Under which condition will Thiothrix bacteria predominate in an activated sludge process?

Thiothrix bacteria are commonly associated with elevated sulfide levels in wastewater treatment processes. They are known as filamentous bacteria that can thrive in anaerobic or microaerophilic conditions, where sulfide is often present. This ability allows them to oxidize sulfide, a potentially toxic compound, using oxygen or nitrate as an electron acceptor.

In the context of an activated sludge process, the presence of high sulfide levels creates an environment conducive to the growth of Thiothrix, as they can utilize these conditions to proliferate. Their predominance often indicates a shift in the microbial community resulting from the specific chemistry of the wastewater being treated, particularly when sulfide is being produced in abundance.

The other conditions listed, such as low dissolved oxygen (DO), low food-to-microorganism ratio (F:M), and high pH, can influence bacterial growth but do not specifically promote the growth of Thiothrix. Instead, these conditions might favor different types of microorganisms or lead to imbalances that reduce overall treatment efficiency. Therefore, it is the presence of high levels of sulfide that most directly correlates with the dominance of Thiothrix bacteria in activated sludge systems.