(1) Standard trickling filters. Sludge in sedimentation tanks following
standard trickling filters must be removed often enough to prevent rising sludge. Once
daily may be enough in cold weather if sludge is stable. Denitrification may occur in
warm weather, causing release of nitrogen gas and rising sludge. Sludge removal
every 3 hours may be necessary for control of rising sludge and during the filter
(2) High-capacity filters. Sludge in sedimentation tanks following high-
capacity filters must be removed at least once each shift, or more frequently, as
required. This sludge becomes septic much more rapidly than that from standard filters.
At some installations, sludge can be removed continuously.
c. Filter Effluent Recirculation. The amount of recirculation for any particular
filtration process (see Figure 2-16) depends upon a number of factors. Experience is
necessary to determine the most effective method and rate of recirculation. The best
guide is recirculation of filter effluent in a sufficient amount to provide 1 ppm minimum
dissolved oxygen in the settling tank receiving recirculated effluent. Control samples
are taken in the inside of the effluent weir of the filter ahead of the settling tank. Since
several weeks are needed for the filter to adjust to the changes, each trial must be
continued with appropriate tests for one month before any conclusions are made--
unless, of course, failure becomes obvious.
Section III. TERTIARY WASTEWATER TREATMENT
2-10. CARBON ADSORPTION
a. Columns. Activated carbon is produced by chemically treating charred wood
particles. It can be packed into vertical columns arranged in series or parallel and then
placed so the wastewater to be treated flows through it. Depending on flow conditions,
the carbon particles may float or remain stationary at the bottom of the columns. The
flow may be induced by mechanical pressure or gravity acceleration. Series
configurations achieve more complete organic removal and will be used when carbon
absorption is required to remove 90 percent of the total plant organics. For lower levels
of treatment, staggered parallel columns can produce a greater volumetric flow of
passage while still maintaining removal efficiency.
b. Backwashing. The carbon media can be cleaned by a technique known as
backwashing. This involves reversing the flow of water through the columns so that
collected solids and nonfiltrable residues may be "blown" back through the columns to a
position where they may be collected.
c. Process Design. Where practical, carbon column studies should be
conducted on the waste to be treated to determine the process design. These studies
should test the type of activated carbon that will be used in operating the full-scale plant.