Civil Engineering Reference
In-Depth Information
50
40
0.02 tons/ft
2
30
Not Handleable
Sludge Becomes
Handleable
20
Handleable
10
0.05 tons/tf
2
0
0
10
20
30
40
50
60
Solids Concentration (%)
Fig. 23-7.
Comparison of sludge settled solids concentration with the solids concentration where
a sludge becomes ''handleable'' (Reprinted from
Handbook of Practice: Water Treatment Plant
Waste Management,
by permission. Copyright
1987, American Water Works Association and
AWWARF.)
A more likely problem, especially with alum sludges, is the formation of small
particles from the breakup of alum floc due to shear. Alum sludge is very sensitive to
shear (Fig. 23-9), and a shear (
G
) value equal to 500 / sec can be attained in filter
cakes from the passage of water during vacuum dewatering.
32
Therefore, much of the
demand for conditioning chemicals results from the shear associated with the dewa-
tering process and not because alum sludge is composed of small particles. An im-
portant function of conditioning chemicals is to make the sludge more resistant to
shear.
MINIMIZING RESIDUALS PRODUCTION
Residuals production can be minimized, either by the removal of water to reduce the
volume, or by the reduction of the amount of solids present in the sludge, or by a
combination of the two. If a useful by-product can be obtained from the residual
material, some of the sludge-handling costs can be recovered.
One method of reducing the amount of solids is to reduce the amount of chemicals
used for coagulation and / or softening. The quantity of chemical coagulant used can
be reduced in some WTPs by substituting organic coagulants (polymers) for inorganic
coagulants, either partially or entirely. However, Bishop has cautioned that polymers
