So I am running a near-future (TL8/9) campaign in which the players have just gotten ahold of their own ship. It's basically a space-18-wheeler, for all practical purposes, modelled after the Modular Cutter in the Traveler's setting. I've got the ship design all set up, and then I hit a snag:

p17 in Spaceships says: "Life support provides air, climate control, and water for occupants."Okay. Air, climate control and water. Got it. Check. Then:

p35 "Long-term accommodations provide full life support for an indefinite period. " An indefinite period. I know it's a game, but I can't get quite that hand-wavey. High-efficiency systems that recycle urine into drinkable water? Sure. Using electrolysis to get oxygen (and, perhaps, hydrogen for maneuvering thrusters) from water? Sure thing. But....it can't be indefinite. Ultimately, some sort of raw materials have to be input into the system. Does anyone have any suggestions about how to figure out what/when/how much needs to be added to a life support system to realistically make it semi-realistic? I'm okay with moderate psuedo-science that sounds good, or kind of makes sense. And the players are purchasing food ration stocks, so that's not a problem.

By the way, the ship is meant to be a long-term accommodation for the crew. As I said, like an 18-wheeler rig, on a larger scale.