Innovative Biological and Physico-Chemical Recycling of CO2 in Human Spaceflight
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Human spaceflight beyond Low Earth Orbit requires recycling technologies to handle consumables and waste. State-of-the-art technologies on the ISS are based on physico-chemical processes and enable atmosphere control, water regeneration, carbon dioxide (CO2) removal and reduction. Food is completely provided by resupply. Physico-chemical CO2 treatment means concentration by adsorption and reduction by the Sabatier (or Bosch) process. The next technological step to recycle CO2 is the testing and implementation of biological systems. Especially microalgae based systems can offer an innovative approach to meet the space requirements. Based on photosynthesis, microalgae generate edible biomass from CO2 and release oxygen. An innovative approach is to connect the CO2 photobioreactor inlet with the outlet of a CO2 concentration unit. This is one focus of the spaceflight experiment PBR@LSR prepared by DLR, Airbus DS and the University of Stuttgart. Cultivation in aquatic reservoirs (photobioreactors) reaches up to ten time higher growth rates and lower energy and volume investments than higher plants. Hydroponic basis or soils are not needed. The species Chlorella vulgaris is a promising candidate among a multitude of microalgae species. It is rich in proteins, thus up to 30% of human food needs can be covered by algae biomass. Harvesting and downstream processing of liquid algae medium are engineering issues under micro to partial gravity. An appropriate technology to break the cell wall, such as ultra-sonic handling and cross-flow filtration, depends on the gravity level and space system boundaries. Another innovative physico-chemical technology reducing CO2 is solid oxide electrolysis. It will be useful to treat CO2 in the Martian atmosphere. Crucial components are stable electrode materials not poisoned by carbon monoxide. Requirements, boundaries of biological and physico-chemical recycling technologies are summarized.