UFC 3-280-04
17 DEC 2003
studies on the actual filters (ASTM D2035-80 [1999]). It is necessary to determine the
most effective pretreatment method and dosage to maximize the filtration. Pre-treating
the influent too extensively may result in increased filter clogging and shorter filter runs.
This is particularly the case when polyelectrolytes are overdosed, blinding the media
because the amount of filter aid far exceeds the solids loading that would otherwise be
in the waste stream.
6-2.2
Alum has traditionally been used in filtration as a coagulant to produce a
heavier floc with greater settling velocities. More recently, cationic polymers have been
used alone or in combination with alum or clay. Studies have shown polymer coagula-
tion is more effective and problem-free than alum coagulation. When using alum (versus
polymers), the operator generally must add lime for pH adjustment. Also, alum is more
difficult to use than polymers because various polymer combinations can be tolerated
without significant effect on effluent turbidity. Additionally, the polymer floc is signifi-
cantly more resistant to shear than the more fragile alum floc. The alum floc tends to
trap water, which may result in dewatering problems. When using polymers, though,
surface wash is strongly recommended. Excessive polymer feed to upstream units can
result in carry over to the filtration system, which causes the media to stick together, ex-
cessively high head losses, short filter runs, and eventually blinding. This requires the
media be changed out.
6-2.3
Ferric chloride is also commonly used as a coagulant. Such metal salts can
be used to precipitate phosphorous from waste streams. These fine floc particles do not
necessarily settle well and filtration may be the only way to meet stringent phosphorous
limitations. It is dangerous to use ferric chloride if groundwater is to be re-injected, as
iron tends to precipitate at the well screen, thereby affecting the capacity to re-inject
water.
6-2.4
Polymers or alum may be rapidly mixed with the water directly before the
filtration process, eliminating the need for prolonged sedimentation. Flocculation periods
may be short with high intensity flocculation, or somewhat longer with less intense mix-
ing. This process is called direct filtration. Usually, direct filtration is used with dual me-
dia and multi-media filters. Single medium filters cannot handle the high solid loading
that direct filtration creates. A waste stream needs to be purified to a greater degree,
e.g., sedimentation, before it is introduced to the single media filter. However, too high a
solids loading will clog any filter. Overdosing of polymer can blind the bed. Charge de-
stabilization (i.e., destabilizing the charge of the particle so that the particles will coagu-
late) is the mechanism by which a polymer coagulates. Once charges are destabilized,
excess polymer will unnecessarily add to solids loading. This will result in significant
head loss because greater quantities of solids need to be removed from the influent
stream. Alum, on the other hand, is more forgiving for variable source particle loadings.
Additional information is available in EM 1110-1-4012.
6-2.5
Direct filtration is generally used with dual media filters. When contaminants
are soluble and can be precipitated to form floc particles, direct filtration may be appli-
cable. In direct filtration, the water is rapidly mixed and flocculated, followed directly by
filtration. Direct filtration can be used only when floc can form quickly. In some in-
6-2
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