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A water utility in Birmingham, Alabama conducted an evaluation of 26 dual media sandfilters at four different water treatment facilities to assess the need for systemrehabilitation. The filters were selected as representative of the water utility's 86 filtersbased on influent and effluent piping configurations, underdrain type, media lifts, andperformance data. Each filter underwent a filter evaluation including extensive mediacoring, turbidity profiling, bed expansion tests, hydraulic evaluations, gravel profiles, andmud deposition evaluations.The results of the bed expansion and hydraulic evaluations showed that most filtersexperienced less than 20 percent expansion. Mud deposition results from media coresbefore and after backwash showed little removal of floc from the filter during backwash.Media samples taken from random filters were acid washed resulting in a significantliberation of accumulated metals (Mn, Ca, and Fe) as well as floc. Poor filter backwashperformance did not appear impacted by piping configurations or filter characteristics.Furthermore, the limited efficacy of backwashing on filter media condition did nottranslate into poor filter performance. Filtered turbidity, for all plants, was historicallybelow 0.1 ntu. Filtered turbidities were below 0.1 ntu throughout the evaluation period.Recent coagulant evaluations for improved organics removal at two of these plantsshowed that while the filters generally performed well under normal conditions, changesto the primary coagulant or coagulant aids resulted in problematic filter operation. Theproblems included rapid headloss or turbidity breakthrough despite little evidence ofcarryover during coagulant trials. It was apparent that the accumulation of coagulant andfilter aid in the filter media helped produce low filtered turbidities. However, this buildup of chemical made the process more sensitive to changes in treatment chemical usage.Strategies for improving filter media expansion and hydraulic control have since beendeveloped and are being implemented at each plant to provide a more robust filtrationprocess.Filtered turbidity data does not provide an adequate measure of filter process condition.Filters should be evaluated periodically regardless of historic filtered turbidities. Pendingregulations are requiring improved performance from existing treatment facilities,primarily organics removal in coagulation. Before testing new coagulation strategies,utilities should evaluate filter media condition to avoid the risk of treatment failure. Includes 3 references, tables, figures.

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Edition: Vol. - No. Published: 11/15/2004 Number of Pages: 16File Size: 1 file , 970 KB