Fond du Lac's Operators and Engineers Were Not Afraid to Try Something New. Their Courage Paid Off.

An advanced nutrient removal technology saves a plant in northeast Wisconsin from an expensive expansion and brings multiple benefits.

Fond du Lac's Operators and Engineers Were Not Afraid to Try Something New. Their Courage Paid Off.

Wastewater operators are (clockwise from left) Ryan Masek, Mike Nolde, Kyle Casper, Aaron Hafermann and Alex Krause. Hafermann holds an Imhoff cone with granules.

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The Fond du Lac (Wisconsin) Regional Wastewater Treatment and Resource Recovery Facility must meet stringent phosphorous limits of 0.8 mg/L to 0.19 mg/L by spring 2023.

After submitting a preliminary phosphorus reduction plan to the state in 2013, plant and city officials faced the challenge head-on. In 2014, the city hired the Strand Associates engineering firm, which worked with operators to find an answer.

“Without a technical solution, we faced building a fourth aeration basin to handle increased industrial loading through the headworks and a high sidestream load from our high-strength waste receiving station,” says Cody Schoepke, plant superintendent.

The team evaluated three sidestream deammonification technologies. Two of the footprints required relocating tanks, but not the AnammoPAQ reactor (Ovivo-Paques). “We could use our two centrate equalization tanks and integrate the reactor tank next to them exactly where it belonged,” Schoepke says.

However, there was a hitch: Paques had representation in Europe but no reactor installations in the U.S. Schoepke contacted three plants and received operating information that clinched the team’s confidence in the process. In addition, Paques guaranteed 83% or greater ammonia removal without adding alkalinity, provided specific conditions were met.

Contractors installed the process equipment from April through October 2018, and it functioned as promised. The pilot project was the first sidestream deammonification system in Wisconsin and the first in the U.S. using the Paques system. The project earned the 2020 Engineering Excellence State Finalist award from the American Council of Engineering Companies of Wisconsin.


A rectangular tank with fine-bubble aeration houses a high-loaded anaerobic ammonium oxidizing (anammox) upflow sludge bed reactor. Because of the high specific anammox activity and biomass concentration, deammonification occurs in one pass.

Pumps feed centrate from the equalization tanks to an Astra separator on top of the tanks. “This tilted plate settler captures residual total suspended solids, which will inhibit the bacteria,” Schoepke says. A rotary-lobe pump returns solids to the main treatment line.

Centrate overflows from the separator to the reactor. Inside the tank, 1-5 mm granules support colonies of annamox bacteria, which convert ammonium and nitrite directly into nitrogen gas, and ammonia-oxidizing bacteria. The reactor’s internal settler retains the majority of dense granules (biomass) while floc and nongranulated bacteria are wasted.

Three positive displacement blowers (Aerzen) feed the aeration grid on the bottom of the reactor. Dissolved oxygen, pH, ammonia and nitrate/nitrites probes (s::can Messtechnik GmbH) control the blowers and operating DO. “Paques was uncomfortable using a U.S. probe manufacturer,” Schoepke says. “S::can is prevalent in Europe and works well for us.”


Upgraded in 2008, the 11 mgd (design) activated sludge plant averages 8 mgd from 65,000 residents. Operators use ferric chloride for phosphorous removal, but the chemical has limitations. “Our influent ammonia is 25 to 28 mg/L, and because of the high-strength waste, the digester sidestream contributes 40% of the ammonia load,” Schoepke says. “Since 2014, we’ve experimented with biological phosphorous removal to reduce phosphorous, but needing to nitrify makes it difficult.”

While waiting for the process equipment to arrive from the Netherlands, operators replaced the old centrate feed pumps with Flygt - a Xylem Brand submersible pumps, and the maintenance staff built a hut for the rotary lobe sludge pump (Boerger) next to the location for the Astra separator.

Months before startup, the granular anammox catalyst arrived in 27 300-gallon totes from Denmark and Brazil. To keep the biomass alive, operators monitored the totes daily, adding sodium nitrate if nitrate levels were too low. If conductivity was high, they pumped off supernatant and added fresh effluent. 


“Our operators were integral to startup, which happened on Jan. 8, 2020, during one of the coldest winters on record,” Schoepke says. “The biomass required constant attention as it acclimated.” The team controlled the centrate feed rate and dilution water so as not to overwhelm the bacteria with too much ammonia. They also manually controlled the blowers based on reactor samples drawn every three hours.

Within a week, ammonia levels were less than 200 mg/L in temperatures between 20 and 30 degrees below zero. During the second week, the system achieved consistent 90% or greater ammonia removal. According to Paques, it was the fastest startup in the world.

The team also learned that temperature was crucial for anammox to perform at optimal levels. “In winter, we heat the 54-degree F dilution effluent to between 95 and 98 degrees F,” Schoepke says. “Since the anammox reaction produces heat, the effluent prevents the reactor from overheating in summer.”


The operators’ greatest challenge was almost killing the biomass two months after startup. They were adding 20 gpd of bleach to 225,000 gpd of dilution effluent to keep it fresh. Over a weekend, the bleach feed pump air-blocked. An operator turned up the feed to push out the air, but then forgot to turn it down. By Monday morning, ammonia removal had ceased and the biomass was black with some white spots.

“Replacing the seed catalyst would have been a large expense and taken months to arrive,” Schoepke says. “Instead, we turned off the centrate feed to rest the bacteria while providing air and micronutrients. Revival happened slowly, but we had high ammonia removal again by the second month.”

Even polymers upset the sensitive biomass. While operators weren’t guilty of overfeeding the cationic dry polymer, they did switch to a different type during a trial. “This polymer prevented the release of nitrogen gas, causing the granules to float,” Schoepke says. “My staff pumped them off the surface, and the pump’s shear rate broke the polymer bond.” The polymer eventually made its way through the reactor.


Compared with conventional nitrification/denitrification, the Paques system achieved energy savings of 30% due to lower aeration demand. The reactor’s smaller footprint saved 90% in construction costs and achieved 100% savings in external carbon. Schoepke’s operators love the system because it functions on its own with little supervision: “Their contributions and hard work are the reason it is performing so well.”  


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