TYPICAL
CONSTRUCTION
Conventional gravity and pressure rapid filters
operate downflow. The filter medium is usually a 15-30 in. deep bed of sand or
anthracite. Single or multiple grades of sand or anthracite may be used.
A large particle bed supports the filter media to
prevent fine sand or anthracite from escaping into the underdrain system. The
support bed also serves to distribute backwash water. Typical support beds
consist of 1 8-1 in. gravel or anthracite in graded layers to a depth of 12-16
in.
TYPES
OF MEDIA
Quartz sand, silica sand, anthracite coal, garnet,
magnetite, and other materials may be used as filtration media. Silica sand and
anthracite are the most commonly used types. When silica is not suitable (e.g.,
in filters following a hot process softener where the treated water is intended
for boiler feed), anthracite is usually used.
The size and shape of the filter media affect the
efficiency of the solids removal. Sharp, angular media form large voids and
remove less fine material than rounded media of equivalent size. The media must
be coarse enough to allow solids to penetrate the bed for 2-4 in. Although most
suspended solids are trapped at the surface or in the first 1-2 in. of bed
depth, some penetration is essential to prevent a rapid increase in pressure
drop.
Sand and anthracite for filters are rated by
effective particle size and uniformity. The effective size is such that
approximately 10% of the total grains by weight are smaller and 90% are larger.
Therefore, the effective size is the minimum size of most of the particles.
Uniformity is measured by comparison of effective size to the size at which 60%
of the grains by weight are smaller and 40% are larger. This latter size,
divided by the effective size, is called the uniformity coefficient-the smaller
the uniformity coefficient, the more uniform the media particle sizes.
Finer sands result in shallower zones for the
retention of suspended matter. The most desirable media size depends on the
suspended solids characteristics as well as the effluent quality requirements
and the specific filter design. In general, rapid sand filters use sand with an
effective size of 0.35-0.60 mm (0.014-0.024 in.) and a maximum uniformity
coefficient of 1.7. Coarse media, often 0.6-1.0 mm (0.024-0.04 in.), are used
for closely controlled coagulation and sedimentation.
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