Chlorine-Susceptible and Chlorine-Resistant Type 021N Bacteria Occurring in Bulking Activated Sludges

Two filamentous bacteria causing bulking in two activated sludges were examined. Investigations using morphological features, staining techniques, and fluorescent in situ hybridization identified both filaments as type 021N. However, an examination of the effect of chlorine on the sludges revealed a chlorine-susceptible type 021N in one sludge and a chlorine-resistant type 021N in the other. The performance of the activated sludge process is often determined by the gravity separation between the treated wa-ter and the sludge in the final clarifier. The proliferation of filamentous bacteria in the microbial community of the sludge, referred to as “filamentous bulking, ” has often been reported to hamper this solid-liquid separation (7, 18). Filamentous bulking results in less dense and less settleable sludge flocs, and in severe cases the operation of the plant can be totally compromised. Addition of disinfectants (mostly chlorine based) to the sludge to selectively kill off the causative fila-mentous bacteria is often used in practice as a short-term an