Environmental Engineering FOURTH EDITION

Sludge Treatment and Disposal 209

do

Velocity gradient, - dv

Figure 10-2. Rheograms for three different fluids.

fluids may be described by the equation

du

t = ty + q-,

dY

(10.1)

where

t = shear stress,

ty = yield stress,

q = plastic viscosity, and

du/dy = rate of shear, or the slope of the velocity @)-depth (y) profile.

Although sludges are seldom true plastics, Eq. (10.1) may be used to approximate the
rheogram.
The yield stress may vary from more than 40 dyne/cm2 for 6% raw sludge to only
0.07 dyne/cm2 for a thickened activated sludge. The large differences suggest that
rheological parameters could be used for scale-up purposes. Unfortunately, rheological
characteristics are difficult to measure and such analyses are not even included as
standard methods (U.S. Environmental Protection Agency 1991).
The chemical composition of sludge is important for several reasons. The value
of sludge as fertilizer is dependent on the availability of nitrogen, phosphorus, and
potassium, as well as trace elements. A more important measurement, however, is the
concentration of heavy metals and other toxic substances, which should be kept out
of the food chain and general environment. The range of heavy metal concentrations
in sludges is very large. For example, cadmium concentrations can vary from almost
0 to more than 1OOOmgkg. Industrial discharges are a major source of heavy metals
and toxins in sludges; a single poorly operated industrial firm may contribute enough
toxins to make the sludge worthless as a fertilizer. Although most engineers agree that