Civil Engineering Reference
In-Depth Information
THICKENING AND DEWATERING ALTERNATIVES
Thickening
Thickening, which begins with concentrating the sludge in the bottom of the clarifier,
is an effective and inexpensive method and generally the first phase of reducing sludge
volume and improving sludge dewatering characteristics. Thickening is most effec-
tively accomplished as a separate operation. Thickening will improve the consistency
of feed material for subsequent dewatering units, and in many cases will reduce the
size of dewatering equipment. WTP residuals are most commonly concentrated using
gravity thickeners, but they can also be concentrated in flotation thickeners or by
gravity belt thickeners.
Gravity Thickening
Gravity thickeners are generally circular and are usually con-
crete, although small tanks are sometimes made of steel. They are typically equipped
with rake mechanisms. The floors are conically shaped with a slope of between 10
and 20 percent. The slope allows the rake mechanism to move solids to the discharge
hopper. Gravity thickeners can be either batch feed or continuous flow. Residuals
thickened in gravity thickeners may require conditioning.
Coagulant Sludge Thickening Results.
Coagulant sludge, which is usually withdrawn
from clarifiers at less than 1 percent solids, can be thickened in gravity thickeners to
2 to 3 percent solids. Aluminum and iron hydroxides may be conditioned with the aid
of polymers; but the polymers will have a minimal effect on the ultimate degree of
compression. Polymers will affect particle size and zone settling velocity, and will
likely improve capture efficiency. Settling curves are shown in Figure 23-10 for 0, 10,
and 20 mg / L doses of polymer.
38
Typical design solids loading rates reported for alum sludge thickening are 4.0 lb /
day/ft
2
when sludges are conditioned with polymers. Alum sludges that are mixed
10
0 mg/L POLYMER
9
8
7
10
6
5
20
4
0
20
40
60
80
Settling Time - minutes
Fig. 23-10.
Effects of polymer dosage on settling time (
Source:
Reference 38. Copyright
American Society of Civil Engineers.)
