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Human waste could be used in a hydroponics facility to grow algae in space or Mars in pressurized tanks, and the photosynthesis can be used to convert CO2 to breathable oxygen. A carbon dioxide enriched nitrogen or helium medium would be needed to be processed by the algae and then by humans.

Would the waste from one human and carbon dioxide from Mars sustain the algae to produce enough oxygen for 1 or more human to breath?

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  • $\begingroup$ Radiation is a complicated and different question. An answer will have to address how much oxygen the algae consumes as well. $\endgroup$
    – uhoh
    Dec 17, 2018 at 3:35
  • $\begingroup$ if you can model a closed cycle, than in general you need to create a system which has no loss. There is always some entropy, so the answer is possibly no. $\endgroup$
    – J. Doe
    Dec 17, 2018 at 7:44
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    $\begingroup$ What's the difference between "organic oxygen" and "regular oxygen"? "Organic" means "containting carbon atoms", so "organic oxygen" would be $CO_2$.. Could you please clarify this? $\endgroup$
    – AtmosphericPrisonEscape
    Dec 17, 2018 at 15:35
  • $\begingroup$ @AtmosphericPrisonEscape the oxygen is made by an organic method. $\endgroup$
    – Muze
    Dec 17, 2018 at 16:50
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    $\begingroup$ what you are asking is could you create a closed environment that works indefinitely. The biosphere2 people tried and found out that it was harder than it looked and that atmospheric oxygen content declined over time. The general answer is yes, but the devil is in the details. Read up on biosphere2 to see what some of the problems were. $\endgroup$
    – zeta-band
    Dec 17, 2018 at 19:18

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I believe this is what is behind Elon Musks Mars mission ideas however it has to be a specific type of algae, as this algae has to be fast enough to produce breathable oxygen quickly.

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  • $\begingroup$ Welcome to space stack exchange! Fact-based answers are valued here. Do you have any references to back up your assertion that Elon Musk is planning to use this sort of thing? $\endgroup$
    – Organic Marble
    Dec 19, 2018 at 21:47
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The stoichiometrics of respiration say that the CO2 exhaled by one person, plus their poop, can be converted by algae into 3000 calories of nutrition and 26 moles of oxygen (enough for a day) using 220kWh per day. See Algae Bioreactor for O2 and CO2 (and food?) - Any data on required size and power requirements?

The answer to your question is that a person exhales enough CO2 for the algae system to potentially regenerate all the O2 they require without adding additional CO2.

An algae bioreactor initially seems like a great idea for space travel. It is very elegant to turn waste CO2 and astronaut poop into oxygen and food. Unfortunately, the device’s mass and energy requirements make it impractical for voyages in the inner solar system.

On Mars, the need to produce industrial quantities of oxygen would make photosynthetic oxygen unnecessary. Matt Damon's poop would still be needed for growing potatoes.

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  • $\begingroup$ Gas bubbles ascending in water would not work in zero gravity anyway. $\endgroup$
    – Uwe
    May 3, 2022 at 9:05
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    $\begingroup$ "On Mars, the need to produce industrial quantities of oxygen would make photosynthetic oxygen unnecessary.": particularly since rockets run fuel-rich. Producing 1 full Starship propellant load would involve producing about 50 t of excess oxygen. At 0.84 kg/day/person, that's 160 person-years of oxygen. And then there's metal smelting, which will involve breaking up metal oxides for the metals...oxygen will be a major industrial waste product on Mars. $\endgroup$
    – Christopher James Huff
    May 3, 2022 at 13:51
  • $\begingroup$ @ChristopherJamesHuff When heating metal oxide to get pure metal, you need a chemical agent to remove the oxygen, for instance carbon monoxide as used for iron and copper as well as many other metals. Breaking up a metal oxide by heat alone works with red quick silver oxide but not with all other metals. Getting pure metal by electrolysis is done with aluminum and may work with iron or copper after some research to find the materials for a crucible and the electrodes. $\endgroup$
    – Uwe
    May 5, 2022 at 16:55
  • $\begingroup$ @Uwe molten oxide electrolysis has been demonstrated and is being pursued for commercial production of iron and other metals even on Earth, and the most likely source of carbon monoxide on Mars would be something like the MOXIE process to split oxygen from atmospheric CO2. Both produce oxygen as a byproduct. $\endgroup$
    – Christopher James Huff
    May 5, 2022 at 17:39

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