Wadi, which is solar-powered of course, works like an alternative bottle top. You screw it in place, leave it in the sun (you can measure several bottles at once if they’re close together), and then consult a progress bar on the side. An unhappy smiley indicates bad water. One, two, three, four bars (like on a mobile phone) show water getting cleaner. Finally, when there’s a happy face, you’re good to go.