General Statement of Activated Sludge Processes

A typical wastewater treatment plant is shown schematically in Figure 2. It usually consists of four sections, namely pretreatment, primary treatment, secondary treatment, and tertiary treatment. There are no clear

Table 2 Characteristics of primary and secondary sludges


Primary sludge

Secondary sludge

Total solids (TS) (%)



Volatile solids (% of TS)



Nitrogen (N, % ofTS)



Phosphorous (P2O5, % of TS)



Potash (K2O, % of TS)



Heat value (kJ kg-1, dry basis)






Alkalinity (mg r1 as CaCO3)



Data from Metcalf E (1991) Wastewater Engineering. New York: McGraw-Hill; Chen G, Yue PL, and Mujumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883-916.

Data from Metcalf E (1991) Wastewater Engineering. New York: McGraw-Hill; Chen G, Yue PL, and Mujumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883-916.

Table 3 Sludge indices from settleability tests



Experimental condition

Sludge volume index (SVI)

Sludge density index (SDI) Fractional sludge depth (fsd)

Stirred specific volume index (SSVI)

Dilute sludge volume index (DSVI)

V = settled volume of sludge Vt = total volume

depth of sludge layer fsd — 1

depth of column fsd x 1000


MLSS V <200 ml n = twofold dilutions required

30 min quiescent settlement

As above

30 min settlement under gently stirred conditions (1-2 rpm stirring mechanism)

Dilution at which the settled sludge volume (V) is less than, but closest to 200 ml

Table 4 Types of sewage sludge

Type of sewage sludge


South Africa classification

Raw sludge Primary sludge Secondary sludge

Aerobic digested sludge Anaerobic digested sludge

Chemically conditioned sludge

Heat treated sludge Composted sludge

Unstable, causing odor nuisance and fly-breeding Contains pathogenic organisms

Fully or partly stabilized without significant odor nuisance and fly-breeding Contains pathogenic organisms

Certified to comply certain quality requirements

Type A

Type B

Types C and D (if not certified, type B)

Table 5 Biological groups in activated sludge

Biological group Genus (example) Species (example) Function

Autotrophic microbes Nitrosomonas N. europaea Nitrogen removal

Heterotrophic microbes Thiosphaera pantotropha Simultaneous nitrification and denitrification

Pseudomonas Nitrification and phosphorous removal

Cyanobacteria Anabaena sphaerica Carbon and nitrogen removal

Acinetobacter Gram-negative and others Phosphorous removal Protozoa Secondary colonization

Figure 1 Structure of Anabaena sphaerica.

classification criteria to distinguish between pretreatment and primary treatment in the literature. For simplicity, we combine both sections together in this article. Since the tertiary treatment only involves a small amount of sludge, it is not described in detail.

A photo of the wastewater treatment plant of Buelach, Switzerland is shown in Figure 3, which consists of rectangular primary settlers, octagonal, aeration basins, circular secondary settlers, and anaerobic reaction tanks.

The unit operations in pretreatment and primary treatment, as well as in secondary treatment are explained in the following section.

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