Abstract
The oxic-settling-anoxic (OSA) process, which involves an aerobic tank attached to oxygen- and substrate-deficient 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 on microbial diversity. Generally, reactors with lower oxidation-reduction potential had higher microbial diversity. The main aerobic sequencing batch reactor of OSA (SBROSA) that interchanged sludge with 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 cellular matter, 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.
Highlights
The management of excess sludge constitutes a significant fraction of the total operation cost of biological wastewater treatment
The current study provides a microecological perspective on the mechanism of sludge autolysis in the external reactors: bacteria such as β- and γ-Proteobacteria decrease in the external reactor, thereby producing materials that can be metabolized by hydrolyzing and fermentative bacteria for cell maintenance
principal coordinate analysis (PCoA) of unweighted Unifrac distances demonstrated that redox condition was the most important factor affecting microbial diversity
Summary
The management of excess sludge constitutes a significant fraction (up to 60%) of the total operation cost of biological wastewater treatment. Ocean-dumping and land-filling were the traditional means of disposing of sludge; the former has been banned to protect marine life and while the latter has been restricted due to the high cost of landfill operation Current practices, such as sludge incineration and re-use of sludge as landapplicable biosolids, have some inherent disadvantages. Re-using sludge enables the recovery of organic matter and nutrients, but the conversion of sludge into high-quality biosolids that can be safely used in agricultural applications and the transport of biosolids from metropolitan facilities to farmlands are expensive (Semblante et al, 2014; Tchobanoglus et al, 2003) These concerns emphasize the need to minimize sludge production. The full-scale implementation of these approaches are hindered because they either require significant capital and operating cost or only result in a marginal sludge reduction (Foladori et al, 2010)
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