The SOLAQUA by designer Jason Lam is a concept for a passive way to purify water using both heat and UV rays (so, in other words, leaving water out in the sun). Each petal that extends from the main unit — which folds up as to be easy to carry — contains several 10 liter, clear tubes of water that'll have the water inside entirely and thoroughly bombarded by the sun. Filling the tubes up is easy, as SOLAQUA automatically funnels it down into them, rather than making someone fill the bottles one by one.
Boiling water is one of the easiest methods of disinfecting the liquid, though a filter needs to be used to get out any solid pollutants — which is why the SOLAQUA includes a built-in filter of sari cloth — but the water doesn't even have to be all that hot if it's going to sit out for a while. Boiling just speeds things up. UV rays, as well, are popular for purifying, and the petals of the SOLAQUA would direct them straight through the bottles.
Check out the pictures below for more of the SOLAQUA.