One of the most critical issues in enabling long term space habitation (e.g., ISS, Lunar, Base or Mars Mission) is the ability to recycle water. Ideally water recycling technologies would be able to function in both micro-gravity and reduced gravity, reflecting their use on transit vehicles and Lunar or Martian bases. For any extended space operation, re-supply is both expensive and potentially not feasible (Mars Mission) and technologies should strive to minimize their consumables as well as mass and energy requirements. However, it is also true that potential space water processing technologies should also be highly robust and require minimum crew time for operation. Biological water processors meet many of these attributes and as such have been extensively researched by NASA and NASA funded researchers. As with terrestrial systems that reprocess wastewater to potable water, a combination of technologies is required including biological, physical and chemical in order to meet stringent health requirements. In general, biological reactors are used to remove organic carbon, and in the case of space waste streams, convert the high concentrations of reduced nitrogen (e.g., ammonium and urea) to N2 gas and/or oxidized forms (NO3-). Physical and chemical post processing units typically have high consumables, high energy consumption, and/or produce secondary waste streams. Reducing the load on these units can significantly decrease the system up-mass and volume requirements as well as consumables for the necessary post processing units. In addition, current configurations for biological systems entail a combination of nitrification/denitrification reactors in order to gain further advantages, including reduced oxygen consumption (NO3- as the terminal electron acceptor) versus full aerobic treatment and re-supply of the major make-up gas (N2). However, limited engineering studies have been performed to determine the optimum loading rates or to fully characterize (limiting reactants) the biochemical transformations occurring within the reactors. Two potential micro-gravity compatible biological treatment systems have been operated for extended periods (>3 years). This presentation will discuss the attributes, abilities, and limitations of biological systems for us in micro-gravity applications as pre-treatment for physiochemical systems with an emphasis on kinetic limitations, operational issues caused by micro-gravity compatible configurations, and potential effects on downstream systems.