Page 395 - Water and wastewater engineering

10-12 WATER AND WASTEWATER ENGINEERING

d. For the 1 mm/s particles

1 mm/s
P (100%) 500 %
02 mm/s.
But the particle removal cannot be greater than 100%, so the particle removal is 100%
for the particles settling at 1 mm/s.

Type II Sedimentation
Type II sedimentation is characterized by particles that flocculate during sedimentation. These
types of particles occur in alum or iron coagulation, in wastewater primary sedimentation, and in
settling tanks in trickling filtration.
There is no adequate mathematical relationship that can be used to describe Type II settling.
The Stokes equation cannot be used because the flocculating particles are continually changing in
size and shape. In addition, as water is entrapped in the floc, the specific gravity also changes.

Settling Column Model. Laboratory tests with settling columns serve as a model of the behavior
of flocculant settling. These have value in evaluating of existing settling tanks and in developing
data for plant expansion or modification of existing plants, but are not practical for the design
of new settling tanks because of the difficulty in replicating the properties and concentrations of
particles coming from the coagulation/flocculation process. The following discussion illustrates
both the method of analyzing the column data and the behavior of the flocculant suspension.
A settling column is filled with the suspension to be analyzed. The suspension is allowed to
settle. Samples are withdrawn from sample ports at different elevations at selected time intervals.
The concentration of suspended solids is determined for each sample and the percent removal is
calculated:

⎛ C t ⎞
R% 1 ⎟ ⎠ ( 100 %) (10-20)
⎜
⎝
C 0
where R % percent removal at one depth and time, %
C t concentration at time, t, and given depth, mg/L
C 0 initial concentration, mg/L

Percent removal versus depth is then plotted as shown in Figure 10-8 . The circled numbers are
the calculated percentages. Interpolations are made between these plotted points to construct
curves of equal concentration at reasonable percentages, that is, 5 or 10 percent increments.
Each intersection point of an isoconcentration line and the bottom of the column defines an
overflow rate ( v o ):
H
v o (10-21)
t i
where H height of column, m

t i time defined by intersection of isoconcentration line and bottom of column ( x -axis)
where the subscript, i, refers to the first, second, third, and so on intersection points