LONG-TERM IMPACT OF DISSOLVED O-2 ON THE ACTIVITY OF ANAEROBIC GRANULES

The impact of influent dissolved O-2 On the characteristics of anaerob ic granular sludge was investigated at various dissolved O-2 concentra tions (0.5-8.1 ppm) in 1- and 5-L laboratory-scale upflow anaerobic sl udge bed (UASB)-like anaerobic/aerobic coupled reactors with a synthet ic wastewater (carbon sources containing 75% sucrose and 25% acetate). The rate of dissolved O-2 supplied to the coupled reactor was as high as 0.40 g O-2/L(rx). d, and the anaerobic/aerobic coupled reactors ma intained excellent methanogenic performances at a COD loading rate of 3 g COD/L(rx). d even after the reactors had been operated with dissol ved O-2 for 3 months. The activities of granular sludge on various sub strates (glucose, propionate, and hydrogen) were not impaired, and ace tate activity was even improved over a short term. However, after 3 mo nths of operation, slight declines on the acetoclastic activities of g ranules were observed in the coupled reactor receiving the recirculate d fluid containing 8.1 ppm dissolved O-2. Methane yield in the anaerob ic control reactor and anaerobic/aerobic coupled reactors revealed tha t a significant aerobic elimination (up to 30%) of substrate occurred in the coupled reactors, as expected. The presence of dissolved O-2 in the recirculated fluid resulted in the development of fluffy biolayer s on the granule surface, which imposed a negative impact on the settl eability of granular sludge and caused a slightly higher sludge washou t. This research shows that the anaerobic/aerobic coupled reactor can be successfully operated under O-2-limited conditions and is an ideal engineered ecosystem integrating oxic and anaerobic niches. (C) 1996 J ohn Wiley & Sons, Inc.