| Abstract | The impact of influent dissolved O2 on the characteristics of anaerobic granular sludge was investigated at various dissolved O2 concentrations (0.5–8.1 ppm) in 1- and 5-L laboratory-scale upflow anaerobic sludge bed (UASB)-like anaerobic/aerobic coupled reactors with a synthetic wastewater (carbon sources containing 75% sucrose and 25% acetate). The rate of dissolved O2 supplied to the coupled reactor was as high as 0.40 g O2/Lrx·d, and the anaerobic/aerobic coupled reactors maintained excellent methanogenic performances at a COD loading rate of 3 g COD/Lrx·d even after the reactors had been operated with dissolved O2 for 3 months. The activities of granular sludge on various substrates (glucose, propionate, and hydrogen) were not impaired, and acetate activity was even improved over a short term. However, after 3 months of operation, slight declines on the acetoclastic activities of granules were observed in the coupled reactor receiving the recirculated fluid containing 8.1 ppm dissolved O2. Methane yield in the anaerobic control reactor and anaerobic/aerobic coupled reactors revealed that a significant aerobic elimination (up to 30%) of substrate occurred in the coupled reactors, as expected. The presence of dissolved O2 in the recirculated fluid resulted in the development of fluffy biolayers on the granule surface, which imposed a negative impact on the settleability of granular sludge and caused a slightly higher sludge washout. This research shows that the anaerobic/aerobic coupled reactor can be successfully operated under O2-limited conditions and is an ideal engineered ecosystem integrating oxic and anaerobic niches. |
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