Abstract
Abstract A low operational dissolved oxygen (DO) concentration in activated sludge improves oxygen transfer efficiency. However, it also can promote the growth of filamentous microorganisms that adversely affect sludge settling. In this study, filamentous microorganisms were found to additionally impede oxygen transfer; a previously unrecognized problem. We found that, when the operational DO of the complete-mix activated sludge reactor was reduced from 2.0 to 0.5 mg/L, the improvement in the oxygen transfer efficiency (OTE) was less than expected. Further investigation revealed that the change in OTE was highly correlated to the abundance of filamentous microorganisms and the excessive growth of filamentous microorganisms could reduce the OTE by 50%, even under the same operational DO condition. It was hypothesized that filamentous microorganisms impeded oxygen transfer mainly by increasing the mixed liquor viscosity due to their long filaments and large hydrodynamic volume. The results of this study imply that the formation of filamentous microorganisms in activated sludge process must be controlled for improving aeration efficiency and more studies are needed to better understand the effect of microbe types in activated sludge on the oxygen transfer.
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