A New Framework for Analyzing Filamentous Bulking in Activated Sludge: Roles for Kinetics and Diffusion

Filamentous bulking is caused by the excessive growth of filaments over floc formers, and their competition was previously described using kinetic selection. However, recent studies reported that instead of kinetic selection, diffusion limitation inside the flocs may be the crucial factor in microbial selection. To clarify the roles of these factors in explaining filamentous bulking, a new conceptual qualitative framework was developed in this study. We hypothesize that the growth rates of filaments and floc formers are affected by the combination of kinetic selection and substrate diffusion limitation. Three different regions (bulking, transitional, and non-bulking region) based on substrate concentration are suggested. In the bulking region, kinetic selection controls the growth rate process and favors filaments. In the non-bulking region, kinetic selection also is controlling, and favors the growth of floc formers. However, in the transitional region, substrate diffusion limitation, determined by the floc size, plays an important role in causing bulking. To test this framework, sequencing batch reactors (SBRs) were operated with influent COD of 100, 300, 600 and 1000 mg/L, and the sludge settleability was measured at various floc size distributions developed using different mixing strengths. The experimental data in the bulking and transitional regions supported the proposed framework.