A water utility in Birmingham, Alabama conducted an evaluation of 26 dual media sand
filters at four different water treatment facilities to assess the need for system
rehabilitation. The filters were selected as representative of the water utility's 86 filters
based on influent and effluent piping configurations, underdrain type, media lifts, and
performance data. Each filter underwent a filter evaluation including extensive media
coring, turbidity profiling, bed expansion tests, hydraulic evaluations, gravel profiles, and
mud deposition evaluations.
The results of the bed expansion and hydraulic evaluations showed that most filters
experienced less than 20 percent expansion. Mud deposition results from media cores
before 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 significant
liberation of accumulated metals (Mn, Ca, and Fe) as well as floc. Poor filter backwash
performance did not appear impacted by piping configurations or filter characteristics.
Furthermore, the limited efficacy of backwashing on filter media condition did not
translate into poor filter performance. Filtered turbidity, for all plants, was historically
below 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 plants
showed that while the filters generally performed well under normal conditions, changes
to the primary coagulant or coagulant aids resulted in problematic filter operation. The
problems included rapid headloss or turbidity breakthrough despite little evidence of
carryover during coagulant trials. It was apparent that the accumulation of coagulant and
filter aid in the filter media helped produce low filtered turbidities. However, this build
up of chemical made the process more sensitive to changes in treatment chemical usage.
Strategies for improving filter media expansion and hydraulic control have since been
developed and are being implemented at each plant to provide a more robust filtration
process.
Filtered turbidity data does not provide an adequate measure of filter process condition.
Filters should be evaluated periodically regardless of historic filtered turbidities. Pending
regulations 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.
| Edition : | Vol. - No. |
| File Size : | 1
file |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 16 |
| Published : | 11/15/2004 |
