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•Case Study: Small Plant Corrects Big Problem

Even smaller wastewater treatment plants confront a big challenge in handling foreign material and the thicker sludge so often associated with the return activated sludge (RAS) process.

Take the case of the municipal plant serving Sherrill, New York. The state permit rates the plant at 950,000 gallons per day but hydraulically it can successfully process up to 3.5 MGD following installation of higher efficiency submersible pumps. The flow averages 500,000 to 600,000 gallons per day except during rain events when infiltration significantly increases the inflow. This led to replacing the existing pumps at the headworks with ITT Flygt submersible pumps, according to Chris Dunn, plant superintendent. He would turn to an even more advanced type of pump developed by ITT Flygt to address a different problem last year.

"We were initially concerned a few years ago with improving our hydraulic capacity during rain events because of the infiltration," he said. "The replacement pumps we acquired gave us that needed capacity increase."

As presently configured, the wastewater stream enters the plant where it initially undergoes grit removal and passes through a communitor that shreds any solids. The influent then reaches the two, 5-HP and two, 18-HP ITT Flygt raw sewage pumps equipped with variable speed drives. The waste stream advances next into one of two, 400,000-gallon tanks with mechanical aerators. The aerated liquor, laden with suspended solids, then advance by gravity into two, 150,000-gallon settling tanks covered by domes where it undergoes detention for a length of time governed by the inflow. The12-HP RAS pumps then draw the settled sludge -- sometimes with a solids content as high as 3% -- from the bottom of the clarifier to recycle for additional aeration and eventual discharge.

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•Case Study: Small Plant Corrects Big Problem