Prior to completion of the Fox Metro Water Reclamation District (originally named the Aurora Sanitary District) in 1929, waste from the surrounding community was discharged directly into the Fox River. Since that time, Fox Metro has met the needs of the expanding community through innovation, forethought and technology. Today, Fox Metro is a highly efficient facility capable of routinely treating 42 million gallons of wastewater per day and serving over 250,000 people.
One of the ways that Fox Metro is able to make the best use of its resources is with the implementation of Danfoss Drives. The plant was recently faced with upgrading some of its aging equipment and turned to engineering firm Walter E. Deuchler Associates.
The main pump building housed three constant pressure piston pumps that maintained flow of primary and thickened sludge to other parts of the plant. The sludge was being pumped over 1800 lateral feet through pipes that had been originally rated for 150 PSI. The piston pumps could not maintain constant pressure in the pipes, so the pressure would often peak around 100 PSI, causing breaks in the shear pins of the piston pumps.
Deuchler Associates recommended replacing one of the piston pumps with a rotary lobe pump connected to a 125 HP VLT® 8000 AQUA variable frequency drive. The drive, which is housed in a NEMA 4X, air-conditioned enclosure, runs the motor based on feedback from a Danfoss pressure transmitter and pressure switch near the outfeed of the pump. “We can now maintain an optimal flow rate at around 55 PSI,” says Ryan Cramer, the Project Engineer at Walter E. Deuchler Associates responsible for the project. The pressure transmitter communicates with the VFD to ensure that the pressure in the discharge piping does not exceed 90 PSI by allowing the VFD to speed up or ramp down accordingly. A Danfoss pressure switch also lets the drive know if the system reaches 120 PSI, indicating a blockage. In the event that pressure reaches this level, the drive will shut down the pump to prevent damage to the piping. The drive can then slow or stop the pumps to prevent damage to the pipes. “The drive takes all the guesswork out of it,” says Cramer. The implementation of the new equipment was so successful in reducing maintenance requirements that the remaining two piston pumps were replaced with identical setups.
In addition to reduced maintenance, VLT 8000 Series drives also provide greater flexibility to the system. “The variable frequency drives give us a measure of control that we didn’t have previously,” says Cramer. With new lobes in the pumps, they can be run at slower speeds, he explains. “The drives currently run the motors at 45 Hz, but as the lobes wear, we can increase it to 60 Hz to match the existing flow rate.” The drives also provide a 30-second initial ramp of the motors to pressurize the system more gently.
One of the secondary benefits of the new system is the reduction of risk of injuries to maintenance personnel. The old piston pumps required regular lubrication of its moving parts. The lubricant made the floor extremely slippery, increasing the likelihood of slips and falls. With a more stable and reliable system, maintenance personnel are safer and can focus their efforts elsewhere, and the Fox Metro treatment facility is better able to serve its community.