Recycling Oxygen?

We put this question to Mark Hempsell, Senior Lecturer, University of Bristol... 

The oxygen isn't strictly recycled.

The carbon dioxide that the humans breathe out is filtered out of the air with a molecular sieve and then simply dumped overboard.

The oxygen is created from the water that comes from the air and from the washbasins and from the loos.

After that water is cleaned up it is electrolysed. An electric current is passed through it and it is separated into hydrogen and oxygen.

The hydrogen is dumped overboard; the oxygen is fed into the cabin for the crew to breathe.