Redox Control Bioreactor : A Unique Biological Water Processor

The redox control bioreactor (RCB) is a new hollow fiber membrane bioreactor (HFMBR) design in which oxygen and hydrogen gases are provided simultaneously through separate arrays of juxtaposed hollow fiber (HF) membranes. This study applied the RCB for completely autotrophic conversion of ammonia to N2 through nitrification with O2 and denitrification using hydrogen as an electron donor (i.e., autohydrogentrophic denitrification). The hypothesis of this research was that efficient biofilm utilization of O2 and H2 at respective HFs would limit transport of these gases to bulk fluid, thereby enabling completely autotrophic ammonia conversion to N2 through the co-occurrence of ammonia oxidation (O2–HF biofilms) and autohydrogenotrophic denitrification (H2–HF biofilms). A prototype RCB was fabricated and operated for 215 days on a synthetic, organic-free feedstream containing 217 mg L−1 NH–N. When O2 and H2 were simultaneously supplied, the RCB achieved a steady NH–N removal flux of 5.8 g m−2 day−1 normalized to O2–HF surface area with a concomitant removal flux of 4.4 g m−2 day−1 (NO+NO)–N based on H2–HF surface area. The significance of H2 supply was confirmed by an increase in effluent NO–N when H2 supply was discontinued and a decline in NO–N when H2 supply was restarted. Increases in H2 pressure caused decreased ammonia utilization, suggesting that excess H2 interfered with nitrification. Microprobe profiling across radial transects revealed significant gradients in dissolved O2 on spatial scales of 1 mm or less. Physiological and molecular analysis of biofilms confirmed that structurally and functionally distinct biofilms developed on adjacent, juxtaposed fibers. Biotechnol. Bioeng. 2008;99: 830–845. © 2007 Wiley Periodicals, Inc.