Civil Engineering Reference
In-Depth Information
0.125
37.2%
Settled Solids
Concentration
13.1%
0.10
Sludge Type
Settled Solids (%)
Alum
Iron
Lime and Iron
Lime and Iron
Lime
6.7
13.1
22.3
26.1
37.2
22.3%
0.075
6.7%
26.1%
0.05
Range of shear
values where sludge
becomes handleable
0.025
0
0
10
20
30
40
50
60
70
Solids concentration (%)
Fig. 23-6.
Variation in shear strength with sludge moisture content (
Source:
Reference 28. Re-
printed from
Journal AWWA,
Vol. 65, No. 6 (June 1973), by permission. Copyright
1973,
American Water Works Association.)
tons / ft
2
. The data in Figures 23-6 and 23-7 also show that alum sludges generally
fall in the settled solids range of 7 percent and below. Therefore, solids concentrations
of 15 to 20 percent may be sufficient to produce a handleable sludge. In contrast,
some softening sludges may require concentrations above 50 percent before they can
be handled.
According to the theory of filtration, the resistance of sludges to filtration is a
function of particle size of the flocs in the sludge cake. Although there is no precise
relationship between particle size and cake resistance, measurements of particle size
generally support this theory.
Data presented in Figure 23-8 for various metal hydroxide sludges show the rela-
tionship between mean floc size and specific resistance. These data were measured
using a HIAC particle counter and indicate that unconditioned alum sludge has a mean
floc size of 20
or less.
31
Conditioning chemicals also influence particle size. There is typically an increase
in particle size caused by the addition of polymer and an associated decrease in the
specific resistance to filtration.
Although the average particle size is the primary factor in determining sludge fil-
tration behavior, two other factors are important. If the particle distribution is bimodal
(i.e., if there are two major sizes of particles, small and large), the sludge is susceptible
to ''blinding,'' which is defined as the migration of fines through the cake, resulting
in much lower cake permeability near the filtering surface. This occurrence has been
documented for certain wastewater sludges but does not appear to be common for
WTP residuals.
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