Characterization and Modeling of Fundamental Parameters of a Membrane-Aerated Biological Reactor

The expansion of human presence in space necessitates a reliable, sustainable water source. Current water recovery systems rely on physical and chemical methods that require resupply and also create hazardous end products. Membrane aerated biological reactors (MABRs) provide an efficient and sustainable alternative process for treating a space-based waste stream. The undertaken research examined the relationships between gas flow, loading rate, and membrane density through the use of bench and full scale MABRs and the development and calibration of theoretical models. The completion of this research resulted in the essential characterization of MABR systems and the subsequent development of theoretical modeling equations that can be used as predictive tools. The results of the research were centered around several areas: specific surface area, gas transfer, system performance and inhibition, and integration of experimental results into development of theoretical model. Specific Surface Area: Gas transfer, biomass thickness, and surface loading are all directly influenced by specific surface area. The optimal specific surface area provides the required oxygen transfer without creating issues with long term operation due to hydraulic short circuiting or failure due to biofilm channeling, bridging, or clogging. The results of the research assessed a range of specific surface areas; however the 100 m2/m3 specific surface area was able to maximize reaction rates with the least density. Gas Transfer: Biological treatment involves the production of gases including CO2, N2, and others. Since two phase flow is problematic in microgravity environments, it must be prevented either through pressurization or sufficient gas stripping to prevent super saturation. In addition to providing oxygen transfer to the biofilm, the membranes also allow efficient gas stripping, due to the low partial pressures of the biogenic gases in the lumen. The choice of lumen gas: air, O2, or a blend of the two, is also an important parameter in system design. The research verified operation with air, O2, and a blend and assessed the impact of oxygen partial pressure on system performance. Another important result of the work was the analysis of the potential inhibitory impact of increased partial pressure of O2 and CO2 due to pressurized operation. In summary O2 partial pressure above 1.4 atm illustrated an inhibition of both nitrification and carbon oxidation, however, no impact was seen from elevated CO2 partial pressure up to 0.3 atm. System Performance: Understanding the fundamental performance of the MABR systems treating the space based waste stream allows for future implementation of the MABR into the space based wastewater architecture. Based on this work, treatment efficiencies of >90% carbon oxidation and 60% nitrification can be expected at a 3 day hydraulic retention time (θ). Volumetric conversion rates of 196 g-C/m3-d and 194 g-N/m3-d suggest that 0.05 m3 per crew member will be required for 70% conversion of NHx. Another important result of the research was the assessment of stable nitrite accumulation within the reactor systems. Assessment of the potential factors contributing to the inhibition of nitrite oxidation led to the determination of free nitrous acid concentrations as the most probable cause of the inhibition. These results also provide potential for further terrestrial applications of the technology as the reactor effluent has a favorable composition of NHx and NO2- for potential integration with the anammox process, which can result in significant oxygen and carbon savings in total nitrogen removal processes. Theoretical Model: The experimental results were incorporated into the development of the kinetic and rate expressions for a theoretical model within the AQUASIM platform. The assessment of the nitrite oxidation inhibition led to the iteration of the model to include terms to account for pH and carbonate system and nitrite oxidation inhibition.