The oxic-settling-anoxic (OSA) process, which involves an aerobic tank attached to oxygen- and substratedeficient external anoxic reactors, minimizes sludge production in biological wastewater treatment. In this study, the microbial community structure of OSA was determined. Principal coordinate analysis showed that among the three operational factors, i.e., (i) redox condition, (ii) external reactor sludge retention time (SRText), and (iii) sludge interchange between aerobic and anoxic reactors, redox condition had the greatest impact onmicrobial diversity.Generally, reactorswith lower oxidation-reduction potential had highermicrobial diversity. The main aerobic sequencing batch reactor of OSA (SBROSA) that interchanged sludgewith an external anoxic reactor had greater microbial diversity than SBRcontrol which did not have sludge interchange. SBROSA sustained high abundance of the slow-growing nitrifying bacteria (e.g., Nitrospirales and Nitrosomondales) and consequently exhibited reduced sludge yield. Specific groups of bacteria facilitated sludge autolysis in the external reactors. Hydrolyzing (e.g., Bacteroidetes and Chloroflexi) and fermentative (e.g., Firmicutes) bacteria, which can break down cellularmatter, proliferated in both the external aerobic/anoxic and anoxic reactors. Sludge autolysis in the anoxic reactor was enhanced with the increase of predatory bacteria (e.g., order Myxobacteriales and genus Bdellovibrio) that can contribute to biomass decay. Furthermore, β- and γ-Proteobacteria were identified as the bacterial phyla that primarily underwent decay in the external reactors.
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