UFC 3-280-04
17 DEC 2003
chemical interaction, electrostatic forces, electro-kinetic forces, or van der Waals forces
are strong enough to cause particles to deviate from streamlines. Adsorption is not be-
lieved to be a significant removal mechanism under normal filtration conditions.
3-1.2.4 The effect of filter rates on the quality of filtrate can vary widely, depending on
application. Both the waste stream and any upstream pretreatment (e.g., polymer addi-
tion) can result in changes in the acceptable range of feed rates. Generally, large solids
are removed initially at the surface by straining. As the hydraulic gradient increases,
these flocs may break up and penetrate further into the filter media. As the solids be-
come lodged between the media grains, the void space decreases, and resistance to
flow increases. The rate of flow increases through the larger openings and lessens
though the clogged openings. There is little or no deposition in the channels where ve-
locities are high. Backwash is initiated when the resistance increases to a limiting level
or breakthrough occurs.
3-1.3
Filtration Applications. When specifying a filtration system, it should be
noted that numerous solids separation techniques other than filtration may be applicable
to a given situation. More importantly, from a design standpoint, filtration may not be the
most efficient means of removing solids and the design professional should be aware of
its limitations.
Ideally, the design professional should have a detailed knowledge of all avail-
able options to make proper design decisions. However, Table 3-1 provides a compara-
tive summary of solids separation techniques that can be used as a starting place for
assessing options. Similarly, Figure 3-1 provides a visual representation of available
treatment options based on particle size.
Table 3-1. Comparative Summary of Solids Separation Techniques
Product parameters
Favorable field conditions
Unit
Solid in
Liquid in
Suitability of
Solids
Solids
Operation
Liquid
Solid
Filtrate for use as
Concentration
Characteristics
Stream
Stream
Backwash Water
Filtration
Fair to Good
Good
Good
High to medium
Light, coarse to
med. floc. fine
Sedimentation
Fair to
Poor
CCD*, low efficiency
Medium to low
Dense, medium or
excellent
flocculated fine
Centrifugation
Fair
Poor
Fair to excellent
Medium to low
Dense fine
Cycloning
Poor
Poor
Poor
Low to medium
Dense, coarse to
medium
Screening
Poor
Poor to fair
Poor
High to medium
Coarse to medium
(DSM)†
Ultrafiltration
Excellent
Poor to Fair
Poor
Low
Very Fine
* Counter current decantation
†
Dutch State Mines
Source: Swilzbin (1996)
3-2
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