The MELiSSA loop as a regenerative life support system has to maximize the recovery of the main elements, as a key to develop a closed and self-sustainable process. Nitrogen is one of these main elements, and several alternatives for the use of Nitrogen in the MELiSSA loop are possible, both in terms of technology as well as the specific compartment. There is already vast knowledge on the several possibilities to process the Nitrogen sources from a human crew, also within the MELiSSA Consortium. Starting from this, the proposed PhD project would first analyse the Nitrogen loop in the context of the MELISSA loop, considering the inputs, the intermediate bioprocessing and the outputs and their point of use. For this, the required forms of Nitrogen (le, ammonium or nitrates) In the different MELISSA Compartments should be considered. From that analysis, the project will also comprise experimental work on alternative bioprocessing technologies of Interest, for example in the treatment of urine, or in the alternatives of operation conditions in the nitrification compartment of the MELiSSA loop. This work would be done at laboratory scale. The final output of the PhD work would be a proposal for the treatment and recovery of Nitrogen in the MELiSSA loop.