The water purifier project I describe in Chapter 10 is a good project because it’s relatively easy, foolproof, and cheap. Or as cheap as you want to make it. You can make a small purifier or a big one with valves and drippers and all kinds of features. You can purify a gallon of water a day or a thousand, and the concept is the same. You can use salvaged parts, so if your game is to get everything for free, here you go.
This project is good if you’ve got a remote cabin (off-grid enthusiasts!) and have access to a creek that’s of dubious origin (all together now — what do bears do in the woods?). Or if you just want to purify your local tap water, you can use this project for that purpose, too.