UFC 3-280-04
17 DEC 2003
stances, flocculating periods as short as 3 minutes are used. Flocculation is conducted
in a static in-line mixer, eliminating the need for rapid mix tanks. The designer must
consider adequate flocculation times. (See EPA 815-R-99-010.)
6-2.6
Systems filtering physicalchemical flocs tend to use lower filter rates and
finer media than those filtering biological floc. This may be because biological flocs tend
to be stronger and more resistant to shear than chemical flocs. But, because of the
strength and character of the biological floc, greater surface filtration occurs, resulting in
excessive head loss because the floc does not penetrate the bed. Polymer filter aids
may be added to the filter influent to strengthen weak chemical flocs, to permit higher
flow rates, but backwash rates, surface wash, or air scour, or all three, may be needed
owing to higher attachment forces to the media.
6-3
PROCESS CONTROL OPTIONS. The major filter functions that require
monitoring and control are head loss, influent and effluent quality (turbidity), flow rate
through the filters, and backwash sequence, rate, and duration. Generally, filtration
systems should be equipped with an influent and effluent turbidimeter, head loss and
flow indicators, and backwash timers. These parameters may be controlled through
constant pressure filtration, constant-rate filtration (effluent rate control), and declining-
rate filtration. Declining-rate filtration is often the preferred method for control of granular
media filters for potable water, but constant rate filtration may also be preferred in
HTRW applications as there are not large flow variations. Generally, package filtration
systems will already have control systems. The designer will only need specify special
needs that are different from the standard control system. Process controls can be
specified based on capital cost considerations, as well as on the availability of opera-
tions and maintenance personnel to manage less automated systems.
6-3.1
In constant-pressure filtration, the total available pressure drop is applied
across the filter throughout the filter run. The control mechanism is compressed gas
maintained at a constant pressure. This maintenance of the total available pressure
drop results in this constant pressure providing the driving force. Because the driving
force stays constant, the flow will decrease as the filter bed becomes clogged with sol-
ids. Constant-pressure filtration is not often used.
6-3.2
Traditional constant-rate filtration is achieved by adjusting the effluent flow
rate through the filter so that it is kept constant by means of the effluent flow rate valve.
Control may be achieved directly or indirectly. Direct control is difficult because varying
influent will greatly affect control needs. Indirect control is usually achieved by a set
point controller linked to a pneumatic or hydraulic valve operator. Significant head may
be lost in the controller. The plant flow is equally divided among the plant filters by
means of a venturi and modulating butterfly valve. The venturi element communicates
with the controller, which adjusts the butterfly valve to ensure that each filter is filtering
an equal volume of the influent water. A level element is installed to signal when exces-
sive head has built up and backwash must begin.
6-3.3
Influent flow splitting on gravity filters can achieve constant rate filtration by
dividing the flow among filters via a flow splitting tank or channel. The water level over
6-3
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