Composition and dynamics of the bacterial community in aerobic granular sludge reactors

Doktorsavhandling, 2017

The aerobic granular sludge (AGS) technology is probably the future standard for wastewater treatment, due to its low footprint and low energy consumption. Although achieving granulation is usually not a challenge anymore, our understanding of the community assembly during start-up, and of the microbial ecology of these reactors in general, is incomplete. Earlier studies have shown that high removal efficiency and stable process performance in bioreactors are dependent on the microbial community composition. High functional redundancy, which is a result of high evenness and phylogenetic variability, was found to be the key to a resilient bioreactor. The research presented in this thesis aimed to expand on the current knowledge about the composition and dynamics of the bacterial community in AGS reactors, using molecular biology techniques including qPCR, T-RFLP and Illumina MiSeq. The harsh wash-out conditions, typical for the start-up of AGS reactors, were found to drastically decrease the abundance of nitrifiers. A stepwise decrease of settling time enabled better retention of nitrifying organisms, but seemed to have no long term effects on the general composition of the community. The community assembly was affected mainly by deterministic factors – e.g. short settling time – during start-up, and the stochastic components became evident only when the strong selection pressure decreased. Reactors with different operational parameters were found to be dominated by different taxa, but high functional redundancy was observed within all key guilds in all reactors. Certain functional groups – denitrifiers, EPS and PHA producers – were over-represented in granular sludge compared to the flocculated seed sludge. The typical AGS reactor operation seems to favor these traits, but whether these are necessary for successful granulation and reactor operation depends on the intended function of the reactor. At present, microbial ecology studies – including our experiments – on AGS processes are predominantly of descriptive nature. However, it was suggested that in the future microbial ecology may provide a tool to predict or even design diversity and thus process stability. The increasing availability of next generation sequencing methods allows us to study the rare biosphere in AGS reactors and gain insights in how we can integrate ecology in biotechnology.