224 ENVIRONMENTAL ENGINEERING
i- Sludge
Figure 10-14. Capillary suction time apparatus.and onto a blotter. The speed at which the water is taken up by the blotter is measured
by timing the spread of water over a set distance. This time, in seconds, is correlated
to specific resistance. A short time indicates a highly filterable sludge; a long time
suggests slow, problematic filtration.
A major drawback of the CST test is that, unlike the specific resistance to filtration,
the CST is dependent on the solids concentration. Using the Darcy equation it is
possible to derive an expression that appears to be an excellent measure of sludge
filterability,
x=4(?), (10.6)where
x = sludge filterability (kg2 m4/s2),
4 = dimensionless instrument constant, specific for each CST apparatus,
,LA = fluid (not sludge) viscosity (P),
X = suspended solids concentration (mg/L), and
t = capillary suction time (s).The sludge filterability term (x) can be used for design as specific resistance to filtration.
Centrifugation became popular in wastewater treatment only after organic poly-
mers were available for sludge conditioning. Although the centrifuge will work on
any sludge, most unconditioned sludges cannot be centrifuged with greater than 60
or 70% solids recovery. The centrifuge most widely used is the solid bowl decanter,
which consists of a bullet-shaped body rotating on its axis. The sludge is placed into the
bowl, and the solids settle out under about 500 to 1000 gravities (centrifugally applied)
and are scraped out of the bowl by a screw conveyor (Fig. 10-15). Although laboratory
tests are of some value in estimating centrifuge applicability, tests with continuous
models are considerably better and highly recommended whenever possible.