Activated Sludge Foam

Microbial foam is produced in the aeration tank (Figure 20.1) and is discharged from the aeration tank to other treatment units such as the secondary clarifier, thickener, and dewatering facilities. As foam is discharged from the aeration tank to other treatment units, the entrapped air and gas bubbles escape, and the foam collapses. The collapsed foam often is referred to as scum (Figure 20.2).

TABLE 20.1 Microbial Foam Production in the Activated Sludge Process

Contributing Biological Activity

Texture and Color

Erratic sludge wasting rates

Concentric circles of viscous dark brown and viscous light

brown foam

Foam-producing filamentous bacteria

Viscous chocolate brown

Nutrient deficiency, old sludge

Greasy gray

Nutrient deficiency, young sludge

Billowy white

Sludge aging

Billowy white to crisp white to crisp brown to viscous dark

brown

Slug discharge of soluble cBOD

Billowy white

Zoogloeal growth (viscous floc)

Billowy white

TABLE 20.2 Microbial Foam Production in the Anaerobic Digester

TABLE 20.2 Microbial Foam Production in the Anaerobic Digester

Increase in alkalinity

Increase in carbon dioxide

Increase in fatty acids, including start-up condition

Particulate surfactants

Polymers

Temperature fluctuations

FIGURE 20.1 Foam on an aeration tank.

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