The engineering of long-duration habitation
A closed loop is never quite closed. The engineering is in managing the leak.
A long-duration habitat is a machine for keeping a small volume of atmosphere and water in a state that a human body will tolerate, indefinitely, while that body works against it every minute by breathing, sweating and shedding.
The numbers that matter
Water recovery on our orbital station exceeds 96%. Oxygen is regenerated from carbon dioxide and water. The remaining few percent — the loss — is made up from stored reserves and, increasingly, from resources processed on site.
That last four percent is where the engineering lives. Closing it further costs mass, power and complexity that rise steeply as you approach 100%. The right target is not zero loss. It is loss small enough that local resources can cover it.
Sustainability in a habitat is not a philosophy. It is a mass balance.
Designing for the failure
Every loop has a bypass. Every consumable has a reserve sized for the worst realistic case plus a margin. The habitat is designed to degrade gracefully — to give the crew days, not minutes, to respond to any single failure.