An on-line NAD(P)H fluorometer has been used to investigate the fluorescence profile of a simulated anaerobidanoxidoxic biological nutrient removal (BNR) process and its dependency on operating parameters, such as the concentrations of organic substrate (acetate) and activated sludge. As expected from the different rates of NAD-(P)H oxidation a t different stages, the order of fluorescence levels was found to be anaerobic > anoxic > oxic. The effects of the acetate and sludge concentrations were observed most clearly at the anoxic stage: a n increase in either concentration led to faster recovery of fluorescence from the drop caused by Nos-addition. A higher acetate concentration also resulted in a more significant, continuous rise in fluorescence, corresponding well with the enhanced Pod3release, a t the anaerobic stage and a slower initial fluorescence decrease at the oxic stage, indicating the dependency of the decrease on substrate availability. The ratio of the two fluorescence drops caused by Nos-addition and aeration, Al/Az, obtained with the plant's sludge was found to be 0.7 0.1, which is similar to those of pure nitrate-respiring cultures (e.g., Pseudomonas aeruginosa and Escherichia coli). The dominant majority of nonnitrifiers in the sludge therefore are capable of nitrate respiration, and the nitrifiers either contribute little to the overall fluorescence or perform nitrate respiration as the rest of the sludge population.
The effect of interfacial surfactant molecules on oxygen transfer through oil/water phase boundary has been studied in FlurO(2) (TM) emulsions, i.e., perfluorocarbon (PFC) emulsions developed as oxygen carriers in cell culture. Measurements of oxygen permeability were made with a polarographic oxygen electrode in pure PFCs and in emulsions with various PFC volume fractions. Comparison of the experimental results with the theoretically derived values of relative oxygen permeability clearly indicates that the mass transfer resistance caused by the interfacial surfactant layer in PFC emulsions is insignificant. Therefore, oxygen dissolved in the enclosed PFC phase is readily available to cells growing in the aqueous media and FlurO(2) emulsions with very fine emulsion particles (< 0.2 microm) can be used to effectively enhance gas/liquid interfacial oxygen transfer in bioreactors. The inadequacy in describing mass transfer in heterogeneous systems, such as the PFC emulsions, by conventional concentration-based oxygen diffusion coefficients has also been discussed.
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