Activated sludge waste water processing is one of the most common technologies used in municipal waste water treatment facilities. Bioregenerative waste water treatment research at the NASA Johnson Space Center has, however, focused on the use of attached growth bioreactors due to their advantageous solids retention capability. The development of cross flow filtration methodologies in recent years has provided a means for significantly increasing solids retention time of activated sludge reactors. The settling tank traditionally used in municipal activated sludge processes for biomass retention is replaced with a microgravity compatible cross-flow biomass filter. The resulting activated sludge reactor has entirely independent solids and hydraulic retention times that may be modified as necessary to enhance processing performance. This paper describes the development of such an activated sludge bioreactor and the performance characteristics achieved to date.Results of an activated sludge reactor without biomass filtration indicated that at a Hydraulic Retention Time (HRT) of 5 days, TOC reduction was maintained at or above 95% and ammonium conversion at about 75%. When the HRT was reduced to 2 days, TOC reduction remained above 90%, but ammonium nitrification dropped to less than 10%. With the addition of the biomass cross-flow filter at an HRT of 2 days, the activated sludge reactor was able to maintain a TOC reduction of greater than 91% and an ammonium conversion near 60%. Ammonium conversion rates were more than double that of the filterless configuration even at the 5 day HRT. Flow across the filter element decreased significantly (3.2 to 1 L/day) over the weekly regeneration cycle implemented, but no permanent loss of performance was observed. Results were complicated by the development of simultaneous nitrification and denitrification reactions occurring in the bioreactor. This novel behavior is thought to have hampered the ability of the ASR to completely convert ammonium in the waste water feed stream.