WEMCO Prerostal™ System is chosen for New Festus Wastewater Plant

When the design team for the City of Festus started working on the new wastewater treatment plant, they had to overcome a number of unique constraints. The plant site was surrounded by a wetlands and railroad tracks making the site area small.

A key component of the plant was a new influent pumping station. The new station would need to provide a variable output of 2.0 mgd up to a maximum flow rate of 12.5 mgd. The "Prerostal" automatic flow matching system from Wemco Pump was chosen for its ability to achieve the variable flow without the use of complex variable speed drives or sophisticated controls. The Prerostal System consists of the Wemco-Hidrostal screw-centrifugal pump installed in a patented fiberglass prerotation basin. The prerotation basin is installed in the field by the contractor and causes the fluid to enter the impeller eye at a variable angle of attack, or prerotate, depending on the liquid level in the wetwell. As the degree of prerotation increases, the pumped flow rate decreases until it reaches a state of equilibrium where the pumped flow rate is equal to the influent rate. This process occurs while the pump and motor operates at a constant rotational speed.

For the Festus project, four 10" model F10K pumps with 64.3 hp "immersible" motors were selected by the design consultant. Bently Green, P.E., Project Manager for Horner & Shifrin said, "For this application, we felt VFD's were not going to give us the reliable operation from a hydraulic standpoint, and there was not enough room at the site to construct a 6-pump or larger (constant speed) station. The Wemco Prerotation pumpsallowed us to meet a wide peak-to-average hydraulic condition with only four pumps." The Festus installation will be the largest Prerostal system in the area. Start-up is anticipated for Spring 2004. For more information call (636) 532-1600 or visit www.wemcopump.com.

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