Which term describes a condition that makes sludge float rather than settle?

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

Answer: (C)

Bulking reflects a problem with how well sludge settles in the clarifier. When conditions favor filamentous bacteria, the solids form fluffy, loose flocs that don’t compact properly. Those filaments can extend through the mixed liquor, holding water and resisting settling, so the sludge tends to float or remain suspended rather than form a clear blanket. This leads to poor clarification and higher suspended solids in the effluent.

The other terms don’t describe this settling issue. Biomass is a general term for the microorganisms present, chlorination is a disinfection step, and CBOD measures the amount of biodegradable organic carbon, not how well sludge settles.