208 ENVIRONMENTAL ENGINEERING
lhble 10-1. Sludge Characteristics
Physical ChemicalSolids Volatile Yield Plastic Nas Pas Kas
Sludge conc. solids strength viscosity N P205 K20
me (m@Y (dyne/cm2> (g/cm-s) (a) (%o) (%)Water - - (^0) 0.01 ---
Rawprimary 60,000 60 40 0.3 2.5 1.5 0.4
Mixeddigested 80,000 40 15 0.9 4.0 1.4 0.2
Waste activated 15,000 70 0.1 0.06 4.0 3.0 0.5
- 2.0 2.0 -
2.0 3.0 -
Alum, ppt 20,000 40 -
Lime, ppt 200,000 18 - -
uNote that sludge with 10.000 mg/L solids is approximately 1% solids.the sludge changes from aerobic to anaerobic (or vice versa). Not surprisingly, it is
quite difficult to design sludge-handling equipment because the sludge may change in
some significant characteristic in only a few hours.
The third “law” is constantly violated. Tables showing “average values” for “aver-
age sludges” are useful for illustrative and comparative purposes, and as such, are
included in this chapter for general information; however, they should not be used for
treatment design. Instead, you need to determine the specific and unique characteristics
of the sludge that needs treatment. For illustrative purposes only, Table 10-1 shows
characteristics of hypothetical, “average sludges.’’ The first characteristic, solids con-
centration, is perhaps the most important variable, defining the volume of sludge to be
handled and determining whether the sludge behaves as a liquid or a solid.
The second characteristic, volatile solids, is also important to sludge disposal.
Disposal is difficult if the sludge contains high concentrations of volatile solids because
gases and odors are produced as the sludge outgasses and the volatile substances are
degraded. The volatile solids parameter is often interpreted as a biological rather than a
physical characteristic, the assumption being that volatile suspended solids are a gross
measure of the viable biomass. Another important parameter, especially in regard to
ultimate disposal, is the concentration of pathogens, both bacteriological and viral.
The primary clarifier seems to act as a viral and bacteriological concentrator, with a
substantial fraction of these microorganisms existing in the sludge instead of the liquid
effluent.
Rheological characteristics (degree of plasticity) are one of only a few truly fun-
damental physical parameters of sludges. Two-phase mixtures like sludges, however,
are almost without exception non-Newtonian and thixotropic. Sludges tend to act as
pseudoplastics, with an apparent yield stress and a plastic viscosity. The rheological
behavior of a pseudoplastic fluid is defined by a rheogram shown as Fig. 10-2. The
term thixotropic relates to the time dependence of the rheological properties. Sludges
tend to act more like plastic fluids as the solids concentration increases. True plastic