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7.18 CHAPTER SEVEN

Drive assembly
- Mechanical skimming
mechanism (optional)
I
Scum weir t
[-- (optional) ~l ~-- Floating bridge
Guide wheels

Siphon pipes
Effluent troughs

t__ Fiberglass floats
I
Collection headers (with patented
- -
degasing system)
- - Individual sludge valves
Sludge return

Control panel
FIGURE 7.10 Floating bridge-type collector. (Courtesy of Leopold.)

ket that is too deep. If underflow is removed too quickly for too long an interval, the draw-
off "postholes." That is, less viscous liquid breaks through, and dense sludge accumulates
in the basin, overloading the desludging equipment.
Drawing off underflow at regular intervals is best, either manually, with a programmed
blowdown valve, or using a transfer pump. Such a program must be completely adjustable
and programmed by the design engineer during early operation to fit plant operation. Man-
ual drawoff control by guess and "gut feeling" often leads to operating problems.