The Money-Saving Capabilities of Ammonia-Based Aeration Control

Ammonia-based aeration control promises additional energy savings in the activated sludge process at Chicago’s Stickney Water Reclamation Plant.

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About half of a typical clean-water plant’s energy goes to aeration. It follows that pumping more air than necessary into the activated sludge process wastes substantial electricity.

The team at the Metropolitan Water Reclamation District of Greater Chicago’s Stickney Water Reclamation Plant has embarked on a project to optimize aeration and so cut the blowers’ slice of the energy consumption pie by about 25 percent.

They’re doing it by installing ammonia-based aeration control, a more efficient method than the traditional control based on dissolved oxygen (DO). “Ultimately, we want to be positioned so that when the system says there is too much air going out, we can keep dialing down the output from the blowers,” says Joe Cummings, assistant operations manager at the Stickney plant. “That’s where the real money is saved.”

The district’s engineering consultant, Donohue & Associates, estimates simple payback on the project at 1.9 years, including the cost of control technology needed to achieve the desired blower turndown.

Major facility

The Stickney plant, in the middle of Cook County about 15 miles southwest of downtown Chicago, is the largest of seven MWRD treatment plants. Its nearly 400 employees work in 193 buildings on a 413-acre site.

The plant handles an average flow of 700 mgd and a peak flow of 1,440 mgd, discharging to the Chicago Sanitary and Ship Canal. It serves central Chicago and 46 other communities within a 260-square-mile area that is home to 2.3 million people.

The plant has four treatment batteries, each with its own aeration tanks and secondary clarifiers. Each battery has eight aeration tanks, and each tank includes four passes. Each tank has one air main to passes 1 and 2 and a second air main to passes 3 and 4.

“Roughly 50 percent of our electric bill comes from the blowers feeding the activated sludge process,” Cummings says.

The way things were

Historically, the Stickney plant’s aeration process has run on a combination of airflow control and DO control, by way of a distributed control system (DCS). “Passes 1 and 2 are set for airflow control,” says Cummings. “We’re telling the blowers to put out as much air as possible. One reason for that is we’re doing biological phosphorus removal and we did not install new tankage to meet the phosphorus reduction needs — we used the existing tanks.

“Because of that we had to cut the air in the channels that feed the aeration tanks and for roughly the first half of the first pass of each tank. That produces an anoxic zone so that the polyphosphate accumulating organisms (PAOs) go through that zone. Once they get into the aerobic zone, we want to provide as much oxygen as possible, so we run passes 1 and 2 on flow control.”

Passes 3 and 4 operate on DO control by way of an automated feedback loop. “The plant NPDES permit requires a minimum 4.0 mg/L of DO in final effluent,” says Cummings. “We don’t have post-aeration facilities, although there is some post-aeration net gain through turbulence in our channels. For the tanks on DO control, we have a setpoint of 4.0 mg/L.”

Eliminating excess

The aeration control project calls for installation of DO and ammonia probes at additional locations in the tanks so that those two parameters are being monitored throughout the process. “Basically, by the time the water exits the tanks, we want to deliver the amount of air that’s needed both to meet our minimum effluent DO and to get our ammonia down below our permit limits by the end of the tank,” Cummings says. Monthly average ammonia permit limits are 2.5 mg/L from April through October and 4.0 mg/L from November through March.

Fenghua Yang, senior environmental research scientist, observes, “We want to get our ammonia down to below our permit limit near the end of the tank instead of one-half to two-thirds of the way through the tank, as commonly happens. Because when that happens, then through the rest of the tank you continue to add oxygen that isn’t necessary for nitrification.”

The new control strategy will also benefit the biological phosphorus removal process. “For bio-P, we have an anoxic/anaerobic zone,” says Yang. “And if we have low DO at the end of the aeration tank, then there is low DO in the return activated sludge, which recycles to the head of the aeration tanks.”

The control project, costing an estimated $300,000, will replace flow control with DO control for passes 1 and 2 of each tank and provide ammonia-based aeration control for the entire aeration system. Each pair of aeration tanks will share one ammonia probe at the two-thirds mark along the tank length, a DO probe at the end of pass 2 to control the airflow to passes 1 and 2, and another DO probe at the end of pass 3 to control the airflow to passes 3 and 4.

The probes (all from YSI, a xylem brand) were to be installed starting in early 2015 in all four batteries. “In the meantime, we’ll be programming our DCS so we’re ready once everything is installed,” says Jeff Majka, senior electrical engineer.

Yang adds, “We will need to run through a variety of control scenarios to determine the best DO and ammonia setpoints to meet our effluent permit, yet still achieve the best air savings. We would like to do a pilot study to look at the actual air savings we can obtain and the potential problems we may encounter.”

The ammonia-based aeration control strategy is just one of many innovative projects underway at the Stickney plant. Says Cummings, “You could write a whole magazine about the things we’re doing here.”



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