The aerobic biodegradation of quinoline, a two-ring nitrogen heterocycle, offers an outstanding example of when structured modeling including cosubstrates is required for a biofilm system. In this case, the cosubstrate is oxygen, which is used as a direct cosubstrate in oxygenase reactions and as a primary electron acceptor in respiration. Quinoline biodegradation is numerically simulated as occurring in five key steps, two of which involve oxygen as a direct cosubstrate. Modeling evaluation of experimental results from a laboratory-scale biofilm column shows that the oxygenation steps are much more sensitive to low oxygen concentrations than are steps in which oxygen only participates through respiration. The result of this differential oxygen sensitivity is that the first intermediate product, 2-hydroxyquinoline, builds up, because its degradation through an oxygenase reaction is slowed preferentially by oxygen depletion.

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