UFC 3-280-04
17 DEC 2003
6-4.3
In dual and multi-media beds, mudballs formed in the filter remain above the
coalsand interface where they are subject to auxiliary scouring, an advantage over
single media beds where mudballs tend to sink to the bottom of the bed.
6-4.4
When the head loss at any level in the filter bed exceeds the static head to
that point, a head condition below the atmospheric level (vacuum) occurs. This is com-
monly called a negative head condition and can cause air binding of the filter. When
negative head conditions occur, dissolved gases in the water are released and gas
bubbles are formed within the filter bed. These trapped gas bubbles cause additional
head losses and aggravate the problem even further. Air binding problems are most
prevalent when there is insufficient water depth over the media and at times when the
surface water is saturated with atmospheric gases because of the rising water tem-
peratures in the spring.
6-4.5
If pressure taps were provided at various depths in the filter, it would be
possible to establish the relative head loss at different points in the bed. Figure 6-1 is a
diagram of representative pressure curves through the filter bed during filtration. The
hydrostatic pressure line shown in the diagram indicates the static pressure at each
depth in the filter based on the water level in the filter and the available filtering head.
Head loss through the filter bed can be considered as the difference between the hy-
drostatic pressure line and the representative pressure curves at the respective points
along the pressure axis. Curve C1 represents the pressure in a clean bed at a specific
filtering rate and h1 is the clean bed head loss. The shape of the curve shows that the
clean bed head loss is proportional to the depth of the media. Curve C2 represents the
pressure during clogging of the media. The upper portion of the curve shows a de-
crease in pressure owing to the removal and storage of particles in the upper portion of
the media, and correspondingly, an increase in filter bed head loss. The point where the
C2 curve turns and is parallel to the C1 curve represents the depth of particle penetra-
tion into the bed; this is applicable to the other curves as well. Curve C3 shows the
pressure conditions at turbidity breakthrough. At no point does the curve parallel the
clean bed curve, thus indicating that the particles have penetrated through the full depth
of the bed. Curve C4 represents the pressure with a clogged bed condition causing air
binding. There is a large pressure drop in the upper portion of the bed, which is less
than the static pressure to that point. This pressure is less than the atmospheric pres-
sure and will result in dissolved gases from the water being released and forming air
bubbles in the bed, even though filtering head is still available. Air binding is of particular
concern at an HTRW site because hazardous volatile emissions may result from the air
binding.
6-4.6
An additional operation issue is the time required for a filter to operate effec-
tively immediately following backwash. Filter operation varies from manufacturer to
manufacturer, but on some systems, after backwash, some time is needed for the fil-
trate turbidity to drop to an acceptable level (filter ripening). Three methods have been
used to deal with this initial quality problem: filter to waste, slow initial filtration, or poly-
mer conditioning. Filter to waste (returning the initial filter run to the head of the treat-
ment system) requires additional treatment capacity and may present a disposal issue
on HTRW sites. Although unusual, it can be good practice where biological growth is a
6-6
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