Mathematical modelling of bioreactors in the MELiSSA regenerative life support system

Abstract

MELiSSA (Micro-Ecological Life Support System Alternative) is a project conceived to develop a closed regenerative life support system where the integration of six biological compartments in a loop can enable to provide life support functions in long-term human space missions. The present thesis is focused on developing different models to describe the performance of some of the compartments in MELiSSA loop in stand-alone operation as well as considering the connection between different compartments in different gas liquid and solid phases. In a first step, a model for the description of the operation of the packed-bed nitrifying compartment 3 has been developed. This reactor operates in axenic conditions with two nitrification strains Nitrosomonas europaea and Nitrobacter winogradsky. The model is structured in different levels: the global reactor hydrodynamics and the biofilm that considers the growth of the different species based on a simplified 1-D diffusion model and includes a simplified approach to simulate biofilm consolidation. The validity of this model has been tested by using experimental data from the bioreactor operation during a period of 2 years in which different concentrations and loads of ammonium have been tested to characterize the continuous operation of the reactor. The obtained results fit reasonably well the experimental data and final biomass concentration profiles observed at the end of the operational period. This is especially relevant considering the limitation to obtain biofilm samples during the long-term operation in order not to perturbate the packed-bed structure. In a second phase, a model has been also created to describe the operation of the external loop air-lift photobioreactor colonized with Limnospira indica, corresponding to compartment 4a in the MPP. This model integrates existing knowledge on light transfer in the photobioreactor liquid, gas-liquid mass transfer of oxygen and carbon dioxide and the growth kinetics of microalgae. In a third phase, a simplified model has also been developed for the oxygen consumption and carbon dioxide production in the gas phase of the crew compartment. This compartment is configured in the MPP by an animal isolator with a cohort of rats. To develop the model, the consumption coefficients have been calculated based on experimental data. Then, the knowledge generated in the development of these last two models has been integrated with the MPP control system to support the gas-phase integration of these two compartments (the photobioreactor and the animal compartment) in continuous operation. Given an oxygen set point in the animal compartment, the control system increases the illumination in the photobioreactor to reach the necessary oxygen production in order to maintain the oxygen level in the animal compartment. Finally, the complete set of models has been validated by using two series of long-term operation experimental data of the complete system (photobioreactor and animal compartment) in closed gas loop. The developed models reproduce with high accuracy the experimental profile of the photobioreactor and the global system dynamics associated to the introduction of a real living crew in the gas close loop. This provides a proof of concept of the high potential of mathematical models to understand and support such type of complex systems dynamics. The results obtained in the present thesis represent a step forward in the development of the MELiSSA loop. The knowledge generated in the formulation of the proposed models can be used for the definition of simplified knowledge-based control systems critical for future development steps as well as their use in the study of future integration scenarios as a tool to simulate the performance of different loop architecture options and operational conditions.

Date of Award	4 Jul 2022
Original language	English
Supervisor	Francesc Godia Casablancas (Director) & Arnau Jimenez, Carolina (Director)