Effect of Recycle and Axial Mixing on Microbial Selection in Activated Sludge

Abstract

A mathematical model was developed to simulate the effect of axial mixing and recycle on competition between filamentous and floc-forming microbial populations based on the Monod equation and the plug flow with recycle flow model, and a matched artificial recycle ratio was defined to include the effects of both axial mixing and recycle flow on the residence time distribution. The model predicted that low recycle ratios (R 10) eventually eliminated the competitive advantage of the floc-formers over the filaments. The feed concentration was also an important factor in microbial selection with a predominantly filamentous biomass predicted when R> 0.1 and Si< 100 mg/L as COD. An operating diagram prepared based on the model output showed that a high recycle ratio and/or low feed concentration favored filamentous domination, while floc-formers dominated at a low recycle ratio and/or high feed concentration. Strategies to control or...