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More details about Phase 1 studies and compartment modeling
Mathematical modeling and mass balances:
In the overall strategy of the MELiSSA project, the development and the analysis of the loop and its compartments is associated with the development of reliable models for each compartment.  
First, steady state and dynamic models for each compartment are derived, which then are used to describe the overall behavior of the MELiSSA loop.

The mass balance of every compartment is described by one or more stoichiometric equations derived from the known cell metabolic pathway and from experimental results. Six major chemical elements (in terms of mass) are taken into account (C, H, N, O, S, P). For each stoichiometric equation the substrate entering the compartment is assumed to be completely exhausted at the outlet. Each compartment is treated separately with its own inputs and outputs, and the outlet state vector is determined from the inlet one. For steady state simulations, MELiSSA is considered as a semi-closed subsystem, with external constraints.

Two degrees of freedom are used for the steady state simulation: the percentage of microbial biomass and the percentage and composition of the high plant mass in the diet. By varying the values, different behaviors of the loop can be simulated. Following this steady state approach of the loop, a transient model is studied for all compartments. It enables the taking into account of the physical limitations (light, gas transfer) as well as substrate limitation (nutrients) and growth kinetics. These models lead to independent calculation of the mean volumetric growth rate and biomass quality.
Control and Automation
The steady states simulations as well as the transient models described above are very useful in predicting the overall mass balance and they are the most efficient way to improve the efficiency of the loop, but they cannot be directly used to build a robust control strategy of the MELiSSA.
A dynamic simulator of the loop is being progressively developed and implemented with MATLAB-SIMULINK® and ECOSIMPRO® software. The dynamic simulator consists of the five compartments of the loop, their local control blocks, which have been divided into two control levels, and an upper control block. The simulator is based on non-linear first principles models. These models are dynamic and can be used as internal model in the control laws. As they are non linear, a predictive control strategy based on classic predictive control principles has been implemented for the top levels of the control hierarchy. Predictive control is based on the use of an internal model to determine a reference trajectory and initiation of an auto-compensation procedure if deviations from the target performance are predicted.

The control architecture of the MELiSSA loop is composed of three levels: a global control for optimization of the loop performance, a local control for optimizing the performance of the compartment and a basic control level with a PID (Proportional, Integral, Derivative) character. This strategy has been successfully applied to and validated for the Spirulina compartment (compartment IVa).
Engineering of the Waste Compartment (EWC)
Compartment I, or the waste compartment, is the first one in the MELiSSA loop. It is designed as an anaerobic thermophilic bioreactor that will be used to assure the first step of the waste treatment.

Its design and optimization are of major importance with regard to the overall efficiency of the MELiSSA loop. Special attention is paid to process biosafety. Within the frame of this study, an additional biological technology to increase waste biodegradation is also being considered.
Total Conversion and Biosafe study (MAP (Microgravity Application Program))
This study concerns the development of side technologies to increase the degradation rate of the waste material. Special attention is paid to degradation of fibrous recalcitrant material not completely degraded in the thermophilic compartment (compartment I). Biosafety of the process and increased conversion of the waste material are the main topics of interest.
Higher plants
The Canadian MELiSSA partner is investigating the metabolic behavior of a plant canopy using closed and partially open higher plant chambers. Funded by the Canadian Space Agency (CSA), the investigation gives insight to the response of the canopy to environmental conditions such as photosynthetically active radiation and pressure. The gathered information is used to advance top layer modeling of a plant canopy and to design plant growth chambers.
Immobilised Biomass Monitoring
Although it is well known that biotechnology is more and more widely used in industrial arenas, most of the processes still need a basic tool to accurately quantify the biomass. Nowadays, most of the processes use indirect measurements that mostly end up with the application of standard recipes.

In the case of immobilised biomass (i.e the MELiSSA nitrifying compartment) or in the case of very high level of biomass (i.e. the liquefying compartment), these indirect measurements are really an insufficient method.

For the last 5 years ESA and NTE have been working on the applications of the multi-frequency impedance measurement technology for the immobilised biomass monitoring. The activities presented in the final report are aimed to go one step further and present the interest of the developed technology for industrial processes.

Contact us in order to obtain the final report.
FOOD (Fungus On Orbit Demonstration)
In this study a new compartment, colonised by fungi, is proposed to improve the fibre degradation in the liquefying compartment. Fungi have an excellent ability to degrade organic fibres using ligninolytic enzymes and could also contribute to improving the nutritional value of the crew diet.

During the development of the study various Pleurotus and Lentinus mushroom strains were cultivated using as substrate the output of the liquefying compartment.

Contact us in order to get the final report.

Last update: 22 November 2007

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