17 DEC 2003
SGm = specific gravity of the media.
In the filter bed, there is a range of grain sizes as determined from the sieve analysis of
the media. The particles do not all fluidize at the same superficial velocity; smaller parti-
cles fluidize at a lower velocity than the larger particles. Therefore, to assure complete
bed fluidization, it is necessary to check fluidizing velocity of the coarser grains. The d90
size, that is, the particle size corresponding to the sieve opening size for which 90% of
the grains are smaller, is typically used for this. The d90 size can be used as an accept-
able approximation for the deq size of the largest particle in the bed. It is necessary to
know the density of the media particles, and this can be found by running a specific
gravity test on the media. This is especially important in multi-media applications where
a lighter medium is used as a cap. The fluidizing velocity depends on the temperature of
the water as well, as the density and viscosity of the water are factors of the equations.
Higher water temperatures require higher fluidizing velocities. Once the required fluidiz-
ing velocity is calculated, a safety factor of 1.3 is normally used to assure that an ade-
quate wash rate is provided.
Continuous Backwash Filtration. Two types of continuous backwash sys-
tems are commercially available: the traveling bridge filter and the upflow or downflow
deep bed granular media filter.
5-1.4.1 Traveling Bridge Filter. The traveling bridge filter is a gravity filter divided up
into several individual filter cells. A hood travels horizontally along the cells, backwash-
ing individual cells while the other cells continue to filter water. The influent floods the
bed to a depth of 600 mm (2 ft), flows via gravity through the media and exits through
effluent ports. Typically, the media bed ranges from approximately 300 to 600 mm (12
to 24 in.) deep and may consist of single or dual media. Typically, wastewater applica-
tions use dual media with silica sand, 0.55 to 0.65 mm, and a uniformity coefficient of
1.5 under an anthracite layer sized at 1.1 mm with a 1.5 uniformity coefficient. The filter
functions at the surface rather than at depth.
5-22.214.171.124 The low terminal head loss (usually less than 610 mm [2 ft]) creates the sur-
face filtration. Concurrent with the filtering, a hood travels along a track system. The
hood isolates an individual cell for backwashing. A backwash pump draws filtered water
from the effluent chamber, pumping the water back through the effluent port to fluidize
and backwash the bed. Another pump picks up wash water collected in the hood and
discharges it to the wash water trough. Bed cleaning enhancements may include air
scour or hydraulic spray jets to supplement backwash, or a scarifier blade to plow the
media and loosen the solids mat as the hood moves into position to backwash. Back-
wash may be triggered by a certain head loss measured by water level probes, started
automatically by a timer, or started manually. Figure 5-6 shows a traveling bridge filter
system in cross section.