UFC 3-280-04
17 DEC 2003
Figure 5-3. Appropriate Filter Backwash Rates
at a Water Temperature of 20 Degrees Celsius
5-1.3.5.3 Pressure Loss and Fluidizing Velocity. Pressure loss during backwash in-
cludes loss through the underdrain orifices or plates, loss through the expanded filter
bed, loss through the gravel layer, friction and minor losses in underdrain channels and
piping from the source of backwash supply, and elevation differences to the top edge of
the wash water trough. The most significant pressure loss is usually that through the
underdrain. Head losses through the underdrain system are obtained from the manu-
facturer. Loss through the gravel layer may be estimated by treating the gravel layer as
porous media. Loss through valves and piping may be calculated using standard fluid
flow equations.
Backwash fluidization in a granular media filter bed can be described as the
upward flow of water through the media with sufficient velocity to suspend the grains in
the water. As the rate of backwashing is increased the head loss through the media is
linear until the rate is reached where the head loss is equal to the weight of the media
grains in water. At this point, no further increase in head loss will occur. Figure 5-4
shows head loss vs. surficial velocity. As the flow rate is increased further, the media
expand and provide a larger flow area that can accommodate the higher flow without
additional head loss. A typical curve for fluidization of granular media is shown in Figure
5-5.
5-20
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