Oxygen Mass Transfer in a Flow-through Hollow-fiber Membrane Aeration Reactor
The increasing use of membrane-aerated biofilm reactors in wastewater treatment necessitates more accurate understanding of their oxygen transfer capabilities for design and operation purposes. The oxygen mass transfer in a hollow-fiber membrane-aerated biofilm reactor (HFMBR) was evaluated using conventional clean water testing, and results were compared to performance data and oxygen microsensor profiles when an active nitrifying biofilm was present on the fibers. Using a flow-through configuration (as opposed to sealed end fibers), conventional mass transfer testing predicted oxygen transfer efficiencies between 6% and 40%, depending on air flow rate through the fibers and lumen pressure. The mass transfer coefficient (average = 2.8 x10−4 cm/sec) was found to be largely independent of air flow rate but had a slight positive correlation with lumen pressure (correlation coefficient = 0.5), possibly due to radial deflection of the siloxane fibers. With an active nitrifying biofilm (predominantly nitritation), significantly higher oxygen transfer efficiencies were observed, between 40% and 62%, as measured by outlet gas oxygen concentration, ammonia consumption stoichiometry, and oxidized nitrogen production stoichiometry, all of which were in reasonable agreement. Oxygen microsensor profiles in active biofilm and on bare fibers suggest that one reason for this discrepancy is the reduced surface dissolved oxygen (DO) concentration at the membrane-biofilm interface. Bulk liquid DO measurements lead to an overestimation of the mass transfer driving force and, as a result, an underestimation of the mass transfer coefficient (KOL). These results demonstrate substantial limitations when applying conventional oxygen transfer testing methods (based on bulk liquid DO concentration) to design of membrane-aerated biofilm reactors.
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Document Type: Research Article
Publication date: 2007-10-01
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