A space mission crew generates a large amount of waste, be it directly as a result of their bodily functions or indirectly from their packaged food, drinks and clothes. While the handling of the supply of resources and resulting waste is currently dealt with frequent resupply missions, it is a costly approach and not feasible in more distant missions, for instance on Mars. Those outposts will rely on on-site food and oxygen generation using the crew’s waste as source of starting materials such as carbon, oxygen, nitrogen, phosphorus, etc. The aim of the waste treatment unit is to serve as a first treatment step in the MELISSA regenerative life support systems, converting complex organic polymers into simpler molecules that can be used in subsequent units, such as volatile fatty acids (e.g. acetate, propionate, butyrate), ammonium, carbon dioxide, etc.
The so- called Waste compartment bioreactor is populated with a mixed microbial community able to anaerobically process complex organic waste in a two-step process, combining hydrolysis and acidogenic fermentations (see figure below). It is operated at thermophilic conditions, providing not only enhanced hydrolysis, but also sanitation as temperatures of 55°C inactivate most human pathogens.
The waste compartment is a critical and challenging step that needs to deal with solid waste of heterogeneous nature. As a result, several R&D challenges need to be addressed to ensure its optimal and safe operation.
Firstly, the waste compartment unit is currently operated using an enriched, but undefined, mixed microbial community, able to degrade different types of substrates. It is important to understand how these bio-transformations take place, by which organisms and under what conditions. Only by fully comprehending the systems it will be possible to adapt and control its function and, ultimately, move into a defined mixed culture.
Secondly, current research activities have demonstrated it is possible to process complex wastes in the MELISSA waste compartment. However, degradation efficiency should be improved to maximize the recovery of resources within the regenerative life support system. In line with that, volatile fatty acid profile obtained should be adequate for further conversion in the subsequent units, especially the C2 compartment or the photosynthetic ones C4 A&B.
Lastly, future research should demonstrate the stability of the community (both genetic and in terms of function) under space conditions (as discussed below).
Space conditions pose a challenge to biological processes in many ways. Microgravity affects gas-liquid mass transfer, space radiation can damage microbial cells or result evolutions altering its genotype (i.e. microgravity, radiation, …). Although, today there is no yet evidence on their impact on the waste treatment compartment microbiome. Future astro-microbiology activities within MELISSA will seek to elucidate whether or not space conditions have any impact on the function or genomic stability of these anaerobic cultures. It is important to highlight that, as opposed to other units relying on pure cultures or simple synthetic communities, the waste compartment microbiome is rich and complex, sometimes relying on the interaction of different organisms.
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The performance of the waste compartment unit has been demonstrated at 5-10 L scale in several research project in Belgium and Spain, over very long period of time. However, the technology has not yet achieved the level of understanding and control needed to be implemented at the MELISSA Pilot Plant in Barcelona, where other compartments have already been demonstrated at large scale and interconnected.
The core processes within the waste compartment unit are an integral part of anaerobic digestion, a state of the art technology for the production of bio-energy (heat & electricity) from waste.
To date there are not yet MELiSSA terrestrial applications connected to the knowledge generated during the waste compartment technology development. However, the current circular economy context and growing interest in the production of bio-chemicals from complex organic materialsare promising and can lead to potential applications in the years to come.
The waste compartment compartment is one of the focuses of the MELISSA PhD training program.