MELISSA Loop: First Estimate of Flow Rates and Concentrations through the Loop 2000-01-2380

MELiSSA is a micro-organism based ecosystem conceived as a tool for understanding the behaviour of artificial ecosystems and developing the technology for a future biological life support system for long term manned space missions. The driving element of MELiSSA is the recovering of oxygen and edible biomass from waste generated by the crew (CO₂, faeces, urea). MELISSA is composed of four microbial compartments. A fifth compartment, a higher plant chamber (HPC), working in parallel with the Spirulina compartment was associated to the MELiSSA loop in order to improve the diet quality for the crew.

In order to perform steady-state simulations of the MELiSSA+HPC loop starting from previous stoichiometric models with an ideal behavior of the loop (complete conversion of key substrates in the compartments) and considering the present knowledge of kinetic limitations it must be taken into account flow rates, substrates and products concentrations and experimental efficiencies of bioreactors in the simulations. Thus, in a first step, the characteristics of the bioreactors developed for the MELiSSA loop (pilot reactors) will be detailed and individual performances of the reactors within their range of operating conditions will be presented. These results will serve as a base for the calculations and the constraints applied on the loop in steady-state simulation.

The simulations proves the ability of MELiSSA +HPC loop to satisfy the habitability constraints accounting for the physiological kinetic limitations in the compartments. The loop recycling efficiency is comparable with that obtained performing stoichiometric simulations. The re-supply mass of 1400 g.day^-1.man^-1 is mainly CO₂ and water and can be reduced if an higher aceto-methanogenesis rate is allowed in compartment I. The volumes of compartment I to IV are respectively 1.5m³, 180 L, 8L and 40 L. The important volume of the photobioreactor of compartment II is dependent on the quantity of AGV produced by compartment I and can thus be reduced if operating conditions of the first compartiment are changed. The total surface of the HPC is about 31 m², but this surface is also dependant of the plants cultivated.