A modified membrane-aerated biofilm reactor (mMABR) was constructed by incorporating two distinct biofilm immobilization media: gas-permeable hollow fiber membranes and high surface area inert bio-media. In order to evaluate the mMABR for space flight applications, a synthetic ersatz early planetary base (EPB) waste stream was supplied as influent to the reactor, and a liquid loading study was conducted at three influent flow rates. On average, percent carbon removal ranged from 90.7% to 93.1% with volumetric conversion rates ranging from 25 ± 3.3 g/m3 d and 95 ± 13.4 g/m3 d. Simultaneous nitrification/denitrification (SND) was achieved in a single reactor. As the liquid loading rate increased from 0.15 mL/min to 0.45 mL/min, the volumetric denitrification rates elevated from 27 ± 3.3 g/m3 d to 65 ± 5.2 g/m3 d. Additionally, it was found that nitrification and denitrification were linearly related with respect to both percent efficiency and volumetric reaction rates. The mMABR provides an equivalent systems mass (ESM) cost comparative alternative to previously studied two-stage biological systems.