Mostly because they don't need to retain it though, a sub doesn't need to reprocess the CO2 into C and O2 but the process can be done. The process is largely the same instead of ejecting it as a waste product continue to break it down into useful products like injecting it into a Sabatier process.
The Sabatier reactor on the space station broke after processing about a thousand liters of water because deodorant and astronaut urine poisoned the catalyst beds.
Everything is easy to do in theory. Recycling carbon in practice is very, very hard if you don't have plants to help you.
Hypothetically... after you have separated the carbon from the oxygen (discounting the human waste which has a significantly more amount of carbon)... what would you do with it as part of a recycling process in a submarine?
In current tech not much comes to mine. Maybe it could be reprocessed into dry graphite lubricant but there's not much need for raw carbon or principally carbon materials in a submarine. That's another reason it just gets tossed there's just not a real use for it. On a hypothetical extraplanetary colony though it's a pretty important material just to keep people alive.
You don't want to be simply reintroducing it into the atmosphere again that would just increase the amount you need to scrub out of the air because now your cycle just reintroduces all the CO2 you remove back into the air along with the stuff the sailors are breathing out. Submarines as far as I can see have basically always used electric cook tops for that reason, at least since they were designed to stay submerged most of the time. UBoats probably didn't because electric power was so limited they probably just didn't cook hot food while submerged.
> The galley was located on the starboard side, between the chiefs' quarters and the wardroom, and was made up of three hotplates and two small electric ovens. It also contained a refrigerator, self-heating soup kettle, provision lockers, and an enamel sink with hot and cold fresh water and hot salt water.
> Fleet subs of the Gato (SS 212) and Balao (SS 285) classes boasted sizable freezer and refrigerator compartments, and their galleys, though diminutive, were well-equipped, generally with two griddles, a deep-fat fryer, two electric ovens, a hefty electric mixer, and a two-gallon coffee urn. Fleet boats usually boasted an ice cream maker as well, even when lack of space in the galley or crew mess made it necessary to install the machine among the bunks in the crew's berthing space.
> The loss of O2 to water means that each time through this removes O2 from the atmosphere which would need to be replenished.
Well you're missing half the equation on the CO2->CH4 process. To get the 2 H2 molecules you'll need to split 2 H2O in all likelihood so you're already getting the 2 O2 from splitting the CO2 and then the 2 H2O so in theory it's oxygen neutral.
That said it's a silly process to go through because there's already absolutely massive electrical supplies available on submarines to power the engines while submerged or to recharge those batteries while on the surface in the case of old subs or modern diesels. Why add a whole "mini" sabatier reactor just to cause more problems with your air quality and recycling when you can just cook electric and toss the captured CO2 overboard.
Neat info about the UBoats. Wonder if they were allowed to use the electric ranges while submerged. UBoats exist in this fascinating in between period where they were both surface and subsurface ships because of the limitation on things like their speed underwater.
The idea of using a combustible gas to cook is archaic and really dumb. It's amazing that people are still cooking this way today in many places. We've had better technology for ages.