Effect of Anoxic Selector Configuration on SVI Control and Bacterial Population Fingerprinting

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Courtesy of Water Environment Federation (WEF)


Anoxic selectors are used in activated sludge systems prior to the aerobic zone to favor floc-forming bacteria growth over filamentous bacteria, which cause poor settling characteristics. However, anoxic selector may not successfully limit the growth of all filamentous organisms, including Microthrix parvicella, Type 0041, Nostocoida limicola II, and Type 1851. Anoxic selectors are expected to be very effective for the removal of readily biodegradable COD (rbCOD) for filamentous control. But there is some concern about the removal efficiency of slowly biodegradable COD (sbCOD) in anoxic selectors and its impact on filamentous growth. This study compared sludge settling characteristics, filamentous microorganism populations, and microbial community fingerprinting in three different anoxic selector configurations:
single-stage, 4-stage, and anoxic- aerobic sequencing batch reactor (SBR) fed with both rbCOD (acetate) and sbCOD (dextrin). The total volume of the anoxic single-stage and 4-stage selectors were equal. Diluted sludge volume index (DSVI) and microscopic observation indicated that better sludge settleability occurred with increased selector staging: S1 to S4 to SBR. Higher hydrolysis and uptake rates of dextrin in the staged selectors appeared to contribute to the better sludge settleability. Bacteria community composition, based on intergenic transcribed spacer (ITS) region of ribosomal DNA, was analyzed by automated ribosomal intergenic spacer analysis (ARISA). The composition in the 4-staged selector system was found to be more similar to SBR than were that for single-staged selector.

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