What type of organisms predominately causes bulking in activated sludge systems?

Bulking in activated sludge systems is primarily caused by filamentous organisms, which are long, thread-like bacteria. These filaments can multiply rapidly under certain conditions, such as nutrient imbalances or improper aeration, leading to an increase in their mass relative to the floc-forming bacteria. When the ratio of filaments to floc-formers increases, the sludge can become less compact and settle poorly, resulting in the "bulking" phenomenon.

Filamentous bacteria thrive in environments with low dissolved oxygen levels or excess nutrients, and they can form a slimy matrix that traps other microorganisms. This affects the overall settling characteristics of the sludge during the sedimentation process, leading to operational challenges in wastewater treatment plants.

Other options such as ammonia oxidizers are important in the nitrification process but do not directly cause bulking. Pretreatment bacteria refer to different stages of wastewater treatment and are not typically associated with the bulking issue. Higher organisms may contribute to the overall ecosystem but are not the primary contributors to bulking problems in activated sludge systems.