Influence of anoxic to aerobic volume ratio on sludge settleability and bacterial community structure in a denitrifying-nitrifying activated sludge system

Biological treatment processes under nitrifying and denitrifying conditions are complex systems, in which different kinds of microbes dynamically cultivate and interact. The sludge bulking in these systems can be controlled by understanding the relationship between reactor performance and microbial diversity. In this study, sludge settleability and bacterial community structure were investigated in a modified Ludzack-Ettinger system as anoxic to aerobic volume ratio increased from 1:3 to 1:2 and finally to 2:1. Results indicated that settleability significantly improved as the anoxic to aerobic volume ratio increased (p<0.01), accompanied by the increase in nitrogen removal efficiency and the change in filamentous bacteria. Polymerase chain reaction-denaturing gradient gel electrophoresis analysis revealed that the diversity and richness of bacterial species increased with a small volume ratio but decreased as the volume ratio increased, because of the low energy obtained from anoxic respiration. Proteobacteria were the predominant group, followed by Bacteroidetes. The influence of the anoxic condition on bacterial community structure was evaluated through correspondence analysis. The changes in bacterial community structure concurred with the reactor performance in terms of nitrogen removal efficiency and settleability. Samples with different settleability values were inclined to cluster into different groups.