Page 398 - Water and wastewater engineering

SEDIMENTATION 10-15

80

70
Suspended solids removal, % 60

50

40

30
0 50 100 150 200
Overflow rate, m/d
FIGURE 10-10
Suspended solids removal versus overflow rate. ( Source: Davis & Cornwell, 2008.)

f. Applying the scale-up factors yields
.
t (54 min )(1 75 ) 94 5 or 95 min
.
o
.
v (50 m/d )(065 32 5 m/d
(
.
)
o
Comments:
1. As implied by the shape of the isoconcentration lines, and, conceptually, the trajectory of
the particles, the settling velocity increases as the particles travel through the tank.
2. The depth of the tank is important because flocculant particles tend to grow in size.
Thus, a greater depth facilitates the growth process.
Type III and Type IV Sedimentation
When the water contains a high concentration of particles (for example, greater than 1,000 mg/L)
both Type III ( hindered settling or zone settling) and Type IV ( compression settling) occur along
with discrete and flocculant settling. Zone settling occurs in lime-softening sedimentation, activated-
sludge sedimentation, and sludge thickeners.
When a concentrated suspension of uniform concentration is placed in a column or graduated
cylinder, Type II, III, and IV take place over time as illustrated in Figure 10-11 . With a high par-
ticle concentration the free area between the particles is reduced. This causes greater interparticle
fluid velocities that reduce the settling velocity below that of the individual particles. Because of
the high concentration of particles, the liquid tends to move up through the interstices between
the particles. As a result, the particles that are in contact with one another tend to settle as a zone
or “blanket.” The particles in contact tend to maintain the same relative position. This results in a
relatively clear layer above the settling mass of settling particles. This phenomenon is known as