Microbial Community Assembly during Aerobic Granulation

Licentiate thesis, 2017

Environmental deterioration together with the increasingly restrictive legislation of water quality standards have led to a demand for compact, efficient and less energy consuming technologies for wastewater treatment. Aerobic granular membrane bioreactors (AGMBRs) combine the advantages of aerobic granular sludge and membrane filtration. Although recent studies have shown that AGMBRs are promising, research is needed to develop these systems. One important aspect is the development and stability of aerobic granules in the reactor. The objective of this thesis is to unravel the ecological mechanisms behind the granulation process and the resulting granular structure. The impact of the washout dynamics applied in sequencing batch reactors (SBRs) on the bacterial community, the microbial spatial organization in granules and the reproducibility of the granulation process were analyzed. High throughput sequencing was used to analyze the microbial community structure and dynamics during granulation. Fluorescence in-situ hybridization (FISH) analysis was performed to study the microbial spatial distribution. Results showed that the bacterial community composition of granules and the biomass in the effluent from three parallel SBRs subjected to high washout conditions and different organic loading was very similar but not identical. Retention ratios predicted the spatial location of the taxa in the granules. Bacteria having lower relative abundance in the effluent were located in the granular interior whereas bacteria growing on the granular surface were more susceptible to erosion. Using FISH analyses of intact granules, ammonia oxidising bacteria (AOB) were found in the inner locations of the granules indicating that both oxygen and ammonia were transported across/into the granule allowing aerobic metabolism also in the interior. Interestingly, the predatory bacteria Bdellovibrio sp. was found inside the granules and seemed to be preferentially attacking AOB. Moreover, the reproducibility analysis of three replicate SBRs showed some statistical differences in the reactor performance and in the microbial community diversity and structure. Both deterministic and stochastic processes seemed to be involved in the microbial community assemblage during granulation. Biodiversity decreased due to habitat specialization and competitive exclusion during the acclimatization of the sludge to the reactor conditions. When the washout dynamics were stronger the community assemblage was niche-oriented and once the selection pressures decreased, stochastic processes became more evident, especially for intermediate and rare species. Acinetobacter sp., Comamonadaceae and Rhodocyclaceae families together with the polymer producers Thauera sp., Flavobacterium sp. and the family Xanthomonadaceae seemed to have important roles during the first stages of granulation.