Evaluating the effect of dissolved oxygen on ammonia-oxidizing bacterial communities in activated sludge

The effect of dissolved oxygen (DO) on the communities of ammonia-oxidizing bacteria (AOB) in activated sludge was evaluated in lab-scale and full-scale reactors using the amoA gene as the basis for phylogenetic comparisons. Under controlled laboratory conditions, two chemostats seeded with activated sludge from the same source were operated with high-DO (8.5 mg/L) and low-DO (0.24 and 0.12 mg/L) concentrations for a period of 300 days. At the end of the operation period, the chemostats had enriched AOB communities that belonged to the Nitrosomonas europaea lineage, but were differentiable based on phylogenetic and kinetic analyses. The low-DO chemostat harbored the growth of two different groups within this lineage, differentiable by the amoA sequence comparison and by terminal fragment signatures. The difference in oxygen affinity between high-DO and low-DO enrichments was demonstrated by evaluating the growth kinetics as a function of oxygen concentration. The low-DO enrichment had a higher growth rate at DO concentrations below 4.7 mg/L, but the growth rate significantly decreased at higher DO concentrations, for which the high-DO enrichment experienced higher growth rates. In addition, the dynamic changes in AOB populations in two parallel trains within one full-scale treatment plant were evaluated in response to a significant reduction of DO in one of the treatment trains. Only the train operated with DO concentrations below 1 mg/L favored the establishment of a population of AOB related to the N. europaea lineage.