Biological processes are currently being investigated for use in spacecraft wastewater treatment. In a biological wastewater processor, microorganisms are used to degrade organic and inorganic contaminants to carbon dioxide, water, and other metabolic products. One step in the process is nitrification, in which ammonium ions in the wastewater stream are converted to nitrate ions. Traditional reactor designs for nitrification, which include continuously stirred tank reactors and trickling filters, are unsuitable for use on spacecraft due to their reliance on gravity for aeration. A tubular reactor for aerobic nitrification in a microgravity environment has been developed to allow use of biological systems for wastewater treatment on spacecraft. The tubular reactor uses a 3.2 mm ID tube 305 m long with co-current air and wastewater flow. Aerobic microbes grow on the tube walls. Because of the small tube diameter and the high surface tension of the wastewater, the air/wastewater flow is gravity independent. Thus it is expected that the fluid flow and biological performance will be identical in Earth-normal gravity and in flight.