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When heavily silted waters are encountered, clarifiers are used in place of the
normal coagulating and settling tanks.
The flocculated water is admitted to the clarifier tank through rectangular ports
constructed in the lower part of the wall and arranged in such a manner as to
impart a continuous rotary upward movement. Water which has already made one or
two revolutions of the tank comes in contact with water newly entering with the
result that the freshly formed floc of the new water can attach itself to any
small floc in the stream so increasing the inertia of the particles and
enhancing the rate of deposition of sludge on the floor of the tank. The sludge
is removed from the tank by means of a scraper mechanism. With very heavily
silted waters it may be necessary to desludge every few hours or even
continuously with more normal waters once or twice per day is sufficient. The
scraper mechanism sweeps the deposited mud into a sump at the centre from which
it is drawn off by a sludge pipe either by static head of the water or by a
sludge pump. The desludging is carried out while the plant is in operation.
It is advisable not to allow the sludge to accumulate in the sedimentation tank
unnecessarily.
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