City of Nogales, Ariz., and a $1.65 million contribution from the U.S. Section of the IBWC.

The treated and cleansed water leaves the plant, south of Rio Rico and east of Interstate 19, and gushes into the Santa Cruz River, which flows through Rio Rico, Tumac�¡cori and Tubac, where it sinks underground.

The addition of new air scrubbers and fans at the newly expanded headworks, the area where wastewater first enters the plant, was part of the upgrade.

Dan Urie, operator supervisor, told the River Watch volunteers during a May 22 visit that 65 percent to 70 percent of the sewage entering the plant comes from Nogales, Sonora, Mexico, with the rest from Nogales, Ariz., and portions of Rio Rico.

“It’s normal domestic sewage. Some people think it’s a huge toxic load coming across the border, and it’s not. There are a few heavy metals, just as anywhere in America.

“It goes through a screening process that takes out anything down to three-quarters of an inch in diameter. A vortex grit removal removes rocks, sand, seeds and that type of thing. Then it goes through a fine screen down to a quarter inch,” Urie said.

Next, the wastewater enters new aeration basins. There, bacteria work to separate the heavy sludge from the liquid. “We provide the oxygen and the right growing conditions for the bacteria. The sewage stays in there under aeration about 12 hours. Then it goes off to a secondary clarifier, which is a big settling tank.

“The goal of the activated sludge is that it converts solids into a product that will sell. As a part of the aeration basin process, we do biological nutrient removal here. That was one of the major upgrades and major water quality improvements,” Urie said.

The sludge is moved into aerobic digesters for another 60 days. “At that point it’s a pretty stable inert process. There are no pathogens left alive. It’s thickened to 20 percent dry solids and it looks very much like peat moss,” he said.

The sludge is “put into a landfill or applied to land as a soil amendment and a weak fertilizer. It goes on fields growing grass, hay or alfalfa, anything that’s not a food product,” Urie said.

The clean water passes through the original sand filters installed in 1995 and is then disinfected and discharged to the river. “By July of this year, we’ll no longer be using any chlorine. We’re going to use ultraviolent light for disinfection,” he said.

Urie said that formerly, the treated water that was sent into the river still had suspended solids in it. “Typically, we were discharging about 30 milligrams a liter on average. As of yesterday it was one milligram a liter. It’s about a 30-fold decrease” in suspended solids.

In turn, that encourages the return of wildlife. “There are frogs in the water, moss in the water, and all sorts of insect life starting to grow. We’ve seen an increase in the number of birds. Now insects can hatch in that water; it’s no longer toxic to them,” he said.

“There are very limited health risks with this water. It’s no different from swimming in any stream in America.” However, Urie doesn’t advocate swimming but said wading in it is fine.

John Light, area operations manager for the IBWC, said, “I’m extremely satisfied with the level of improvement from the old facility’s level of effluent to the new level of effluent. I think it’s dramatic. It’s beneficial to the whole community.”

Sass agreed: “It’s very heartening that the government has been responsive and that so many different agencies and groups have been working together towards cleaning up what’s really the heart of the valley. The benefits will accrue to the whole population for years.”

An open house is scheduled Friday, June 23, at 9:30 a.m. to celebrate the completion of the project, and the public is invited.