Civil Engineering Reference
In-Depth Information
indicates that this bed was too shallow and too coarse, and that a better media selection
for the particular application would have been 3 inches (76 mm) of 0.4 to 0.8 mm
garnet, 9 inches (229 mm) of 0.6 to 0.8 mm sand, and 24 inches (610 mm) of 1 to
2 mm anthracite.
A key factor in constructing a satisfactory mixed-media bed is careful control of
the size distribution of each component medium. Rarely is the size distribution of
commercially available materials adequate for construction of a good mixed-media
filter. A common problem is failure to remove excessive amounts of fine materials
when the media is installed. These fines can be removed by placing the media in the
filter, backwashing it, draining the filter, and skimming the upper surface. The pro-
cedure is repeated until field sieve analyses indicate that an adequate particle size
distribution has been obtained. The second medium is then added and the procedure
repeated. The third medium is then added and the entire procedure repeated. Some-
times, 10 to 20 percent of the materials may have to be skimmed and discarded to
achieve the proper particle-size distribution.
Dual Media
As compared to mixed-media, the dual-media (anthracite-sand) filter has lower head-
losses through the media and less resistance to breakthrough because it is made up of
coarser particles and has less total surface area of particles. The mixed-media filter is
capable of producing lower finished water turbidities than the dual-media. These dif-
ferences are greater and become more pronounced when the difficulty of the filtration
application increases. In polishing highly pretreated waters, the differences are not so
great, and some designers prefer to use anthracite-sand media.
Typically, anthracite-sand filters consist of a coarse layer of anthracite about 18-
24 inches (500-700 mm) deep above a fine layer of sand about 8-12 inches (200-
300 mm) thick. Some mixing of anthracite and sand at their interface is desirable to
avoid excessive accumulation of floc, which occurs at this point in beds graded to
produce well-defined layers of sand and anthracite. Such intermixing reduces the void
size in the lower portion of the anthracite layer, forcing it to remove floc that otherwise
might pass through the anthracite layer. Typical gradations of sand and anthracite for
use in dual-media filters are given in Table 12-4.
''Capping'' Sand Filters with Anthracite
One inexpensive method to improve rapid sand filter performance is to remove several
(4-8) inches of sand from a bed and replace it with an equal depth of anthracite. This
produces a layered-type bed that has some of the advantages of a dual-media bed. The
design provides for some intermixing at the interface, and is superior in performance
to a single medium.
49
At Sheboygan, comparisons were made between anthracite-capped sand filters and
sand alone, under various raw-water conditions, including comparisons made during
the algae season. The capped filters were operated at 3 gpm / sq ft (7 m / h) and the
sand at 2 gpm / sq ft (5 m / h). It was found that ''the more adverse the applied water
conditions, relative to algae and floc, the more dramatic are the results obtained with
anthracite capped filter runs.'' With good water conditions, the anthracite cap improved
filter runs by a ratio of 2 to 1. The worst water conditions may give 10-to-1 improve-
ment in filter runs. Through the use of capped filters, short filter runs can be eliminated.
