Ahead of the Curve

Software improves the speed and quality of power measurement, helping a Wisconsin sanitary district to conserve energy and reduce costs
Ahead of the Curve

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When the Western Racine County (Wis.) Sewerage District upgraded the capacity of its activated sludge extended aeration treatment plant from 1.3 to 2.5 mgd in 2005, the expansion doubled the budget and the electric bill.

The upgrade included a new sewage receiving station, a mechanical bar screen to replace a comminutor, a second oxidation ditch and clarifier, and a UV disinfection system to replace the chlorine system.

Superintendent Jeff Bratz at first was too busy getting the plant running to think about energy. “As I found free time, I realized that we had to find creative ways to save money if we were to pay for the facility on a diminishing budget,” he says.

The upgrade increased organic capacity from 2,300 to 3,578 pounds per day of biodegradable material, but the anticipated growth never happened. Bratz wondered why he was using twice as much power to process only a slight increase in organic loading.

To track energy consumption, Bratz applied for a grant from the state’s Focus on Energy program to help purchase EnergyVIEW software from IntelliSys Information Systems. Linked to the plant’s SCADA system, the application meters, measures, stores and analyzes energy data. In the first three months, the $5,000 program gave Bratz the tools to save an estimated $20,000 to $25,000 per year on energy.

 

Real-time analysis

The only hardware Bratz had to install was a metering device on the main power line to the plant to record the draw for kilowatts and report in kilowatt-hours. A real-time display enabled him to trend transducer outputs in high resolution.

The trend pan-and-zoom features gave him second-by-second analysis of real-time data. The software also allowed him to generate yearly, monthly, weekly, and daily charts, configure 15-minute peak demand or hourly data by the minute, and set warnings and alarms if preset values were about to be exceeded.

“Real-time monitoring and reporting enabled me to see and record how much energy we were using at any time of the day and approximately how much it cost,” says Bratz. “I kind of knew which processes were using the most electricity, but it’s easier to convince the sewer commission to take action when they can see it on paper.”

The bulk of consumption occurred in the two oxidation ditches and three sludge tank mixers. The ditches each have two 40 hp and two 20 hp motors. “I had 240 horsepower running constantly and no way to slow down the motors when there was excess dissolved oxygen at night,” says Bratz.

Contractors installed variable-speed drives on the eight motors and attached them to oxygen probes in the tanks to maintain a constant DO level. “We’re anticipating an energy savings of 20 percent on the ditches,” says Bratz. “That’s huge, provided the utility doesn’t raise the rates to compensate for the income it is losing.”

The work, completed by the end of 2010, qualified the plant for a $27,000 grant from WE Energies, the local electric utility, to offset the cost of the equipment.

 

Power booster

The software also showed that the sludge mixers each drew 60 amps. The facility normally operated at a peak demand of 200 kW or less, but mixing sludge increased demand by 180 kW.

“The utility charges $11.35 per kilowatt for peak demand and $1.76 per kilowatt for non-peak demand,” says Bratz. “That’s a difference of $9.59. It’s almost 10 times cheaper to mix sludge at night. While this appears to be a no-brainer, I had never analyzed it. If I had to haul sludge tomorrow, I simply turned on the mixers.

“I think many operators do those things unconsciously. I did until the program put the evidence right in my face. It was a ‘Holy cow! Why didn’t I think of that before?’ moment.” The program displays demand in graph and number form.

This year, Bratz is analyzing the power usage of two screw pumps that bring influent into the plant. “I operate one at a time, but the 75 hp motor runs constantly,” he says. “We’re investigating the possibility of shutting down the big motor and running a smaller pump at night when flows are lower.”

Bratz is also investigating other changes. “The return pumps run continuously in the lift stations,” he says. “Maybe we can turn them on and off. Maybe I can run the dissolved air floatation unit for sludge thickening half a day instead of a whole day. EnergyVIEW makes it possible to look at so many things.”



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