Welcome back. We have designed systems to provide clean air, pure water, and fresh food. But any habitat, whether in orbit or on Earth, is subject to the fundamental laws of entropy: things break down, and trash is created. A truly sustainable system must not only support life but also flawlessly manage its own decay and waste.

Welcome back. After examining the life-support loops for air and water, we now arrive at the system that fuels the crew: food. Growing food in a closed environment is a monumental task, moving beyond basic survival to the challenge of creating a truly sustainable and nourishing habitat. Our two case studies, the ISS and Biosphere 2, approached this problem from opposite ends of the spectrum, providing us with invaluable insights.

Welcome back to our series where we ground our vision for a closed-loop city in the lessons from real-world experiments. Having secured a breathable atmosphere, we now address the solvent of life itself: water. In any sealed environment, recycling water isn't just a good idea—it's the only path to long-term survival.