Civil Engineering Reference
In-Depth Information
150
100
50
0
0
2
4
6
8
r(-10
)m/kg
Fig. 23-5.
Effect of magnesium concentration on the specific resistance of softening sludges
(
Source:
Reference 28. Reprinted from
Journal AWWA,
Vol. 65, No. 6 (June 1973), by permission.
Copyright
1973, American Water Works Association.)
The solids concentration of a dewatered sludge by itself is a poor indicator of its
handleability. While an alum sludge may be sufficiently dewatered for handling at 30
to 40 percent solids, a lime sludge dewatered in a lagoon to 50 percent solids may
not be handleable with earthmoving equipment. Many utilities have reported that lime
sludge cakes in the 50 to 65 percent moisture content range are sticky and difficult to
discharge cleanly from dump trucks. The character of lime sludges at varying solids
contents is generalized as shown in Table 23-15.
Shear stress is an important parameter in determining the handleability of a sludge.
The undrained shear strength of various water treatment residuals, shown in Figure
23-6, varies significantly with the solids content. Figure 23-6 also shows that the
sludge settled solids concentration can provide a reasonable estimate of the range of
solids concentrations where a sludge makes the transition from a liquid to a handleable
solid. This condition is clearly presented in Figure 23-7, where the solids concentration
needed to produce a handleable sludge occurs in a shear stress range of 0.02 to 0.05
TABLE 23-15. Lime Sludge Characteristics
Solids Content, %
Sludge Character
0-10
Liquid
25-35
Viscous liquid
40-50
Semisolid, toothpaste consistency
60-70
Crumbly cake
