Immobilized cell bioreactor (ICB) technology is being investigated under a multiyear, company-funded program to evaluate its applicability as a primary water processor for treatment of wastewater streams in a regenerative life support system (RLSS). Biological wastewater treatment methods may offer several advantages for long-duration space missions. Most contaminants found in life support wastewater can be aerobically converted by microorganisms to simple chemical molecules, such as carbon dioxide, water, ammonia, and nitrate. Thus, one unit could process several, possibly all, wastewater streams. This type of biochemical process can be operated at or near ambient temperature and pressure, requiring very little energy. In addition, bioreactor configurations are relatively simple.The objective of this paper is to describe two aspects of the ongoing ICB research: 1) performance stability over an extended time period, and 2) new reactor designs. Two packed-bed bioreactors were operated over 7 months at a hydraulic retention time (HRT) of 24 hours and exhibited consistent performance throughout this period in terms of total organic carbon (TOC) degradation, urea degradation, and solids production. However, it is desirable to lower the required HRT, by increasing the overall processing rate, in order to minimize size and weight of an ICB for a life support system. Two alternate bench-scale bioreactor designs were investigated and their effect on the HRT determined. A greater than four-fold decrease in HRT, compared with the previous bioreactors, is reported.