URINIS aims to test the feasibility and efficiency of the urine nitrification bioprocess under space conditions on ground and in space. First the experiment requirements for the flight tests were defined (URINIS ground). Next, this URINIS-A proposal describes the flight hardware optimization and biological ground experiments that need to be performed prior to the scheduled URINIS-A experiment in the ISS. The purpose of URINIS-A is to investigate the impact of storage and culture conditions, micro-gravity and radiation on microorganisms (monocultures and synthetic communities) involved in urine nitrification, more specifically with regard to their activity and biofilm formation. URINIS-A is the precursor for the final test of the bioprocess in a bioreactor set-up in ISS (URINIS-B). URINIS functions as a proof-of-principle for a specific and crucial process (nitrification) needed for long-duration human Space exploration (MELiSSA-project).