Abstract
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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.