The use of the plastic soda bottles and UV/heat from the sun works better in the middle latitudes. You can check these web sites for additional information on the technique, as well as the use of solar stills.


http://www.sodis.ch/

http://solarcooking.org/soda-bottle-pasteurizer.htm

http://solarcooking.org/


As to the use of natural plants, etc., I am sure there are plants/herbs out there that have microbial cidal and or static effects, the problem would be they may not be effective against a broad range of bacteria, viruses and parasites. Even if you found one or more, there would be a high probability they would also be toxic to humans in the concentrations necessary to be effective against pathogenic (disease causing) organisms.

Pete