to empty the grit dumpster three times a week. With the new screen, we empty it just once a week.” Removing this abrasive grit early in the process reduces wear and tear on pumps and avoids other downstream headaches, such as grit collecting in the digesters, which can reduce their capacity, Shepherd notes. The higher level of effective debris removal—including both large objects and smaller debris, such as leaves—does require more frequent emptying of the collection box at the headworks. But Shepherd says this is a worthwhile trade-off, given the dramatic reduction in down - stream debris. Shepherd says the FlexRake IQ2™ has also lived up to its promise of adaptability to varying flow levels. “It’s been very reactive to the flows. There haven’t been any backups or slowdowns in the wet well,” he says. “We haven’t had any issues with flow restriction.” What’s the biggest surprise for Shepherd? “I never realized how much grit was passing through. I never expected grit to be removed by the bar screen,” he says, noting this has reduced the amount of grit passing downstream to clarifiers and digesters. “Pumping sludge has been so much easier.” Based on this early experience, Shepherd says Saginaw Charter Township WWTP is now considering acquiring a second FlexRake IQ2™ for the headworks serving Thomas Township to maximize the plant’s resilience. “The bar screen removes debris as efficiently at 20 MGD as it does at 4 MGD,” Shepherd says. “That’s something other equipment has not been able to do.”
The Upper Occoquan Water Reclamation Plant in Centreville, Virginia reclaims over 30 million gallons of water per day, providing a source of potable water for a population of more than 300,000 across por - tions of Fairfax and Prince William Counties. Its chemical treatment system relies on a steady supply of pebble lime that raises pH to levels at which phosphorous can be removed. The lime is conveyed pneu - WRP CONVEYING PEBBLE LIME PNEUMATICALLY ENDS ELBOW FAILURE
matically to six indoor silos during offloading, but it previously wore through elbow walls due to its three to four percent grit content and high conveying velocities. In one typical failure, Robert Forgione, P.E. and his team at the water reclamation plant needed to clean up pebble lime and dust that had escaped from a failed elbow in the conveying line. While the hole in the elbow wall measured only 1/2-inch in diameter, it had spewed the abrasive powder over the building until the high pressure conveying system could be shut down. The blowout created a potential slip-and- fall situation, threatened acceptable indoor air quality levels, and put electronic controls at risk. He and his team found themselves dealing with elbow failures every six months. "We had to fix this situation," said Forgione, the director of the operations and maintenance division for the Upper Occoquan Service Authority (UOSA), but conveying high volumes of the abrasive material proved too demanding for every
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June 2022 csengineermag.com
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