Which condition is not listed as a cause of filamentous growth?

• A. Nutrient Deficiencies
• B. Low DO
• C. Low F/M Ratio
• D. High pH

Answer: (D)

Filamentous growth in activated sludge happens when certain bacteria extend long filaments that link together and trap solids, leading to poor settling or bulking. This tends to occur under specific stress conditions. Nutrient deficiencies create a nutrient-limited environment in which some filamentous bacteria that can scavenge scarce substrates thrive and form long filaments. Low dissolved oxygen reduces the overall activity of fast-growing organisms and shifts the balance toward filamentous taxa that tolerate lower oxygen, promoting filament formation that interferes with settling. A low food-to-microorganism ratio means there’s less substrate available per cell, selecting for slower-growing organisms, including filamentous types, which develop extensive filaments that bridge between flocs.

High pH isn’t typically cited as a cause of filamentous overgrowth in this context, even though pH can affect general microbial processes.