EFFECT OF RECYCLE AND AXIAL MIXING ON MICROBIAL SELECTION IN ACTIVATED-SLUDGE

A mathematical model was developed to simulate the effect of axial mix ing and recycle on competition between filamentous and floc-forming mi crobial populations based on the Monod equation and the plug flow with recycle flow model, and a matched artificial recycle ratio was define d to include the effects of both axial mixing and recycle flow on the residence time distribution. The model predicted that low recycle rati os (R < 1.0), implying low axial dispersion, favored floc-formers; inc reasing R (to R > 10) eventually eliminated the competitive advantage of the floc-formers over the filaments. The feed concentration was als o an important factor in microbial selection with a predominantly fila mentous biomass predicted when R > 0.1 and S-i < 100 mg/L as GOD. An o perating diagram prepared based on the model output showed that a high recycle ratio and/or low feed concentration favored filamentous domin ation, while floc-formers dominated at a low recycle ratio and/or high feed concentration. Strategies to control or prevent bulking include the compartmentalization of or the reduction of mixing power within th e aeration tank to reduce axial mixing, as well as reducing the actual recycle rate. Experimental data and field observations reported by ot her researchers were consistent with the model results.