The hydrogen-based membrane biofilm reactor (MBfR) is efficient in removing nitrate in oligotrophic water, but pH increase due to denitrification is a tough problem in practice. To control pH, we investigated a novel MBfR with double bundles of hollow fibers (one for H-2 supply and the other for CO2 supply). CO2 was used for pH control and as the carbon source. The Fick's first law and steady state permeation tests were adopted to determine the CO2 permeability of polyvinyl chloride (PVC) hollow fibers used in this study. The CO2 diffusion coefficient in the membrane was 3.8375 x 10(-7) m(2)/d @ 25 degrees C. The CO2 diffusion coefficient was verified by a nitrate bioreduction experiment in the MBfR. The calculated supply of CO2 based on the CO2 diffusion coefficient is consistent with the experimental results: The total nitrogen removal rate was more than 99% and the pH is stable at around 7.45. CO2 diffused through the membrane is sufficient to be the sole carbon source. We confirmed that the novel MBfR with double bundles was stable and efficient. This research provides direct guidance to the design and operation of this new type MBfR. (C) 2015 Elsevier B.V. All rights reserved.