To solve that dilemma, Professor Bosscher had located a simple, clear tube with a daub of wax at one end. The tube hangs on a string inside a water jar with the wax end up, and once the water around it becomes hot enough to kill the bacteria — about 150 degrees Fahrenheit — the wax melts, running from the top part of the tube to the lower end. All villagers have to do is look at the tube to be sure the water is safe to drink. To use it again, they simply invert it".
For you borderless non-engineers, wax can be formulated to have a a variety of melting points and to melt either very slowly or relatively quickly (all at once). The trick is to choose a wax that indicates that surrounding water has been exposed to the sun long enough to ensure adequate sterility. Obviously the tube is cheap and lasts through many uses -- an ordinate of systainability. And any old jar or clear bottle will do. [Thanks to Shauns' comment, (below) an error in the original version of this paragraph has been corrected. In essence, sterility is a function of ultraviolet exposure as much or more than heat.]
Water.+ Bottle. + Sun.+ Tube.+ Time. = Water Free of Disease Causing Organisms
Too bad there weren't a whole bunch of these and some basic charcoal filters stored in the New Orleans hurricane shelters. Elegant simplicity might work in developed nations too.