Same way humans survive anywhere. They find what they need, and use it and reuse it as many times as possible. It ain't like Earth. Earth makes waste and then just - leaves it, from what I hear. Giant mountains of the stuff. Poluting the water. There ain't room for that on Mars, or in the Belt. Oxygen is breathed, then carbon dioxide is recycled. Every drop of water you drink has been through nearly every other person in your habitat, at one time or another. Purified and reused. What losses we take we make up for by mining for Ice in the belt, or around Saturn.

It ain't easy, but it ain't a death sentence.