Cold-Water Nitrification

A Bio-Dome mobile ammonia removal system proves itself in testing through a Midwestern winter
Cold-Water Nitrification
The Bio-Dome mobile unit sits on the north side of the Gresham Municipal Utilities western lagoon.

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The wastewater treatment lagoon at Gresham (Wis.) Municipal Utilities needed a $350,000 upgrade to comply with the state Department of Natural Resources’ new ammonia-nitrogen daily maximum limit of 1.8 mg/l. The utilities considered installing a carbon dioxide bubbling system to reach the limit.

Then Jason Metz of Marshall-Bond Pumps proposed a pilot project to manager Art Bahr. Wastewater Compliance Systems (WCS) wanted to document the performance of the Bio-Dome ammonia removal system to see how well the submerged bioreactor functioned in cold weather. If it worked, it could save the village thousands of dollars per year in maintenance costs.

Gresham was the first Midwest utility to test the technology. Operators, working closely with Metz and Kraig Johnson and Taylor Reynolds of WCS sampled the influent and effluent weekly from October 2010 through April 2011. The mobile unit nitrified ammonia even in water at 34.6 degrees F to levels below 1 mg/l.

 

Plug-and-play

The utilities, operated under the village of Gresham, have 280 sewer and water customers with wastewater flows of 60,000 gpd. The 12-foot-deep, two-cell, two-acre facultative lagoon was installed in 1982. In 2008, effluent ammonia averaged 14 mg/l, twice the permit level.

“The plug-and-play unit arrived in an insulated and heated 20-foot cargo container,” says Bahr. “All we did was run a new 120-volt power line on a 20-amp breaker.” Inside the container was a 1,500-gallon tank with Bio-Dome 2800, an air compressor with flowmeter, influent pump, and four-channel temperature data logger. A WCS representative helped in placement, installation, startup and training.

Village workers set the container on the north side of the western lagoon and anchored the pump to an old surface aerator pontoon, 40 feet from shore but close to the effluent side. “At this point, wastewater has already flowed over the coarse-bubble aerators, removing a significant amount of BOD and TSS,” says Bahr.

The crew wrapped the 1-inch influent and effluent lines in heat tape, then insulated and covered them with a protective shell to prevent freezing. The influent line was anchored to the pontoon. Flow from the air compressor optimized microbial growth on the plastic media. Installation took three days.

“The dome, which normally sits on the lagoon floor, is 6 feet in diameter and 5 feet high and weighs 850 pounds,” says Bahr. “Inside are four concentrically nested domes filled with media providing 2,800 square feet of surface area.” The unit secures to a concrete base.

 

Bioreactor basics

Wastewater enters the bottom of the nested domes and flows to the top. Air injected at the bottom is distributed by bubble tubes on both sides of each dome. Besides providing oxygen for the microorganisms, bubbles flowing out of the unit agitate the water, delaying sludge buildup in the lagoon.

To achieve greater process control, the pump time-dosed the bioreactor every three hours. Measuring the flow enabled WCS to adjust the pump-on time to create different hydraulic retention times (HRT). “Sampling began with a 10-day HRT and was reduced four times,” says Bahr. “We drew the final sample with a one-day HRT.”

The unit ran for two weeks before sampling. Operators checked it three times a week, drawing influent and effluent samples on Wednesdays. A certified lab analyzed them for BOD, TSS, ammonia, nitrates and nitrites, TKN, TN, pH and, once a month, alkalinity. Tank water temperature, pH, and dissolved oxygen also were measured.

 

Trial conditions

Operating conditions changed during the trial. WCS installed an inlet drop pipe to keep wastewater from short-circuiting between the tank inlet and outlet, and an overflow sensor. In February, operators installed a solenoid valve to cycle air for denitrification.

“With the air cycled off for two hours per day, we reduced the effective HRT from 96 to 88 hours,” says Bahr. “Later that month, we dropped it to 44 hours, then 22 hours in March.”

The overflow sensor tripped twice that month as ice slush clogged the discharge line, preventing the tank from draining fast enough to keep up with the influent. A one-day spike in influent BOD in February produced a more robust biofilm, enabling ammonia effluent levels to drop below 1 mg/l in March. Alkalinity levels for influent averaged 235 mg/l and effluent 124 mg/l.

“Our BOD and TSS levels suffered because the lowest test resolution used by the laboratory was 2 mg/l,” says Bahr. “Consequently, many samples were reported simply as less than 2 mg/l. TSS was consistently less than 4 mg/l, well below the expected limit for the village.”

The average reduction in TN was 2.23 mg/l with a high of 7.5 mg/l, indicating the anoxic microclimates within the bioreactor. “Adding the solenoid valve on the air line didn’t improve the denitrification rate, probably because of very low BOD levels and DO levels at 10 mg/l,” says Bahr. “Reducing DO levels through shorter HRTs or longer air-off periods and increasing the carbon source by adding more fresh influent should induce more denitrification.”

WSC recommended 17 bioreactors in each lagoon cell. The cost was similar to that of the carbon dioxide system, but required less operation and maintenance.

“When compared with other technologies for cold-weather nitrification, Bio-Domes are an affordable, simple green solution to a long-term problem,” says Bahr. “They will help us meet and exceed our regulatory requirements, while minimizing chemical use and maintenance.”



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