QUANTIFYING SPECIES-SPECIFIC FILAMENTOUS BULKING THRESHOLDS USING MOLECULAR AND REACTOR STUDIES

Activated sludge bulking is normally considered to occur when the sludge volume index (SVI) reaches 150 mL/g. Previous work has showed that above a certain level of total extended filament length (TEFL), the SVI increases rapidly. This filament length level can thus be considered the threshold for bulking to occur due to all the filamentous bacteria present. In this study, the quantitative FISH method was combined with lab-scale reactor experiments to measure threshold values of filament length for bulking caused by a specific filamentous species. Three lab-scale reactors were operated to allow tracking of filaments before and during the bulking episodes: one sequencing batch reactor (SBR) with fast feeding (Reactor A); one SBR with slow feeding (Reactor B); and one completely mixed reactor with continuous feeding (CMR, Reactor C). One week after the addition of S. natans, Reactor C suddenly experienced bulking, and the SVI increased from 92.9 mL/g to 198.2 mL/g in one day. Reactor B began bulking after 4 more days. The SVI for Reactor A remained constant at 40 mL/g. Interestingly, fluorescence in situ hybridization (FISH) analysis showed that instead of S. natans, Eikelboom Type 1851 was the dominant filament in the sludge during the whole course of bulking in Reactors B and C. The quantitative FISH showed that SVI and filament length of Type 1851 strongly correlated with each other (r=0.98), which suggests that Type 1851 proliferation may be the cause of bulking in reactor C. The threshold level for the onset of bulking occurs at 8 × 108 μm/mL total filament length, or around 8 × 108 μm/mL extended filament length.