The nitrifying component of a micro-ecological life support system alternative (MELISSA) based on microorganisms and higher plants was studied. The MELISSA system consists of an interconnected loop of bioreactors to allow the recycling of the organic wastes generated in a closed environment. Conversion of ammonia into nitrates in such a system was improved by selection of microorganisms, immobilization techniques, reactor type and operation conditions. An axenic mixed culture of Nitrosomonas europaea and Nitrobacter winogradskyi, immobilized by surface attachment on polystyrene beads, was used for nitrification in packed-bed reactors at both bench and pilot scale. Hydrodynamics, mass transfer and nitrification capacity of the reactors were analysed. Mixing and mass transfer rate were enhanced by recirculation of the liquid phase and aeration flow-rate, achieving a liquid flow distribution close to a well-mixed tank and without oxygen limitation for standard operational conditions of the nitrifying unit. Ammonium conversion ranged from 95 to 100% when the oxygen concentration was maintained above 80% of saturation. The maximum surface removal rates were measured as 1.91 gN-NH4+ m(-2) day(-1) at pilot scale and 1.77 gN-NH4+ m(-2) day(-1) at bench scale. Successful scale-up of a packed-bed bioreactor has been carried out. Good stability and reproducibility were observed for more than 400 days. (C) 2004 Society of Chemical Industry.