Strategic Filter Backwashing Techniques and Resulting Particle Passage
Publication: Journal of Environmental Engineering
Volume 131, Issue 4
Abstract
A new filter backwashing strategy, extended terminal subfluidization wash (ETSW), aimed at reducing particle passage into the filtered water during the initial filter ripening sequence immediately following backwashing has been evaluated on pilot-scale filters at a conventional water treatment plant using alum as the sole coagulant. Turbidities and particle counts were measured following backwashes extending across a range of ETSW flow rates. The ETSW procedure was effective in significantly reducing the passage of particles through filters that originated from the backwash remnant water flowing out of the filter during the first of the filter run. A secondary turbidity peak associated with the filter influent water leaving the filter was observed following all fluidization backwashes with or without the addition of air-scour or ETSW. The secondary peak was termed the “additional collector” peak and was attributed to cold-water impaired (CWI) alum coagulation at the plant. The secondary peak was not observed with the particle counter as it was with the turbidimeter, which indicates the particles passing were predominantly particles in diameter. A backwash procedure consisting of only subfluidization water flow was evaluated because it was expected to leave significantly higher number of particles attached to the media grains to serve as “additional collectors.” The subfluidization wash procedure virtually eliminated all filter ripening peaks in spite of the CWI coagulation conditions.
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Acknowledgments
Special thanks go to Kelly Comstock, Rob Isabel, and Andrew Tartaglia in the Atlanta office of Camp Dresser & McKee Inc. (CDM) for their very valuable assistance in coordinating this study, making available the use of their pilot-scale filtration units, and providing support in the operation of the equipment in the field. Patrick Pherson of the Cobb County-Marietta Water Authority (CCMWA) and Mike Leonard of the Atlanta-Fulton County Water Resources Commission (AFCWRC) also assisted in coordinating and planning this research. The gracious help of the operations and support staffs at the Wyckoff and Atlanta-Fulton County Water Treatment Plants during this study is also greatly appreciated.
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Information & Authors
Information
Published In
Journal of Environmental Engineering
Volume 131 • Issue 4 • April 2005
Pages: 535 - 547
Copyright
© 2005 ASCE.
History
Received: May 22, 2003
Accepted: Apr 2, 2004
Published online: Apr 1, 2005
Published in print: Apr 2005
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