Civil Engineering Reference
In-Depth Information
FILTER PROBLEMS AND POTENTIAL SOLUTIONS
Problems in filter operation and performance can be caused by poor design or poor
operation. However, advances in the engineering design of filters and filter controls
and appurtenances have made water filtration an inherently stable, extremely efficient,
and highly reliable unit treatment process. With proper design and good operation, all
the problems are easily solved.
Some potential filter problems are:
Surface clogging and cracking
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Short runs due to rapid increases in headloss
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Short runs due to floc breakthrough and high effluent turbidity
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Variations in effluent quality with changes in applied water flow rate or quality
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Gravel displacement or mounding
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Mudball formation
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Growth of filter grains, bed shrinkage, and media pulling away from sidewalls
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Sand leakage
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Loss of media
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Negative head and air binding
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Air leakage into the system
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Surface Clogging and Cracking
These conditions are usually caused by rapid ac-
cumulations of solids on the top surface of the fine media. This is not normally a
problem in dual- or mixed-media filters because of the greater porosity of their top
surface, compared to sand. Also, when a filter aid is used with dual- or mixed-media
filters, the dosage can be reduced as necessary (or eliminated) to allow particulates to
penetrate deeper into the bed. In other words, regulation of the polymer dosage to the
filter influent gives some control over the effective porosity of the filter, to accom-
modate changes in incoming floc characteristics.
Rapid Increases in Headloss
This is related to the problem just discussed. Dual-
and mixed-media beds collect particulates throughout the depth of the bed, rather than
mostly at the surface of the bed as with a sand or other surface-type filters, and are
much less susceptible to this problem than are the surface-type filters. Also, the flex-
ibility provided by use of a polymer as a filter aid allows control of the rate of headloss
buildup through dosage changes.
Floc Breakthrough
Floc breakthrough can be avoided by increasing filter aid dos-
ages or converting rapid-sand filters to dual-media filters or mixed-media filter units.
As mentioned earlier, this is one important point of superiority of mixed-media and
dual-media over sand filters. It arises because of the much greater surface area of the
grains in a mixed-media or dual-media filter compared to sand. The finest medium is
40 to 80 mesh in a mixed-media bed (10 percent of the total bed), 40 to 50 mesh (9
percent of total) in a sand bed, and 40 to 50 mesh (5 percent of total) in a dual-media
filter. The finest medium (garnet) in a mixed-media bed has the additional advantage
not only of being finer but also of being located at the very bottom of the filter where
the applied load is lightest, and where it can serve its intended purpose as a polishing
