Urea Hydrolysis, Nitrification and COD Removal of Synthetic Urine in a Continuous Packed-bed Bioreactor with a Synthetic Microbial Community at the MELiSSA Pilot Plant

Urine is an important resource of water and nutrients in space. MELiSSA project focusses on the development and integration of a system providing life support for long-term human missions in space. It consists of a loop of compartments with specific functions operating in continuous mode integrated one to each other. This concept is demonstrated in the MELiSSA Pilot Plant. One of the compartments at the MELiSSA Pilot Plant is a 5.9 L packed bed bioreactor designed to perform urea hydrolysis, complete nitrification and Chemical Oxygen Demand removal of the crew urine, resulting in a stable and safe nutrient solution suitable for plant and microalgae cultivation. The bacterial consortium facilitating this process comprises heterotrophic bacteria for urea hydrolysis and COD removal, and nitrifying bacteria for ammonium oxidation to nitrate. Nitrification of synthetic urine at different concentrations (2.5-10%) was studied at the MELiSSA Pilot Plant to ensure feasibility under controlled conditions. A six-bacteria consortium was tested using a defined medium in continuous mode under axenic conditions. Nitrogen and COD loads were progressively increased from 2.5% to 10% synthetic urine as influent at 20 hours hydraulic residence time. The results demonstrated the consortium capacity for full nitrification, achieving a maximum urea degradation rate of 180 mg N·L-1·d-1 at a 10% urine. No accumulation of ammonium nor nitrite was observed. COD removal averaged 50%, with a maximum COD removal rate of 550 mg O2·L-1·d-1 at a 10% urine concentration. These findings represent a significant step forward in developing a biological process for the treatment of human urine. However, further optimization is required to enhance the nitrogen recovery efficiency and maximize the process performance. The study provides a solid foundation for integrating urine as a resource in the MELiSSA loop, marking a relevant advancement toward sustainable and circular Life Support Systems.