Standing on a steel bridge suspended over a massive concrete basin at the city of Hays sewage treatment plant, Jason Riegel looked down past his feet into 4.5 million gallons of water swirling below him.

“This is the magic step, this is where the magic happens, right here in this basin,” said Riegel, water reclamation and reuse superintendent at the facility, where a $28.6 million modernization and expansion was officially completed Sept. 15.

Construction of the huge circular basin was a key part of the upgrade to the sewer plant, officially known as the Chetolah Creek Water Reclamation and Reuse Facility, off US-40 highway.

With the basin 22 feet deep and 185 feet across, it makes Hays home to the largest single wastewater aeration basin in the state. And a unique one it is, too, said Riegel, dictated in part by a tight construction schedule and budget.

A gigantic concrete hole in the ground, the basin has four huge submerged mixers with blades of polymer resin that stir the sewer liquids that flow into it. The stirring, along with oxygen continuously injected from the bottom of the basin by diffusers, makes the liquid a bubbly, happy environment for zillions and zillions of microscopic critters to thrive, transforming the liquid into clean water. Simply, the micro-organisms come to the plant right from the human digestive system.

“It’s pretty much our farm,” Riegel said of the process. “We’ve got these living, breathing organisms doing all this work for us. The organisms break it all down.”

Aeration isn’t unique at a treatment plant, and Hays’ old facility, with a much smaller basin, had that too. But this design is one-of-a-kind not only for its massive size, but also its arrangement: Four submerged 7-foot-radius mixing blades in the basin itself move 25,000 gallons of liquid a minute. Designed uniquely for Hays by Kansas City-based engineering firm Burns & McDonnell, the project took two years to build and was completed two months ahead of schedule. According to B&M, the design saved the city $2 million compared to a previous design.

It also allows the city to meet modern sewer plant standards of the Kansas Department of Health and Environment and the Environmental Protection Agency.

With the Hays plant facing a new permit cycle, the agencies required a biological nutrient removal process, which takes out more nitrogen and phosphorus than the old chemical treatment process.

The point is to reduce the toxic nutrients flowing into the nation’s waterways, thereby preventing harmful and destructive algal blooms, like those choking out life in the Gulf of Mexico.

“The quality of water that’s leaving this plant exceeds expectations on everybody’s end,” Riegel said. “We’ve had several visitors come in and want to see what we’re doing, want to see how it operates.”

A year ago today, the old plant discharged 426 pounds of nitrogen a day; now it discharges 21, Riegel said. And in 2018 it discharged 66 pounds of phosphorus a day; now it’s 13, he said.

The Chetolah Creek facility treats everything going down a toilet, tub or sink in Hays, as well as some floor drains, converging at the plant via the city’s 116 miles of sewer line.

That’s 2 million gallons arriving each day. Upon leaving, the treated water ultimately flows into the Missouri River, via Chetolah Creek, Big Creek, the Smoky Hill River and Kanopolis Reservoir.

Visitors include treatment plant operators throughout the state, and even a university biochemistry professor from Canada. Riegel welcomes everyone.

“We’ve kind of become a showcase for this style of plant: single cyclic aeration,” he said. “There were people that didn’t think it would work.”

Including Riegel. Mechanical mixers aren’t unique, but normally are arranged in a lazy-river-type layout, where liquids leave a basin and enter a winding canal to encounter mixing.

“I was confident the basin was going to be mixed, but I didn’t know if it would treat that well,” Riegel confessed. “I was skeptical that we were going to get there, but now I’m a believer … We’re pretty proud of it, pretty proud of the numbers it puts out.”

The old plant got rid of ammonia and suspended solids, using some aeration, and lots of chemicals. The new plant, which may save as much as a third on energy costs, uses aeration, micro bugs and ultraviolet disinfection to pull out ammonia, suspended solids, nitrogen and phosphorous, which area farmers can use.

The way it works, the liquids in the basin come into contact with air, which releases ammonia. The bugs, which are recycled through the system, enter the basin from a no-oxygen environment.

“So when they get here to the basin they’re oxygen starved and they’re hungry. So they’re hangry bugs, so they do a lot of work for us,” Riegel said.

Ammonia is converted to nitrate, then nitrites, then nitrogen gas, which is off-gassed in the system. The last step is disinfection with a big dose of ultraviolet for any of the treated water that’s reused for groundskeeping at the Hays Municipal Golf Course, the city’s sports fields, and at the plant.

The facility crew measures and tests throughout the treatment process, to see what’s working, what can be changed, if the micro-organisms are happy and flourishing, and what can be ramped up to improve the water treatment even more.

“We’re still learning every day. We’re still trying to find that sweet spot,” Riegel said. “With wastewater, I make an adjustment, it’s biological, that biology doesn’t move very fast, especially in a basin this size. So we make a change and we give it two or three weeks, and we’re looking at those numbers really close during those two or three weeks, to see if we’re going the right direction.”