Important sludge properties include compositions, indices, and types. Compositions of general municipal sludges classified by organic and inorganic components, and solid fractions are listed in Table 1; the characteristics of primary and secondary sludges are listed in Table 2. The classification of municipal sludges is provided later.
Definitions of sludge indices and experimental conditions for the determination of these indices are listed in Table 3. These indices are widely used in the study of sludge settleability.
There are different ways to classify sludges. They can be conveniently classified based on treatment techniques, which is adopted in this article. In South Africa, sewage sludges are grouped into four types: types A-D. Both classification techniques can be loosely related to each other with certain overlaps, which are shown in Table 4. The sludge treatment techniques listed in Table 4 are further explained in later sections.
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