Anaerobic Digesters at WWTPs: factors affecting stability and community composition revealed by time series analyses.

Introduction: Anaerobic digestion (AD) at wastewater treatment plants (WWTPs) is carried out by groups of microorganisms, active in complex metabolic networks. Recent studies of community structure have shown that there are two large fractions, growing and non-growing, the latter being up > 40% of the bacteria, all immigrating from influent and surplus activated sludge (Jiang et al., 2021). Therefore, it is important to focus on the growing fraction to better understand ecology and ecophysiology of process-critical species.

Aim: The aim of this study was to describe community composition and stability in mesophilic and thermophilic AD, and to investigate the fate of specific growing species during a change from mesophilic to thermophilic conditions by using MiDAS database, which has collected timeseries for >5 years.

Methodology: AD samples were obtained every two weeks at Danish WWTPs from 2015-2019. Bacteria and archaea were identified by 16S rRNA gene amplicon sequencing (Karst et al., 2016; Kirkegaard et al., 2017). MiDAS 3 taxonomy was used with species-level resolution (Nierychlo et al., 2020).

Results & Conclusions: Microbial communities at species-level showed clear differences between AD reactor types. Non-growing taxa, such as Microthrix and Tetrasphaera, were only present because they were continuously added by food streams. Between growing species, almost only uncharacterised genera and species were among most abundant in both mesophilic and thermophilic ADs. For all reactors, a very high level of community stability was observed, with only minor changes over the 5 years period. However, this was not the case with non-growing fraction, which showed large fluctuations. A detailed study of an AD at Aalborg East WWTP, which was perturbated from mesophilic to thermophilic conditions, showed clear changes in community. This concludes importance of focus on growing bacteria fraction when investigating AD, and importance of temperature changes on top abundant bacteria.