MICROBIAL SUCCESSION IN ACTIVATED SLUDGE: ECOLOGICAL PRINCIPLES LINK COMMUNITY DIVERSITY AND OPERATING PERFORMANCE

Heterotrophic biomass in activated sludge wastewater treatment systems often is described according to the kinetics of substrate utilization. Typically, two “groups” of microorganisms are described, namely floc-formers and filaments. Previous studies have attempted to correlate bioreactor operation with the relative abundance of these two “groups” of microorganisms. For example, plug flow bioreactors with high substrate concentrations have been shown to favor floc-formers (e.g., Monod-type r-strategists) while completely mixed bioreactors with low substrate concentrations have been shown to favor filaments (e.g., Monod-type K-strategists). The objective of this study was to determine if additional factors impact the diversity of the microbial community in activated sludge bioreactors. To meet this objective, we have begun to systematically explore correlations between the relative abundance of representative model microorganisms and bioreactor operating strategies. For these studies, fluorescence in situ hybridizations with 16S rRNA-targeted oligonucleotide probes have been used to identify and enumerate Acinetobacter spp. in samples from laboratory-scale activated sludge wastewater treatment systems. Preliminary results suggest that long idle stages, as would be expected in secondary clarifiers operated with significant sludge blankets, correlate with an elevated relative abundance of Acinetobacter spp. This elevated relative abundance appears to be independent of the initial concentration of growth limiting substrate in the bioreactors (i.e., long idle stages result in selective enrichment of Acinetobacter spp. in both plug flow and completely mixed bioreactors). Based upon these results, we hypothesize that the inclusion of long idle stages in bioreactor operation provides a selective pressure that dictates the composition of the microbial community independent of the initial concentration of growth limiting substrate. These results have broad implications for biological wastewater treatment – suggesting that the current two “groups” of heterotrophic microorganisms may be insufficient to capture the essential diversity of microorganisms in activated sludge bioreactors.