What is the main cause of the "bulking" phenomenon in activated sludge?

The primary cause of bulking in activated sludge systems is the excessive growth of filamentous bacteria. In a well-operated activated sludge system, the microorganisms in the mixed liquor settle to form a clear separation between the settled sludge and the supernatant after the settling process. However, when there's an overgrowth of filamentous bacteria, these organisms do not settle properly. The filamentous bacteria can form long, intertwining strands that disrupt the settling process, leading to poor clarification and the bulkiness of the sludge.

This bulking phenomenon results in an increased volume of sludge that impacts the treatment process negatively by reducing the efficiency of the system, creating issues like poor effluent quality and potential operational problems for wastewater treatment facilities.

Insufficient aeration, low organic loading, and high temperatures can contribute to the overall health of the sludge and its performance, but they are not the direct cause of bulking. Filamentous bacteria's dominance directly alters the physical composition of the sludge, making it the main factor in this phenomenon.