Which term describes sludge that floats rather than settles in secondary sewage treatment?

• A. Bulking
• B. Biomass
• C. Chlorination
• D. Cavitation

Answer: (A)

Bulking is the condition described when sludge in the secondary treatment stage floats rather than settles. This happens when the sludge blanket becomes fluffy, often due to filamentous bacteria or imbalances in the mixed liquor that reduce settling speed. As a result, solids don’t form a dense layer and can rise or be carried out with the effluent, disrupting clarifier performance.

Biomass refers to the overall living microbial mass in the system and isn’t specifically about settling behavior. Chlorination is the disinfection step used after treatment, not a settling issue. Cavitation involves the formation of vapor bubbles from rapid pressure changes in pumps or mixers, not the characteristics of sludge settling.