UFC 3-280-04
17 DEC 2003
5-1.5
Advantages and Disadvantages in Granular Media Filtration Systems.
Gravity filtration systems are the simplest granular media filtration systems. Reasonably
long filter runs can be achieved, but there is the possibility of negative gauge developing
within the filter bed, resulting in "air binding." Air binding problems typically result where
particle removal is occurring in only the top few inches of the filter bed and the entire
depth of the bed is not being used for removal. When the head loss at any level in the
filter exceeds the static head to that point, a head condition below the atmospheric level
(vacuum or negative gauge) occurs. This is commonly called a negative head condition
and can cause air binding of the filter. When a negative head condition exists, dissolved
gases in the water are released and gas bubbles are formed within the filter bed. These
trapped gas bubbles cause additional head losses, aggravating the problems even fur-
ther.
5-1.5.1 Negative gauge pressure is generally absent in pressure filtration systems.
Pressure filtration systems can be operated at higher terminal head losses, which gen-
erally result in longer filter runs and reduced backwash requirements. High power costs
are associated with pressure filtration systems, limiting their practicality by cost consid-
erations. Additionally, because the elements are enclosed in a steel shell, access for
normal maintenance and observation is limited.
5-1.5.2 The continuous filters have deep beds, allowing for maximum solids capture.
Continuous backwash systems also offer the advantage of avoiding periodic backwash
cycles. This results in continuous, steady state operation with constant pressure drop,
and also eliminates the auxiliary equipment associated with the backwash process. But,
alternative equipment and systems must be installed and operated to clean the media.
For example, special influent dosing mechanisms, the washer chamber, the com-
pressed air system, and the sand lift mechanism must be provided. In addition problems
have been reported with upflow systems' ability to handle high influent solids loading.
5-1.5.3 Where filtrate is being returned for domestic or industrial reuse, special
considerations may apply. Certain states are no longer giving approval for operation of
continuous backwash systems for potable water applications. With the traveling bridge
system especially, but also to a degree with the upflow system, the potential for contact
between influent and effluent water creates a disinfection issue. Similar considerations
may be an issue with HTRW applications. Potential cross-contamination is an issue
whether the designer is dealing with hazardous waste water or with potable water. Spe-
cifically, the most common complaints are the single wall between filtrate and unfiltered
water, no air scour or water wash, no water to waste after backwash, insufficient media
depth, and open filtrate channel. When a unit process designed for removal of a cate-
gory of HTRW contaminants (e.g., air strippers for removal of VOCs) is present up-
stream from the filtration system, cross-contamination concerns would usually not be
relevant. Cross connections are often not of concern between units at HTRW facilities.
5-2
PRECOAT FILTRATION. Precoat filtration systems employ septa that sup-
port the filter medium or filter aid and conduct the filtrate to a collection manifold. Filter
media can vary according to the filtration needs and the feed stream chemistry. Diato-
maceous earth or perlite are generally used as the filter aid. The cycle consists of three
5-28
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