[overhauling the question to get rid of misconceptions, which were irrelevant anyway]
In the balance of input/output of the system ISS is, I spotted something that looks like build-up of stockpile of oxygen.
ISS receives large shipments of water with every resupply mission - amounts of order of 420kg.
The water is circulated on ISS with very little losses - recycled for drinking or for use in experiments, moisture from air captured, urine purified. Water is also broken up into oxygen and hydrogen through elextrolysis, then the hydrogen, through Sabatier process is used to replace oxygen in carbon dioxide produced by breathing, the resulting methane vented into space. Additionally, dry food is metabolized into carbon dioxide and water, adding even more water to the system.
There's supposedly a shortage of hydrogen for the Sabatier process, extra hydrogen delivered by cargo shipments.
Generally, on input we have big shipments of $H_2O$, some $H_2$ and a lot of organic compounds of food, built mostly from carbon, hydrogen and oxygen.
On output we have the vented $CH_4$, and small amounts of $H_2O$ in brine left after purifying the astronauts' urine.
And so, it seems we have excess oxygen build-up.
Let's try to tally the known loss paths:
- Brine left after reprocessing the urine. I don't know how much of it is produced, but supposedly these are "small amounts".
- Some oxygen is lost with airlock venting during EVA egress, but alongside with the oxygen about 4x as much of nitrogen is lost - and we don't hear of massive shipments of nitrogen, 4x as much as water!
- Leakage of the station - as above, 4x as much nitrogen leaks.
None of these is significant enough to account for the large shipments.
So where does all the extra oxygen go?
Some of my calculations:
(On the losses side I approximate mass of oxygen as mass of water wherever water is lost; in fact it's 16/18 or 88% but since I'm still not getting anywhere into the ballpark, the 12% error is negligible, especially that counting the lost water as oxygen I'm overestimating the losses.)
Human usage: 550 liters of pure oxygen (19 cubic feet) per day. src At 1.326 gram per litre src that's 730 grams per day. Times crew of 6, 4380 grams per day. There are 5-6 resupply missions per yearsrc., so let's say 60 days in between resupplies. 262 kg turns into carbon dioxide.
Half of that is reclaimed through the Sabatier process. src. 131kg of oxygen vented bound into CO2.
About 2kg of urine per person per day.src. 93% reclaimed, 7% lost, so 140g. Six people, 60 days - 50kg lost with brine from urine. Also 200g of fecal losses, not reclaimed AFAIK, x6 x60 - 72kg.
Quest airlock volume: 34 cubic meters src. 34,000 liters of 20% oxygen (80% nitrogen) vented per EVA. So, 6800l, or at 1.326 g/l, 9kg per EVA. About 10 EVA per year; this is very variable, let's round to 2 per resupply src., so 18kg.
If, as David Hammen claims, the Sabatier process is no longer performed, and the airlock wastes negligible amount of air, that's another 131kg minus 18. We're at 384kg max loss vs 373kg of oxygen brought in as 420kg of water brought with resupply, and it seems everything works out to within the error margin. But that is based on several unsourced assumptions: non-reclaimed fecal losses, Sabatier inactive, airlock pumped down. Could someone confirm? Primarily, the Sabatier process, as it would account for 70% of the losses!