Effect of dissolved oxygen on ammonia-oxidizing bacterial communities in activated sludge.

摘要

The hypothesis that dissolved oxygen (DO) concentration is a factor affecting ammonia-oxidizing bacterial (AOB) community structure in activated sludge was tested in several ways. First, two full-scale wastewater treatment plants (WWTPs) were operated with significantly different DO concentrations and the AOB communities were compared using amoA sequence analyses. Second, chemostats were operated with high-DO (8.5 mg/L) and low-DO (0.24 and 0.12 mg/L), and AOB communities were monitored using several molecular techniques. The results from the full-scale and chemostat experiments demonstrated that DO concentrations indeed exerted an ecological pressure that resulted in the establishment of different AOB communities, but a direct correlation between AOB phylogeny and DO could not be established. Additionally, it was found that stable nitrification can be accomplished at DO concentrations as low as 0.12 mg/L and that low-DO selects for AOB having high affinity for oxygen. Two AOB strains were isolated from low-DO chemostats and further studied. Strain ML1 belonged to the Nitrosomonas europaea lineage and showed high affinity for oxygen, while strain NL7 belonged to the Nitrosomonas europaea lineage and showed high affinity for ammonia. The kinetic characterization of these strains was achieved using pure-culture batch experiments plus a co-culture experiment with the two strains sequentially exposed to high- and low-DO conditions. The results from the kinetic analyses suggested the possibility of physiological changes in oxygen affinity when the strains were exposed to different DO conditions. Finally, as a practical evaluation of the effect of low DO operation, the possibility of reducing oxygen supply in a full-scale enhanced biological phosphorus removal plant was evaluated. The retrofitted plant had a modest increase in total nitrogen removal (from 54% to 65%) and favored the establishment of AOB belonging to the Nitrosomonas europaea lineage, but phosphorus removal was not compromised by having DO concentrations below 1.0 mg/L throughout the aerobic stage, a result that suggests a practical way of reducing operational costs of nutrient removal wastewater treatment plants.