A detailed analysis of the mechanisms controlling the acceleration of 2,4-DCP monooxygenation in the two-tank suspended growth process

The mechanisms underlying the observed acceleration of monooxygenation reactions in two-tank accelerator/aerator suspended growth system are evaluated in detail. The accelerator tank is characterized by a very high electron flow through reduced nicotinamide adenine dinucleotide (NADH + H⁺), particularly when the retention-time ratio is small. Only a small fraction of the electron flow was diverted to oxygenation reactions, and the major sinks of NADH + H⁺ were respiration and biomass synthesis. The main producer of NADH + H⁺ is oxidation of acetate, a rapidly degraded electron-donor substrate. The half-maximum-rate concentration for oxygen used in respiration was 0.03 mg/L, while the half-maximum-rate concentration for oxygen used as a cosubstrate in monooxygenation was 0.18 mg/L. Thus, monooxygenations were more sensitive to oxygen limitation than was respiration. The NADH + H⁺ concentration had a direct effect on the monooxygenation kinetics. The rate coefficients for both monooxygenation reactions were directly proportional to the specific growth rate in the accelerator, which supports that the accelerator tank caused an up-regulation of the monooxygenase content. Because the rate coefficients in the aerator tank were much larger than in the one-tank system, even though the specific growth rates were nearly the same, monooxygenases may have carried over from the accelerator tank to the aerator tank. Its higher concentration of 2,4-dichlorophenol (2,4-DCP) and the higher specific growth rate were the main reasons why the accelerator had faster kinetics for 2,4-DCP utilization than did the aerator tank. The apparently higher levels of monooxygenase in both tanks of the two-tank system also appears be a primary reason why its performance was substantially superior to that of the one-tank system in terms of 2,4-DCP removal.