The role of microbial diversity and composition in minimizing sludge production in the oxic-settling-anoxic process

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.