Nine organisations from three different continents teamed up during the SOLWATER project, which was funded by the INCO 2 Programme. The group focused on singlet oxygen, a reactive species with advantageous physiochemical characteristics that enable it to cleanse contaminated water of potentially deadly microorganisms. Work led by the Universidad Complutense de Madrid examined the effectiveness of various photosensitisers, substances that enhance singlet oxygen formation via photocatalysis. Specifically, a number of polyazaheterocyclic ruthenium (Ru) compounds were tested on samples containing high concentrations of two well known bacteria: Escherichia coli and Enterococcus faecalis. The experiments revealed that none of the Ru complexes were effective in the absence of light. On the other hand, bacterial concentrations started dipping when radiation in the visible spectrum was introduced. Furthermore, when the tests were performed in the homogeneous phase, the cationic RDP2+ photosensitiser dramatically reduced the amount of E. coli by two orders of magnitude. The anionic Ru complex, RSD4-, had less of an impact, managing just a 15% reduction in E. coli numbers. This is likely to be due to the fact that the bacteria's membrane also carries a negative charge, thus its interaction with RSD4- is not aided by electrostatic attraction as in the case of RDP2+. The SOLWATER consortium used this knowledge to improve the design of its prototype water decontamination unit.