Environmental Engineering FOURTH EDITION

214 ENVIRONMENTAL, ENGINEERING

Sludge Thickening

Sludge thickening is a process in which the concentrations of solids is increased and
the total sludge volume is decreased, but the sludge still behaves like a liquid instead
of a solid. (In sludge dewatering, described later, the same objectives produce sludges
that behave like a solid.) Thickening commonly produces sludge solids concentrations
in the 3 to 5% range, whereas the point at which sludge begins to have the properties of
a solid is between 15 and 20% solids. The sludge thickening process is gravitational,
using the difference between particle and fluid densities to achieve greater compacting
of solids.
The advantages of sludge thickening are substantial. When sludge with 1% solids
is thickened to 5%, the result is an 80% reduction in volume (Fig. 10-5). Reducing
from 1 % solids to 20% solids, which might be achieved by sludge dewatering, would
result in a 95% reduction in volume. The volume reduction translates into considerable
savings in treatment, handling, and disposal costs.
Two types of nomechanical thickening processes are presently in use: the gravity
thickener and thejotation thickner. A typical gravity thickener is shown in Fig. 10-6.
The influent, or feed, enters into the center of the tank and slowly moves toward the
outer edge of the tank. The sludge solids settle and are removed through a drain in
the bottom of the tank. The effluent flows over the weirs that encircle the tank and is
usually returned to the primary or secondary treatment facility prior to discharging.
The thickening characteristics of sludges have for many years been described by
the sludge volume index (SVI), described as

(lOOO)(volume of sludge after 30 min, mL)

ma of SS

SVI = (9.11)

Treatment plant operators usually consider sludges with an SVI of less than 100 as
well-settling sludges and those with an SVI of greater than 200 as potential problems.
The SVI is useful for estimating the settling ability of activated sludge, and is
without question a valuable tool in running a secondary treatment plant. However, the
SVI also has some drawbacks and potential problems. The SVI is not independent
of solids concentration. Substantial increases in suspended solids can change the SVI

I -- - - - -.

I

.99% i

water I

80% of water removed

I

I I

I

1 % solid 95% water

5% solid

Figure 10-5. Volume reduction owing to sludge thickening.