The application and efficiency of "mixed oxidants" for the treatment of drinking water

This study evaluates the application, benefits and shortcomings of mixed oxidants, a relatively new concept, for disinfection of water supplied to small and rural communities. The results confirm that there are aspects of the production of mixed oxidants that are not well understood and therefore not well defined either. There also seems to be an overlap between the new emerging science of electrochemical activation of water (ECA) and the electrochemical formation of mixed oxidant solutions by electrolytic means. Not enough conclusive evidence was found to either support or discard the claims related to the application of ECA technology. Although the two different ECA devices that were examined, both posed or developed operational problems during the tests, the results obtained indicate that it is possible, based on the redox potential of the solution, to produce an anolyte of consistent quality. Reproducibility of the results was good and it can be expected that this aspect of the performance of the mixed oxidant generators could be equaled or improved in devices which are of better design and construction. Redox potential of the anolyte taken over a period of time showed that the decay in the oxidative power was slow and in the tests it could be kept constant long enough to expose the bacteria to solution with a known redox value. The possible application of mixed oxidants as a disinfectant, was investigated. Although care was taken in the experiments to preserve the mixed oxidant (anolyte) solution to harvest the total potential oxidative power, and the possible synergistic effects of all the oxidants present its effect did not significantly exceed that of chlorine. This was in spite of the fact that the anolyte vapours, and presumably the anolyte solution as well, contained other strong oxidants such as ozone and hydrogen peroxide. It was also confirmed that the catholyte solution did not contain chlorine at concentrations that would have significant microbiocidal properties. This fact is also supported by the low or negative redox potential measured in the catholyte. Results from batch experiments show that the greatest reduction in the bacterial numbers took place within the first minute. In most cases both the anolyte and chlorine killed more than 99% of the bacteria present during the first minute of exposure. Increased contact time did not significantly reduce the numbers of the bacteria any further. In all the batch tests the average percentage reduction in bacterial numbers were 99,55 and 97,84 for the anolyte and chlorine solution respectively after five minutes contact time. The overall percentage reduction in the continuous flow tests were 99,85 for the anolyte and 99,61 for the chlorine. It can therefore be concluded that under the test conditions that prevailed that the mixed oxidant generators examined did not produce anolyte solutions with all the properties as claimed in literature. The units tested did not produce a product that behaved significantly differently from chlorine either. Although it seems as though mixed oxidant solutions of consistent quality can be produced from Electrochemical Activation (ECA) devices, and improvements in the functioning of the equipment can be expected in future, the use of such disinfectant generators in the rural areas would probably not be practical. As with other electrolytic chlorine generators a reliable electricity source is essential whilst, in most rural areas this cannot be guaranteed. The availability of high purity sodium chloride to use in the mixed oxidant generator may also present a problem. A further problem could be the disposal of the alkaline catholyte solution of which a volume equal to about one sixth of the volume of the anolyte, is produced. (Excerpts from the executive summary)