Clean-Water Plant Stays in Permit Compliance During Major Upgrade for Phosphorus Removal

An Idaho team completes a major upgrade for phosphorus removal and a plan for water reuse while keeping the existing plant in permit compliance.

Clean-Water Plant Stays in Permit Compliance During Major Upgrade for Phosphorus Removal

The rotary drum thickeners in the solids handling building (FKC).

It’s been said that making major upgrades to an in-service clean-water plant without incurring violations is a like changing a tire on a car going 60 miles an hour without having a wreck.

Yet that’s just what the staff of the Nampa Wastewater Treatment Plant is doing.

Located 20 miles west of Boise, Nampa is the third-largest city in Idaho. It also runs the state’s oldest and third-largest wastewater treatment plant. The facility (18 mgd design, 11 mgd average) serves about 102,000 people and a population equivalent of 250,000 when industrial and commercial facility flows are included.

Throughout an area called Treasure Valley, state Department of Environmental Quality and U.S. EPA regulators in the early 2000s began requiring cities to upgrade their wastewater systems. In 2009, Nampa began negotiating its future permit limits. In 2010, the city developed a facilities plan and began a phased program of plant improvements to meet new DEQ/EPA phosphorus limits of 0.5 mg/L by 2020 and 0.1 mg/L by 2026.

Controlling temperature

During negotiations with the DEQ, it became clear that a total maximum daily limit study would be required. The city’s permit would have to meet receiving stream temperature limits of 66.2 degrees F in July and August and 67.5 degrees F in September. The instantaneous maximum would be 73 degrees F. The deadline to meet those limits is 2031. These temperature limits are based on Idaho water-quality standards for cold-water fish that live in the receiving stream, according to Andy Zimmerman, Wastewater Division superintendent.

Rather than just install effluent chillers, the city sought input from citizens, industrial users and others. The comments pointed the way to an alternative: discharging to irrigation canals and to local industries instead of to the receiving water, Indian Creek, during summer when effluent temperature is high.  

The city has applied to the DEQ for a Class A recycled water effluent reuse permit. Zimmerman says the permitting process has been going smoothly. The Class A recycled water program has multiple benefits. In warmer months, industries could use Class A water for process and wash water. Commercial entities and farmers could also use it, and residents could water their lawns and gardens with it. Meanwhile, the city preserves its valuable aquifer source for drinking water.

Zimmerman says a key to success will be to provide recycled water when users need it and allow the community to extend the benefits of its water resources. “Our irrigation season here is approximately between April and October,” he says. “That makes a good fit for the reuse water. In the colder months, the effluent still would go to Indian Creek.”

Tackling phosphorus

Meanwhile, the plant team is gearing up to meet the new phosphorus limits, which Zimmerman believes are workable based on experience running their plant during the upgrade transition.

“In May 2019, we removed the trickling filters from our system as other upgrade equipment came online,” he says. “That allowed us to operate in full phosphorus-removal mode. During most of the summer, we’ve biologically met our 2020 permit limit of 0.5 mg/L.”

There have been some hardships along the way. Phasing equipment installation because of funding limitations caused the greatest difficulties. “Ideally, we would have worked in parallel on both the liquid and the solids side so they would get completed simultaneously and we could switch to the phosphorus-removal process instantaneously,” Zimmerman says.

But limited funds meant that wasn’t possible. “Our biggest hardship was to have one process ready to go but be waiting on other processes,” Zimmerman says. Nampa operators were heavily involved in the phasing, getting the processes to work when needed and keeping the plant out of trouble.

Process tuning

“The trickling filter system had three sets of clarifiers: primary, secondary and final,” Zimmerman says. “For the upgrade, we also built a new pumping station. We got away from the tricking filters because of the carbon load they ‘steal’ from phosphorus removal. We did a lot of timed outages, bypasses and similar things to get the system to run. And we never violated our permit.”

While experimenting with phosphorus removal, team members realized that the volume of solids produced might cause a permit violation, so they backed off. Zimmerman explains, “In our aeration basins, we have a flexible aeration zone so we can run it with or without air, depending on whether we’re focusing on phosphorus and ammonia or primarily on ammonia. We can turn that on or off as needed based on the permit limits.

“The tricking filters were phased out because they removed too much of the BOD/COD carbon source that’s required for phosphorus removal. We don’t run the tricking filters at all now.” In the old system, flow went from preliminary treatment to the primary clarifier and then to the tricking filters to reduce BOD/COD loading.

There was a need to deal with sloughings from the tricking filter, which were removed in the secondary clarifier. The secondary effluent, with its lower BOD/COD load, went to a nitrification basin for ammonia removal.

Revamped process

In the upgrade, the trickling filters are still in place and connected via piping to the system. “If something happened, we could go back to them, but that would throw us out of our permit limits because it would use too much of the carbon source.”

The treatment process now has raw sewage flowing into the headworks and grit removal and then to the primary clarifiers. Scum and primary solids are removed, and the primary effluent flows to the aeration basins by way of a new pump station. The secondary effluent pump station in the old system was removed to make room for another aeration basin.

A return activated sludge/waste activated sludge pump sends RAS back to the aeration basin and WAS to the anaerobic digesters, while the final clarifier effluent goes to the chlorine station, the chlorine contact basin, sodium hypochlorite for chlorine neutralization and a post-aeration basin before discharge to the creek or the reuse system (when approved).

The upgraded plant has three anaerobic digesters, and a fourth will be added. The sludge holding tanks and loading dock from the old system have been removed, and the belt presses are being converted to two centrifuges (GEA Group). A dissolved air flotation system has been replaced by rotary drum thickeners. Nampa team members truck the biosolids 35 miles to a landfill where it is mixed with solid waste.

For all these efforts and results, the American Public Works Association Rocky Mountain Chapter selected the Nampa plant as Project of the Year in the category for project values $3 million and up. In addition, the American Council of Engineering Cos. Idaho Chapter named it the 2019 Best Water/Stormwater Project and hailed it the best-in-class solution for meeting nutrient removal and water-quality requirements.

It’s a nice achievement for a team forced to complete a major plant upgrade on the fly.

Up the license ladder

Utility managers have been saying the same thing nearly forever: “It’s hard to find certified operators.”

The wastewater utility in Nampa, Idaho, which also manages the city’s 390-mile collections system, has 20 employees, including an operations supervisor, two lead operators and six operators. The staff operates 20 hours a day in two shifts from 7 a.m. to 2:30 a.m.

To fill the need for certified operators, Andy Zimmerman, superintendent of the Wastewater Division, has built an informal apprenticeship program. The three operations and maintenance flex team members work as trainees for positions that require a state license. Three mechanics and a maintenance supervisor keep the plant running.

“We have an incentive program that provides extra pay for a higher license,” Zimmerman says. “The utility financially helps employees get licensed.” All operators must have at least a Class 1 (lowest) license; lead operators and the operations supervisor need a Class 3 license.

Joe Tague, operations supervisor, holds a Class 4 (highest) license, which the city prefers.

Nampa’s mechanical (maintenance) supervisor must hold at least a Class 2 license.

The incentive program has enticed the other mechanics to earning at least a Class 1 license. Most hires for operators and mechanics are internal hires, and the O&M flex positions are usually filled from outside.


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