What would you do if your state regulatory agency hit you with a stringent nutrient removal standard three years before your plant was due to be upgraded to biological nutrient removal (BNR)?
If you’re the operating staff in Bozeman, Mont., you put on your thinking cap and make process changes to your activated sludge system and cut total nitrogen discharges by 45 percent.
“We appealed the new standards, basing our case on the lack of a compliance schedule, and won an extension,” explains Tom Adams, plant superintendent. “But we asked ourselves, ‘What could we do quickly to reduce our nitrogen discharge?’”
The answer was a $150,000 adjustment to the plant’s complete-mix activated sludge system that has led to a decrease in total nitrogen discharged from 800 pounds per day to just over 400 pounds. “We call it our PNDN (phased nitrification-denitrification) system, and it has given us a lot of bang for the buck,” says Adams.
Hard-working system
The Bozeman Wastewater Reclamation Facility treats more than 5 mgd and serves a population of about 35,000. The treatment train is over 30 years old and is at design capacity, but it performs like a system half its age. BOD and TSS removal rates average 98 to 99 percent, despite very cold winters.
Preliminary treatment includes an Auger-Monster screener-grinder (JWC Environmental) for trash and rags. Grit is collected, washed, and disposed of in the county landfill.
Wastewater then undergoes primary treatment followed by activated sludge and disinfection. Chlorination occurs in an 80-minute retention system; sulfur dioxide is added to the chlorine residual before discharge.
Biosolids are removed, gravity thickened, anaerobically digested, and then injected as a liquid into area farm fields and ranches. The biosolids program has won both state and U.S. EPA awards. Effluent passes to the East Gallatin River, which supports a healthy fish population as it flows into the headwaters of the Missouri River.
A SCADA system built around Allen-Bradley (Rockwell Automation) control logic and a sophisticated Wonderware operator interface controls the entire operation. This system displays real-time process control data including dissolved oxygen, pH, oxidation reduction potential (ORP), and the status of more than 450 other plant components.
Making adjustments
Originally, the activated sludge system consisted of four square basins, operated in parallel as a complete-mix system. Sluice gates connected three of the four basins, though the gates were not used.
Working with the plant’s consulting engineers, HDR Inc. of Omaha, Neb., Adams and his operations team came up with the idea of opening the sluice gates, and running the units as a plug flow system — all four basins operating in series.
“We experimented and tried running the first two basins in an aerobic-anoxic mode, turning the air on for two to three hours, then turning it off for one-and-a-half to two hours,” explains Adams. The plant installed Wilo submersible mixers (Wilo EMU USA LLC) to keep the contents circulating when the air was turned off.
Modifications along the back walls of the basins prevent overflows, and Adams’ staff reprogrammed the plant’s SCADA system ladder logic to accommodate the new aeration cycles. Meanwhile, continuous aeration takes place in the final two basins. “It’s a little like running a sequencing batch reactor, cycling air on and air off, except that the flow is continuous,” says Adams.
“It was a simple process change that altered the metabolic pathways of the micro-organisms, and we weren’t really sure it would work,” Adams explains. “But we felt it was worth a try, so we went ahead and made the change. The results have been really exciting.”
Today, with an average daily ammonia nitrogen limit of 1.52 mg/l, the plant actually discharges an average of 0.2 mg/l or less. This is in addition to lowering total nitrogen levels by 45 percent.
Measures of excellence
A variety of other operational measures contribute to the outstanding performance at the Bozeman plant. The facility recently received the George W. Burke Facility Safety Award from the Water Environment Federation, recognizing a consistently outstanding safety program and safety record.
The plant uses an ORP system supplied by Hach Co. to monitor performance in the biological treatment system. Voltages generated via metal probes placed into the water indicate the ability of the oxidizers in the wastewater to oxidize or reduce contaminants.
Adams’ staff relies on a computerized maintenance management system called JOB Cal from OPS Systems Inc. (now Hach Co.). A calendar displays all the scheduled work, completed work, overdue work, and skipped work for a 52-week period. Using the calendar, staff can close jobs or reschedule, create or delete them. An OPS SQL system from the same company enables the laboratory staff to collect and manage analytical data.
Bozeman staffers pride themselves on avoiding the use of chemicals in the treatment process, although chlorine is sometimes used to knock down filamentous growth in the return activated sludge.
“This plant has a habit of producing filamentous organisms,” says Adams. “We keep a close eye on the clarifiers. One of the most practiced eyes belongs to operations foreman Herb Bartle. He’s meticulous in monitoring and controlling our biological process.”
Upgrade on the way
Still, the plant is at hydraulic capacity and faces recently mandated stringent limits for phosphorus as well as nitrogen. That has led to the largest capital project in the city’s history: a $52 million expansion of the treatment facility to 13 mgd.
Contractors broke ground in January for new headworks facilities and a five-stage Bardenpho process, which combines anoxic, anaerobic and aerobic treatment in five stages for effective biological nutrient removal.
The plans also include UV disinfection for advanced treatment, and energy efficiency measures throughout. New ABS blowers will operate on 30 to 50 percent less electricity than the existing ones. The primary effluent pumping station will be below grade level to save on heating costs, and will be lit by skylights.
“The new plant will be welcome and necessary to meet future needs,” says Adams. “But this old plant has performed exceptionally well. We’re proud of it.”
