Civil Engineering Reference
In-Depth Information
Declining Rate
With DRF, the rate of filtration varies in each filter during a filter
run, between each backwash. The filtration rate usually varies between 70 percent and
130 percent of the average filtration rate to the filter. When a freshly backwashed filter
in a group of filters is put back into service, it will operate at highest average filtration
rate of all the filters until the next filter is backwashed and put back into service. When
the backwashed filter is placed back in service, the filtration rate in the filter under
consideration will decline in a step-wise manner, to a new lower filtration rate. This
rise and fall motion of the water level over the filters will continue along with the
stepped decline in the filtration rate until the filter is the last filter to be backwashed,
at which time it will be filtering at its lowest rate. During this cycle, the water level
in all other filters will rise in unison.
The advantages of this type of filter operation are:
Simpler operation
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Less filtering head is required for filter runs of equal length
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No weir head and free-fall allowance are needed
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No rate-of-flow controller / meter is needed
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Hydraulically responds well to plant flow variations
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The disadvantages are:
Continual manual surveillance or controls must be added.
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Difficult to effectively use a filter aid.
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Filter box must be deeper.
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Less likely to be effective at higher filtration rates.
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Influent header channel and valves must be proportionally larger, and therefore
more expensive.
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The initial high rate of flow into the filter box can cause media migration.
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Operator has little control flexibility.
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Conclusions
Each of the four options for filter control logic is potentially feasible
for overall plant treatment scheme. However, the constant rate rate-of-flow controlled
scheme is considered the most suitable type of control because it:
Provides ability to control flow through each filter.
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Does not cause floc breakup on filter influent water.
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Allows gradual changes of flow through filters.
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Easily accommodates conditioning of influent water with filter aid.
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Filter Backwashing
During the service cycle of filter operation, particulate matter removed from the applied
water accumulates on the surface of the grains of fine media and in the pore spaces
between grains. With continued operation of a filter, materials removed from the water
and stored within the bed reduce the porosity of the bed. This has two effects on filter
operation: it increases the headloss through the filter, and it increases the shearing
