Type of Document Dissertation Author Gilmore, Kevin R Author's Email Address email@example.com URN etd-08182008-093427 Title Treatment of High-Strength Nitrogen Wasetewater With a Hollow-Fiber Membrane-Aerated Biofilm Reactor: A Comprehensive Evaluation Degree PhD Department Civil Engineering Advisory Committee
Advisor Name Title Love, Nancy G. Committee Chair Garland, Jay L. Committee Member Little, John C. Committee Member Smets, Barth F. Committee Member Stevens, Ann M. Committee Member Keywords
- mass transfer
- nitric oxide
- nitrous oxide
- nitrite oxidizing bacteria
- ammonia oxidizing bacteria
- hollow fiber membrane aerated biofilm reactor
- wastewater treatment
- anaerobic ammonia oxidation
Date of Defense 2008-08-04 Availability unrestricted AbstractProtecting the quality and quantity of our water resources requires advanced treatment technologies capable of removing nutrients from wastewater. This research work investigated the capability of one such technology, a hollow-fiber membrane-aerated biofilm reactor (HFMBR), to achieve completely autotrophic nitrogen removal from a wastewater with high nitrogen content.
Because the extent of oxygenation is a key parameter for controlling the metabolic
processes that occur in a wastewater treatment system, the first part of the research
investigated oxygen transfer characteristics of the HFMBR in clean water conditions and
with actively growing biofilm. A mechanistic model for oxygen concentration and flux as
a function of length along the non-porous membrane fibers that comprise the HFMBR
was developed based on material properties and physical dimensions. This model reflects
the diffusion mechanism of non-porous membranes; namely that oxygen follows a sorption-dissolution-diffusion mechanism. This is in contrast to microporous membranes in which oxygen is in the gas phase in the fiber pores up to the membrane surface, resulting in higher biofilm pore liquid dissolved oxygen concentrations. Compared to offgas oxygen analysis from the HFMBR while in operation with biofilm growing, the model overpredicted mass transfer by a factor of approximately 1.3. This was in contrast to empirical mass transfer coefficient-based methods, which were determined using either bulk aqueous phase dissolved oxygen (DO) concentration or the DO concentration at the
membrane-liquid interface, measured with oxygen microsensors. The mass transfer coefficient determined with the DO measured at the interface was the best predictor of actual oxygen transfer under biofilm conditions, while the bulk liquid coefficient underpredicted by a factor of 3. The mechanistic model exhibited sensitivity to
parameters such as the initial lumen oxygen concentration (at the entry to the fiber) and
the diffusion coefficient and partitioning coefficients of oxygen in the silicone membrane
material. The mechanistic model has several advantages over empirical-based methods.
Namely, it does not require experimental determination of KL, it is relatively simple to
solve without the use of advanced mathematical software, and it is based upon selection
of the membrane-biofilm interfacial DO concentration. The last of these is of particular
importance when designing and operating HFMBR systems with redox (aerobic/anoxic/anaerobic) stratification, because the DO concentration will determine the nature of the microenvironments, the microorganisms present, and the metabolisms that occur.
During the second phase of the research, the coupling of two autotrophic metabolisms,
partial nitrification to nitrite (nitritation) and anaerobic ammonium oxidation, was
demonstrated in a single HFMBR. The system successfully treated a high-strength nitrogen wastewater intended to mimic a urine stream from such sources as extended space missions. For the last 250 days of operation, operating with an average oxygen to ammonia flux (JO2/JNH4+) of 3.0 resulted in an average nitrogen removal of 74%, with no external organic carbon added. Control of nitrite-oxidizing bacteria (NOB) presented a challenge that was addressed by maintaining the JO2/JNH4+ below the stoichiometric threshold for complete nitrification to nitrate (4.57 g O2 / g NH4
+). The DO-limiting condition resulted in formation of harmful gaseous emissions of nitrogen oxides (NO, N2O), which could not be prevented by short-term control strategies. Controlling JO2/JNH4+ prevented NOB proliferation long enough to allow an anaerobic ammoniaoxidizing bacteria (AnaerAOB) population to develop and be retained for >250 days. Addition of a supplemental nutrient solution may have contributed to the growth of AnaerAOB by overcoming a possible micronutrient deficiency. Disappearance of the gaseous nitrogen oxide emissions coincided with the onset of anaerobic ammonium oxidation, demonstrating a benefit of coupling these two autotrophic metabolisms in one
reactor. Obvious differences in biofilm density were evident across the biofilm depth, with a region of low density in the middle of the biofilm, suggesting that low cell density or exocellular polymeric substances were primarily present in this region, Microbial community analysis using fluorescence in situ hybridization (FISH) did not reveal consistent trends with respect to length along the fibers, but radial stratification of aerobic ammonia-oxidizing bacteria (AerAOB), NOB, and AnaerAOB were visible in biofilm section samples. AerAOB were largely found in the first 25% of the biofilm near the membrane, AnaerAOB were found in the outer 30%, and NOB were found most often in the mid-depth region of the biofilm. This community structure demonstrates the importance of oxygen availability as a determinant of how microbial groups spatially distribute within an HFMBR biofilm.
The combination of these two aspects of the research, predictive oxygen transfer capability and the effect of oxygen control on performance and populations, provides a foundation for future application of HFMBR technology to a broad range of wastewaters and treatment scenarios.
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