Sludge Aging

Several types of foam are produced through changes in bacterial activity with increasing or decreasing sludge age. Billowy white foam is produced at a young sludge age when the bacterial population or mixed liquor volatile suspended solids (MLVSS) are relatively small (e.g., <1000mg/liter). This small population of bacteria lacks adequate enzymatic activity to degrade the surfactants entering the treatment process. The nondegraded surfactants produce billowy white foam. With increasing sludge age, the bacterial population obtains adequate enzymatic activity and degrades the surfactants. With the degradation of the surfactants, the foam becomes crisp white.

As the bacterial population continues to age (or MLVSS increase), large quantities of oils secreted by the bacteria accumulate in floc particles. The color of the oils darkens crisp white foam to crisp brown foam. Finally, as the bacterial population matures (MLVSS peak), large numbers of slow-growing filamentous organisms such as Microthrix parvicella and Nocardioforms may proliferate. These organisms contribute to the production of viscous chocolate brown or dark brown foam.

Undesired quantities of foam as a result of young or old sludge ages can be corrected by decreasing or increasing the sludge age (wasting rate) of the treatment process. However, sludge wasting rates should be as uniform as possible over a relatively long time period.

FIGURE 20.6 Weak and buoyant young growth surrounding firm and dense old growth in floc particle that has experienced a slug discharge of soluble cBOD.

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