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Tertiary Nitrification Using Membrane Aerated Biofilm Reactors: Process Optimization, Characterization and Model Development

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Title: Tertiary Nitrification Using Membrane Aerated Biofilm Reactors: Process Optimization, Characterization and Model Development
Author: Long, zebo
Department: School of Engineering
Program: Engineering
Advisor: Zhou, Hongde
Abstract: Membrane aerated biofilm reactor (MABR) process allows higher oxygen transfer efficiencies than conventionally aerated processes by employing a gas-permeable membrane for oxygen to diffuse directly into attached biofilm for pollutants oxidation. However, unsolved problems, such as the difficulties in maintaining an optimum biofilm thickness and lack of scale up rules, have hampered its engineering. MABR process for tertiary nitrification holds promise partly because excessive biofilm growth might be avoided. Previous studies have demonstrated effective nitrification in lab-scale reactors treating artificial wastewaters. To address the effects of real secondary effluents, two identical pilot-scale MABRs treating ammonia-supplemented secondary effluent and tap water were examined under similar operational conditions and optimized at three Reynolds numbers. Microbial distribution in biofilms under the optimized conditions was then characterized by fluorescence in-site hybridization (FISH) and confocal laser scanning microscopy (CLSM). Lastly, a mechanistic oxygen transfer rate (OTR) model was developed and evaluated using a bench-scale MABR. Results from pilot studies demonstrated for the first time the feasibility of tertiary nitrification using MABR to treat secondary effluent. Organic carbon in secondary effluent had facilitated a quick formation of biofilm. In contrast, artificial wastewater lack of organic carbon required a reduced mixing to promote initial biomass retention. Equivalent nitrification rates were eventually achieved under different mixing conditions, suggesting that wastewater sources affect the optimum operational conditions. The nitrifying biofilms in both reactors were found to have enhanced the oxygen transfer rates as compared with those of clean membrane modules. The high ratios of nitrifiers to all bacteria in the biofilms confirmed the potential for high-rate nitrification in MABRs. Moreover, it was found that four main genera of nitrifiers coexisted with spatial variations in both reactors. The spatial variations were caused by the oxygen partial pressure gradient along the hollow fibres that had created variable microenvironments for the selective growth of nitrifiers. The proposed mechanistic OTR model accurately predicts OTRs with the measured total mass transfer coefficients (KT), but underestimates OTRs with the estimated KT. The error in estimating membrane resistance was identified as the cause for the underestimated OTRs. A “true” membrane resistance is therefore suggested for model calibration.
Date: 2013-08
Rights: Attribution-ShareAlike 2.5 Canada

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