There is water everywhere, but not a drop to drink. This is partially because water can contain pathogens such as helminthes, protozoa, fungi, bacteria, rickettsiae, viruses and prions. Disinfection methods based on UV/VIS radiation recently have resurged in popularity, note researchers from the University of Illinois at Urbana-Champaign and several other institutions in a review of water purification methods.

The authors imply that current methods of water disinfection need improvement. Although many bacteria can be destroyed by either UV radiation or the application of ozone, followed by combined chlorine treatment, evidence suggests that these chemicals can produce carcinogenic by-products. Moreover, UV radiation and combined chlorine are generally ineffective against viruses.

Light-activated structures could disinfect water more safely and effectively than processes involving ozone or combined chlorine and could do so in a high-throughput way. UV radiation can activate titanium dioxide (TiO₂), or visible light and sunlight can activate titanium oxynitride (TiON), possibly co-doped with palladium or another metal, which would be potentially less expensive than using UV.

In terms of chemical contamination, DNA molecules labeled with a fluorophore and quencher have been used in vitro to obtain parts-per-trillion limits of detection and specificity greater than 1 million times that of other cations. Such a detection system could be combined in a microfluidic device for chemical separation prior to the DNA recognition step.

For reclaiming and reusing water, membrane bioreactors can be used. They contain a membrane filter that keeps together microorganisms that eat contaminating materials. When followed by reverse osmosis and UV disinfection or visible light-triggered catalysis, these bioreactors generally work well, although membrane fouling remains a problem. (Nature, March 20, 2008, pp. 301-310.)