On Autopilot

A test of a real-time nitrogen control documents substantial energy savings and improves process consistency at a Michigan clean-water plant.
On Autopilot
Michael Lunn, Grand Rapids plant manager, checks the monitor screen on the facility’s nitrogen control system supplied by Hach.

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Listening to operators discuss their experiences with nitrogen control at the 2012 WEFTEC Conference convinced manager Michael Lunn of the Grand Rapids (Mich.) Wastewater Treatment Plant that nitrification was coming. It also promised energy savings.

Lunn and Laron Morgan, technical controls supervisor, researched ammonia analyzers for the South Plant treatment train. When the team contacted Hach, the window of opportunity flew open. “They had a real-time nitrogen controller in European facilities and were looking for a pilot plant in the U.S.,” says Lunn.

The city agreed to the test and purchased two AMTAX sc ammonia analyzers, a SOLITAX sc suspended solids probe, the sc1000 digital controller and the RTC-N control module.

The energy-saving project qualified for a rebate program sponsored by Consumers Energy, the local electric utility. Based on energy savings calculated by the utility, the city received a one-time $58,728 rebate check last December. “It was 10 percent more than what I had estimated the savings to be at 8 cents per kWh,” says Lunn. “Annually, the technology should save the city more than 735,000 kWh and $62,000.”

Nutrient control savings

The 61.1 mgd (design) plant averages 42.2 mgd. Its two parallel activated sludge processes are called the North Plant and South Plant. After UV disinfection, effluent discharges to the Grand River. The 29 mgd (design) South Plant averages 20 mgd. It had three parallel 360- by 64- by 15-foot-deep aeration basins, each with five 100 hp turbine draft tube mixers. Spargers on the bottom dispersed fine bubbles from two 500 hp multistage blowers. The system equaled 2,500 hp total connected power. The base connected load was 2,000 hp with one blower in service.

In 2005, the South Plant switched to nutrient control. Modifications included an anoxic zone ahead of the aeration basins and replacement of the original mixers with 9-inch Sanitaire – a Xylem Brand tapered ceramic fine diffusers. The contractor also installed three single-stage 1,000 hp Turblex centrifugal blowers (Siemens Energy) with dual vane control that transfer 91,800 pounds of oxygen per day. Each operates at 9 psi and delivers 20,000 cfm of airflow, drawing eight to 12 run amps. The constructed value of blowers and controls was $1,845,000.

“The annual energy equivalent for the original system was $546,000,” says Lunn. “The new system was $204,800 per year with a projected annual savings of $341,200. It was the best technology available.”

Simple installation

Last March, Hach technicians installed the plug-and-play nitrogen control components and set the proportional-integral-derivative (PID) control loop parameters. Plant operators used the No. 1 aeration basin as the control and the No. 2 basin as the test tank.

“We inserted the solids probe and put an ammonia analyzer at the head and foot of the basin,” says Lunn. “The first test was moving the ammonia setpoint from 2 mg/L to 4 to 6 mg/L to see if it worked. Our permit limit is 8 mg/L.” Over two weeks, the system reacted quickly and accurately.

They then flipped the tank into nitrogen control with limiting setpoints of 1.5 to 5 ppm oxygen, running it for four weeks. At the end of July, operators put all three tanks in ammonia control and tracked it. The system outputs a dissolved oxygen setpoint based on the ammonia load entering and leaving the aeration basin, triggering the blowers to run at the optimal level for nitrification while reducing wasted energy.

Greater flexibility

“The old blower control system wasn’t flexible,” says Lunn. “It accepted one number until someone changed it, so we’d wind up with 0.1 mg/L ammonia because of over-aeration.” Adjusting aeration based on real-time data made the process more consistent, and a command on the plant’s SCADA system enabled operators to switch tanks from DO setpoint to ammonia control and back again.

As testing progressed, the operators changed only two parameters. First, they adjusted the maximum recirculation setting for the plant’s return activated sludge. “The parameter was the default — 300 gpm for a small facility — and we needed 20 mgd,” says Lunn. “Then we smoothed out the proportional gain on the PID to match our plant. Every facility has to do it.”

The success of the pilot study convinced the city to install a Hach system in the North Plant’s four aeration basins. That project also will be eligible for the Consumers Energy rebate program.  



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