Modeling Growth of Filaments and Floc Formers in Activated Sludge Flocs: Integrating the Effects of Kinetics and Diffusion

Abstract

Filamentous bulking is the most common solid-separation problem in wastewater treatment systems. The phenomenon is attributed to competition between filamentous bacteria and floc-forming bacteria, a tradeoff between kinetic selection (KST) and substrate diffusion limitation (SDL). Previously, we explained our contradictory experimental results by both effects and developed a conceptual framework with three regions (bulking, intermediate, and non-bulking). However, within this purely qualitative framework, the range of substrate concentrations for each region was not determined. To test the framework and quantify the critical concentrations for the three regions, we here integrate KST and SDL into a model that simulates the growth rates of filaments and floc formers inside activated sludge flocs. Bulking and non-bulking sludges were produced in laboratory-scale reactors, and the kinetic model parameters were measured by respirometry. We then extended the model to verify our previous experimental results from sequencing batch reactors (SBRs). The effects of oxygen limitation and decay rates of floc formers and filaments on the modeled growth rates were also evaluated. In basic simulations, the model successfully determined the critical values of substrate concentrations in the framework. The results of the extended model were consistent with our experimental data, indicating that large flocs induce bulking whereas small flocs do not. Simulation results were more sensitive to the kinetic parameters than to the diffusion parameters, and the parameters more sensitively affected the growth of floc formers than filament growth.