CAUTION:
Avoid decreasing filter efficiency by destroying the bacterial growth by
chlorination.
7 Allowing the growth to dry by taking the filter out of service for
12 to 48 hours if other adequate filter units are available to continue treatment of the
wastewater generated at the installation.
8 Removing stone and replacing it with a large size if all other
methods fail.
(d) Insects.
1 Filter fly. Several species of flies frequently infest the standard
filter. A fly which is particularly prevalent is commonly called the filter fly. This fly is a
small, mothlike insect belonging to the same family as the Phlebotomus fly. Heavy
infestation with fly larvae comes with thick zoogleal product growth on filter stones when
the air temperature is high. Filter fly larvae assist in the biological action of the filter;
however, a heavy infestation may clog the filter. Also, the fly, which is small enough to
pass through ordinary window screens, is a great source of annoyance to plant
operating personnel and nearby residents because it is attracted to human eyes,
nostrils, mouths, and ears. Breeding in filters is minimized considerably by continuous
application of influent at a high rate and by keeping stone and wall surfaces constantly
wet. Filter flies prefer alternate wet and dry environments. Pupae and larvae
developing under such conditions are continuously washed to the final settling tank
where they may have to be skimmed from the surface. Psychoda flies may become a
nuisance particularly on standard filters operated by intermittent dosage. Serious fly
nuisance may be controlled by: (1) reducing food supply and film development,
thereby removing excessive filter growths ; (2) hosing the inside of the filter walls
vigorously; (3) flooding the filter for about 12 hours every 2 weeks; or (4) using
chlorine or such other chemicals as prescribed by higher technical authority.
2 Water springtail. The water springtail--a small wingless, biting
insect of the order Collembola--may occur in trickling filters. This insect feeds upon the
organic growths in the exposed bed surface and helps keep it clean.
c. Filtration Unit.
(1) Material. The filter bed (see Figure 2-17) consists preferably of clean
quartz sand ranging in size from 0.20 to 0.36 millimeters (mm). Sand grains smaller
than 0.20 mm permit filtering at a lower rate and clogging of the beds; sand grains
larger than 0.36 mm permit a high filtration rate, but allow fine organic material to
penetrate deep into the bed. Filters are most economically constructed if natural sand
deposits can be used, with retaining walls being constructed from topsoil. Underdrains,
if required, are usually of vitrified tile 4 to 6 inches in diameter laid in lines 3 to 4 feet
MD0161
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