UFC 3-280-04
17 DEC 2003
5-1.2.8.1 Dual media filters employ two layers of media of different size and specific
gravity. The flow will contact the lighter, coarser layer first (top size generally greater
than 1 mm), with the finer layer used as a polishing step (reverse gradation). Dual me-
dia filters produce both good effluent quality and deep bed penetration. Grading the
media from coarse to fine allows greater penetration of solids within the bed and greater
removal of solids by the coarse media owing to the consequent increased available re-
moval sites (increased "storage" capacity). Removal in the coarser media results in less
head loss buildup. Dual media filters are the most common arrangement used in prac-
tice. Unless extensive pilot testing is conducted, use of dual media is recommended for
gravity and pressure granular media filters. Because of they way they operate, continu-
ous backwash systems are always designed using a single granular medium. Traveling
bridge filters may use single or dual media, depending on the manufacturer and the ap-
plication, although they usually use single media. These filters are described in Para-
graphs 5-1.3 and 5-1.4.
5-1.2.8.2 The most common dual media filter configuration is crushed anthracite coal
over silica sand. The larger anthracite removes bulk suspended solids; the sand re-
moves finer particles that were carried through the anthracite bed. Other types of dual
media filters have been composed of activated carbon and sand, ion exchange resin
beads and sand, and resin beads and anthracite.
5-1.2.8.3 Multi-media filters operate in the same way as do dual media filters, but have
an additional layer of filtration media, offering a greater potential for tailoring the filter
design to the specific waste stream. A multi-media filter will be the most expensive to
produce and install. Common multi-media beds are composed of anthracite, sand, and
garnet or ilmenite; activated carbon, anthracite, and sand; weighted spherical resin
beads, anthracite, and sand; and activated carbon, sand, and garnet or ilmenite.
5-1.2.8.4 One issue with dual media or multi-media configurations is the effect of inter-
mixing of the media at the interface. The degree of intermixing will depend on the den-
sity, shape, and size differences between the media at the interface. The media may be
graded to maintain a sharp interface (coal size to sand size ratios at the interface of
about 2:1) or substantial intermixing may be allowed (coal size to sand size of about
4:1). Better effluent quality generally results with at least a modest amount of intermix-
ing, which is desirable in dual and multi-media filters. An intermixed bed more closely
approximates the ideal coarse to fine filter bed, eliminating the impervious layer that
may build up at a sharp interface. In practice, some intermixing is unavoidable. No con-
clusive evidence is available to dictate or suggest the ideal or optimum degree of inter-
mixing. One rule-of-thumb is that at least several inches of pure sand should be avail-
able past the zone of intermixing. Intermixing will result in faster head loss buildup owing
to increased suspended solids removal. Typical media design parameters are contained
in Tables 5-3 and 5-4.
5-9
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