Bulking sludge may be caused by hydraulic overload or excessive which type of bacteria?

• A. Filamentous
• B. Cocci
• C. Rods
• D. Hydraulic overload

Answer: (A)

Bulking sludge happens when filamentous bacteria overgrow, producing long, threadlike filaments that stretch between and around sludge particles. This filament network prevents tight packing and dense settling, so the sludge becomes fluffy and settles very slowly, leading to bulking. Hydraulic overload can worsen settling by increasing flow or loading, but the specific bacterial type responsible for the bulking effect is filamentous organisms, which commonly include species like Microthrix parvicella and related filament-formers. The other shapes, cocci or rods, describe bacterial forms but don’t by themselves explain the formation of a loose, poorly settling sludge.