What is the main cause of bulking in activated sludge systems?

Bulking in activated sludge systems is primarily caused by the proliferation of filamentous bacteria. These bacteria tend to form long, stringy filaments that disrupt the settling characteristics of the sludge, preventing it from compacting properly and leading to poor separation in the clarifier. When filamentous bacteria dominate, they can cause the sludge to appear fluffy and prevent it from settling adequately, which is often described as "bulking."

In contrast, high temperatures, excessive organic loads, and high ammonia levels can contribute to the conditions in an activated sludge system but are not the main causes of bulking. For instance, while high temperatures can affect microbial growth rates and possibly encourage the growth of filamentous species, they do not directly lead to bulking. Excessive organic load can overwhelm the system but is not specifically tied to the filamentous growth that causes bulking. High ammonia levels can indeed impact overall system performance, but they do not specifically lead to the unique settling issues associated with bulking caused by filamentous bacteria. Therefore, filamentous bacteria are the key factor to understand when addressing bulking problems in an activated sludge system.