UFC 3-280-03
23 JULY 2003
Compact liquid polymer blending systems typically use polymer delivered in
210-L (55-gallon) drums or even smaller containers. These systems are typically simple
to use, plug-in systems that consist of the polymer storage device (i.e., drum), a
metering pump, a small detention and aging chamber, and a dilution water meter such
as a rotameter. These units maximize solution preparation by slow-mixing and aging in
the detention chamber.
2-4.6.1.4 Chemical Feed Control Systems. The control components and types of
automatic controls required for chemical feed systems are specific to the individual filter
press. Typically, types of controls may include high-low level sensors in the storage and
feed preparation tanks, automatic or manual dosage input controllers, metering pump
controllers, manual on/off switch controllers, and loss of flow sensors. These controls
may be equipped with either light alarms or audible alarms. Chemical feed systems may
be controlled from separate control panels or integrated into a single control panel that
provides overall control for the entire sludge dewatering system. However, either of
these control systems should be interlocked with the sludge feed system to stop adding
chemicals if the sludge feed is discontinued or vice versa. Additional details on specific
types and elements of control systems are provided in Paragraph 2-4.7.
2-4.6.2 Filter Precoat Systems. Precoat systems are an option that can be used to
aid in releasing the sludge cake from the filter media and to alleviate premature blinding
of the media from residual particles. The precoating process involves pumping a slurry
of ash or similar substance to the filter press to provide a thin coating (e.g., 1.6 to 2.5
mm [1/16 to 3/32 inch]) on the filter media before the press is filled with sludge. The
precoat system is most useful for sludges with high biological content or industrial waste
sludges because they have a tendency to stick to the filtration media. It is also useful for
high pressure filter presses (i.e., 1550 kPa [225 psi] ) because of problems with cake
release and filtration media blinding, regardless of the sludge feed characteristics,
because of the "extrusion" effect of solids into the filter media caused by the high
pressure (WEF 1983). A precoat may reduce the overall cycle times and labor required
because removing the residual materials adhering to the filter media will be easier. In
addition, the precoat material can protect the media from mechanical damage caused
by sharp particles contained within the slurry. The precoat system is primarily used in
the fixed-volume recessed filter plate press because the cake release for the variable-
volume recess filter plate press is assisted by a mechanical device that pulls the filter
media down between the plates. Both wet and dry feed precoat systems are typically
used. Figure 2-8 gives schematics of these two systems.
2-4.6.2.1 The dry system shown in Figure 2-8 has typically been used in the past for
larger presses that operate continuously, but it is rarely used today. In the dry system,
water from the filtrate storage tank is circulated by the precoat pump through the filter
press and then returned to the storage tank. While the water continues to circulate and
after the press has been entirely filled with water and all air evacuated, a predetermined
amount of dry precoat material is transferred from storage to the precoat tank. The
filtrate water circulating directly to the press is then diverted through the precoat tank,
which consists of a series of baffles, and forms a slurry with the precoat material. This
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