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Biomass Density Function Correlations in Suspended Growth Activated Sludge Process
Civil and Environmental Engineering
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Density is an important factor that affects activated sludge performance in suspended growth biological wastewater treatment process, but its importance has been neglected for a long time. The density of microorganisms has been proven to being related to sludge settleability and nutrient removal efficiency, however, the research about the density-related microbial abundance of functional bacteria is yet to be investigated. That is the focus of research in this dissertation. This research found that different water resource recovery facilities (WRRFs) with different functions, for example, enhanced biological phosphorus removal and ammonia removal, have large different density distribution pattern among microbial groups present in activated sludge. Previous research has proved higher density of Nitrobacter winogradskyi and polyphosphate accumulating organisms (PAOs) in activated sludge compared to other functional groups. In this research, four main functional microbial groups were quantified in the denser and lighter layers of activated sludge and investigated as how the density of microorganisms affected the community structure. The activated sludge samples were obtained from two different WRRFs with different process functions. According to quantitative-PCR analysis, it was found that in high abundance prokaryotes, like PAOs and Nitrobacter spp., more microorganisms were accumulated in a denser layer of activated sludge in a plant that employed enhanced biological phosphorus removal (EBPR) in the activated sludge process. However, similar observations were not true in the oxidation ditch activated sludge employing nitrification-denitrification without EBPR function. The quantification difference based on microbial density in low abundance prokaryotes was not significant in activated sludge from both plants studied. This means that the low abundance microbial groups were hardly causing a significant effect on biomass density, and hence no big quantifying difference. By the examination of activated sludge using fluorescence in situ hybridization (FISH), it was found that the floc structure is a major reason for variation in abundance of different groups in lighter and heavier layers. For instance, biomass from a non-EBPR oxidation ditch activated sludge process had more filamentous bacteria with large pore sizes (loose structure), and hence the target prokaryotes could have been more scattered. A potential reason is that deflocculation occurred because of such loose structure in biomass, and hence more free cells floated up in a lighter layer causing higher relative abundance. Besides target functional groups, the samples were investigated by sequencing 16S rRNA gene for the study of microbial community structure. The results confirmed that density-based separation employed in this research is a feasible method to characterize the change of microbial community compositions of high abundance functional groups such as PAOs. A significant difference in alpha diversity from richness study was confirmed in lighter and denser biomass fraction. The method developed during this research can also help to distinguish the functional profiling in different biomass samples.