Environmental Engineering Reference
In-Depth Information
Note:
When we state that the rate of agglomeration or flocculation
depends on
velocity gradient
, we refer to the fact that too much mixing can
shear the floc particles, tearing them apart again; the floc then becomes
smaller and more finely dispersed, a situation we are obviously trying to
avoid. For this reason, the velocity gradient must be controlled within a
relatively narrow range.
The theory of flocculation is complex and beyond the needs of this text, but
on an elemental level we can say that flocculation is generally accomplished
by slowly rotating, large-diameter mixers. Recent practice incorporates dis-
persion of the coagulant (flash mixing), flocculation, and sedimentation in a
single unit called a
contact clarifier.
.
Note:
Flocculation is the principal mechanism for removing turbidity
from water.
Sedimentation
In a conventional water treatment plant, the process of coagulation and floc-
culation precedes the sedimentation process for better results and improved
utilization of the settling basins. Sedimentation is then followed by the filtra-
tion process. Filtration occasionally may be preceded only by coagulation, in
which case filtration is provided after only a few minutes of contact, adding
additional stress to the filters. Lack of sedimentation results in less reliable
operation of filters when water quality suddenly changes characteristics (De
Zuane, 1997).
Sedimentation (also known as
clarification)
) is the gravity-induced removal
of particulate matter, chemical floc, precipitates from suspension, and other
settleable solids. Simply stated, sedimentation separates the liquid from the
solids. The process takes place in a rectangular, square, or round tank called
a
settling
or
sedimentation tank
or
basin
. Flow patterns within such basins may
be rectilinear flow in rectangular basins, radial flow in center-feed settling
tanks or square settling tanks, or radial flow or spiral flow in peripheral-feed
settling tanks.
DID YoU KNoW?
An adequate detention time is important for complete sedimentation.
The shorter the detention time, the less the settling and the higher the
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