BTX biodegradation in activated sludge under multiple redox conditions

Activated sludge sequencing batch reactors were used to study BTX biodegradation under anoxic (denitrifying), microaerobic, and aerobic conditions. Toluene and m-xylene were biodegraded under denitrifying conditions, and the loss of these compounds correlated with the activity of reducing enzymes that were capable of oxidizing methyl viologen. Although benzene. p-, and o-xylene were recalcitrant under anoxic treatment, all three were biodegraded under microaerobic [<0.2 mg/L dissolved oxygen (DO)] and nitrate or nitrite (NOx)-supplemented microaerobic conditions. Methyl viologen oxidation potential decreased under all microaerobic conditions while catechol 1,2-dioxygenase (C12O) and catechol 2,3-dioxygenase (C23O) were induced, indicating that the aromatic hydrocarbons were metabolized by aerobic pathways, even in the presence of NOx and in the absence of measurable DO. The degree of C12O and C23O expression under microaerobic conditions was comparable to levels found under aerobic (DO > 4 mg/L) conditions. Benzene, p-, and o-xylene were biodegraded twice as fast under NOx-supplemented compared to NOx-free microaerobic conditions, and specific biodegradation rates under aerobic and NOx-supplemented microaerobic conditions were comparable. Oxidation reduction potential successfully differentiated between the various electron acceptor conditions and proved to be a sensitive indicator.