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Case Study: New Generation Pump Stops Stoppage
January 5, 2006
•Case Study: New Generation Pump Stops Stoppage
Little Rock (AR) Wastewater Utility resolved the recurring clogging problem at Adams Field Wastewater Treatment Plant (WWTP) that often cripples return activated sludge pumps at other plants that added secondary treatment after passage of the Federal Water Pollution Control Act (U.S. Clean Water Act) of 1972.
The facility originated in 1961 designed only for primary treatment and now handles 70 percent of the sewage generated within the utility's service area. The plant was rated at 36 MGD after the addition of secondary treatment in 1972. The plant utilizes the complete mix activated sludge process that distributes influent sewage, return activated sludge and air throughout the aeration tank to maintain a nearly homogenous mix. A uniform organic load from one end of the tank to the other removes approximately 90 percent of the pollutants.
Like many plants modified for activated sludge treatment, the existing infrastructure at the Little Rock facility -- specifically the coarse screens -- enabled rags and other debris to reach and foul the two 35-HP and two 15-HP return activated sludge submersible pumps. Accordingly, the utility's management eagerly agreed to participate in one of ten field tests being conducted by ITT Flygt for an experimental submersible pump equipped with an already successfully proven impeller uniquely engineered for clog-free operation.
"Our old pumps were installed in the 1980s so it seemed like a win/win situation if we could overcome the fouling and clogging problem," Lynn Hyke, Treatment Plant Superintendent, said. "Our practice had been to routinely back-flush the pumps every four to eight hours to remove debris that had passed through the headworks screens during high velocity flows. We had modified the old pumps with variable frequency drives that under normal conditions enabled us to return pump 70 percent of the return sludge from the clarifiers to the aeration tanks. During a rain event, however, the relationship of returned sludge to inflow would decrease. If we experienced a blockage, that would easily reduce the return flow capacity by 25 to 50 percent."
The problem became even more pronounced over long time periods.
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