You don't even have to darken your platypus. Simply fill with water, and place it in the sun over a piece of metal, say aluminum foil. This is basically a means of water purification touted in the third world as SODIS (solar disinfection). Water temps will get into the range needed to kill the bad stuff (160 degrees or so).

I haven't personally tired this, and I would imagine it works best when the bag is exposed at high noon, especially if placed at approximately a 30-40 degree angle. It would certainly save fuel.