Waste Treatment
Objectives
A space mission crew generates a large amount of waste, whether directly from bodily functions or indirectly from packaged food, drinks and clothing. Handling the supply of resources and the resulting waste is currently managed through frequent resupply missions, a costly approach that will not be feasible for more distant missions, such as those to Mars. Those outposts will rely on on-site food and oxygen generation, using the crew’s waste as a source of starting materials such as carbon, oxygen, nitrogen and phosphorus.
The waste treatment unit serves as the first treatment step in the MELiSSA regenerative life support system, converting complex organic polymers into simpler molecules that can be used in subsequent compartments, such as volatile fatty acids (acetate, propionate, butyrate), ammonium and carbon dioxide.
The 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 fermentation. It operates under thermophilic conditions, which provide not only enhanced hydrolysis but also sanitation, since temperatures of 55°C inactivate most human pathogens.
Challenges & Approach to the Research
The waste compartment is a critical and challenging step that has to deal with solid waste of a heterogeneous nature. As a result, several R&D challenges need to be addressed to ensure its optimal and safe operation.
The unit currently runs on an enriched but undefined mixed microbial community, able to degrade different types of substrate. Understanding how these bio-transformations take place, by which organisms and under what conditions, is essential: only by fully understanding the system will it be possible to adapt and control its function and, ultimately, move towards a defined mixed culture.
Current research has demonstrated that complex wastes can be processed in the MELiSSA waste compartment, but degradation efficiency needs to improve to maximise resource recovery within the regenerative life support system. In line with that, the volatile fatty acid profile obtained needs to be well suited to further conversion in the downstream compartments, particularly the C2 compartment and the photosynthetic C4a/C4b compartments.
Future research should also demonstrate the stability of the community, both genetically and functionally, under space conditions.
Flight Experiments
Space conditions challenge biological processes in several ways. Microgravity affects gas-liquid mass transfer, and space radiation can damage microbial cells or drive genetic changes that alter their genotype. There is not yet evidence of the impact of these conditions on the waste treatment compartment’s microbiome specifically, and future astro-microbiology activities within MELiSSA aim to establish whether space conditions affect the function or genomic stability of these anaerobic cultures.
Unlike compartments that rely on pure cultures or simple synthetic communities, the waste compartment’s microbiome is rich and complex, often relying on interactions between different organisms. This makes it:
- Potentially more prone to being affected by space conditions, given the larger number of species present.
- More challenging to study, owing to its inherent microbial richness and trophic interactions.
Ground Demonstration
The performance of the waste compartment unit has been demonstrated at a 5–10 litre scale across several research projects in Belgium and Spain, over a long period of time. The technology has not yet, however, reached the level of understanding and control needed for implementation at the MELiSSA Pilot Plant in Barcelona, where other compartments have already been demonstrated at large scale and interconnected.
Terrestrial Applications
The core processes within the waste compartment unit are an integral part of anaerobic digestion, an established technology for producing bio-energy (heat and electricity) from waste.
There are not yet MELiSSA terrestrial applications directly connected to the knowledge generated through waste compartment technology development. The current circular economy context, and growing interest in producing bio-chemicals from complex organic materials, are promising, though, and could lead to applications in the years to come.
Education & Communication
The waste compartment is one of the focuses of the MELiSSA PhD training programme.