Civil Engineering Reference
In-Depth Information
filters or changing flow in the plant will cause the water level in the filters to rise or
fall accordingly.
The advantages of this type of filter operation are:
Filter effluent valve is a simple on-off type.
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Controls are simple.
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Filter flow or level element is not necessary.
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Filter aid is easy to apply.
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The disadvantages are:
Headloss over weir.
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Channel and weir costs can be high.
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Filter box must be deeper.
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Floc may break up at weir.
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Constant Rate: Rate of Flow Controlled
With this type of control, the water level
in all filters and, generally, the filter header channel are maintained at a constant level.
Flow in all filters is controlled by means of a Venturi or other type of meter and
control valve in the filter effluent piping. A level element monitors the water level in
the influent channel. A controller trims the control valves to maintain each filter flow
to a set point.
During a filter run, the rate-of-flow controller maintains the flow through the filters
at a set rate. As the filter clogs, the headloss through the filter increases, which reduces
the flow. The controller senses the decrease in flow and opens the control valve to
adjust for the increased head in the filter to maintain the set flow rate. The master
level control in the influent channel senses variations in the water level in the channel,
due to increased plant flow or the backwashing of a filter, and increases the set point
flow through the rate-of-flow controllers.
The normal operating level range of this type of filter is about 6 inches. This range
is wide enough so as not to shock the filter with surges.
The advantages of this type of filter operation are:
Operator has full manual and / or automatic control of the filter system.
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Operator can vary the flow through any filter.
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Filter box can be designed shallower.
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Controls can be fully automated.
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No weir head and free-fall allowance needed.
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Can ramp flow at start of filter run.
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Responds gradually to plant flow variation.
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Filter aid is easy to apply.
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The disadvantages are:
Added cost of a rate-of-flow controller / meter
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Additional metering and control equipment needed
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