UFC 3-280-03
23 JULY 2003
slurry is then circulated through the filter press to coat the filter media. The filtrate water
being passed through the precoat tank is circulated at a high rate to ensure uniform and
even precoating. The precoat process typically lasts from 3 to 5 minutes.
2-4.6.2.2 The wet material precoat system shown in Figure 2-8 is more common. The
precoat is prepared before it is introduced into the press by a batch or continuous
mixing operation. The way the press is filled with the wet precoat system is similar to
that for the dry material system. However, after the press is filled with filtration water,
the premixed precoat is pumped to the filter press and distributed uniformly throughout
the filter media.
2-4.6.2.3 Precoat materials commonly used include fly ash, incinerator ash,
diatomaceous earth, cement kiln dust, buffing dust, coal, or coke fines. Typical precoat
materials requirements range from 0.2 to 0.5 kg/m2 (5 to 4 10 lb/100 sq ft), with a typical
design criteria of 0.4 kg/m2 (7.5 lb/100 sq ft) (WPCF 1983). The pumping system should
be designed to complete the filling process in 3 to 5 minutes at a minimum rate of 0.2 to
0.3 L/m2s (0.3 to 0.5 gpm/sq ft). The filtrate storage area is also sized to have a
working volume of one and one-half to twice the capacity of the filter press.
2-4.6.2.4 Overall, the precoat system is an option that may require that equipment and
operation and maintenance costs be economically evaluated versus additional filter
cycle time, frequency of cleaning and washing required, and wear and replacement of
filter media.
2-4.6.3 Wash Systems. Filter media wash systems are an integral part of the filter
press operation. Filter media wash systems are used, as required, following the filtration
cycle to remove residual sludge cake, liquid feed sludge from the feed core (if core
blowing is not used), and solids and grease buildup on and around the filter media to
allow subsequent uninhibited drainage. The removal of these materials is essential to
prevent filter media blinding and to maintain atmospheric pressure between the filter
media and filtrate to alleviate back-pressure buildup. Two types of filter wash systems
are typically used: water and acid. A combination of these can be used where scale
builds up from lime conditioning. In other applications, such as those involving polymers
for conditioning, only water wash systems may be required.
2-4.6.3.1 Water Wash Systems. The surface of the filter media should be washed with
water frequently, typically as often as once every 8 to 10 cycles, to prevent residual
solid buildup (WPCF 1983). The common water wash systems are either manual or
automatic. The manual spray wash system typically consists of a hydraulic reservoir, a
high-pressure wash pump operating up to 13.8 MPa (2000 psig), and a hand-held wand
for directing the spray. The operator directs spray to the areas of observed buildups;
this is very labor-intensive.
Automatic spray wash systems are an option. These are typically included
with the plate shifter mechanism to wash the entire filter media system. They usually
require high-pressure water pump boosters to elevate the pressure of the tap water.
Although the capital cost is much higher than the portable spray-wash system, the labor
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