Hollow-Fiber Membrane Bioreactor for Nitric Oxide Removal

Abstract

This research evaluates the use of a hollow-fiber membrane bioreactor (HFMB) to remove nitric oxide (NO) from combustion gases. In the HFMB, gases containing NO pass through the lumen of microporous hydrophobic hollow-fiber membranes. NO diffuses through membrane pores and partitions into a nitrifying biofilm where it is oxidized to NO₃- and other products. The hollow-fiber membranes serve as a support for the microbial population and provide a large surface area for NO and oxygen mass transfer. In this project, nitrifying bacteria were enriched from activated sludge with a liquid NH₄+/NO₂- feed. After acclimation, an NO/air mixture was introduced to the HFMB at room temperature. NO removal and nitrification rates were investigated at varying liquid velocities. A maximum removal efficiency of 74% was attained at a liquid velocity of 1.5 cm s^-1. The effects of gas composition and temperature on mass transfer and biodegradation were investigated using a synthetic combustion gas (15% CO₂, 5% O₂, 75% N₂, 100 ppm NO) at temperatures between 20°C and 55°C. NO removal efficiency was maintained at approximately 70% regardless of gas composition or temperature. Only 9 to 12% of the eliminated nitrogen was recovered as nitrate nitrogen in the effluent. This may have been due to concurrent nitrification/denitrification using an endogenous carbon source.