THE APPLICATION AND EFFICIENCY OF "MIXED OXIDANTS" FOR THE TREATMENT OF DRINKING WATER
Report No. 832/1100

Executive Summary

Ongoing research into new technologies is required to develop less expensive water treatment processes without sacrificing water quality.

The process of disinfection is one of the areas that needs to be examined continuously to find more efficient disinfectants against presently known micro-organisms and the potentially harmful ones that are discovered.

In addition, growing concerns about the use of chlorine with its potentially harmful by-products place greater emphasis on the need for water utilities to investigate water disinfection alternatives to chlorination. A seemingly advanced technology, new to Western practice but utilised for some time now in many of the Eastern bloc states, has received much acclaim of late. The general concept is that of an electrolytic cell and electrolyte used to generate amounts of anolyte and catholyte, said to contain a variety of oxidants, the mixture of which is referred to as "mixed oxidants". The oxidants include amongst others, chlorine, chlorine dioxide, hydrogen peroxide, ozone and hydroxyl radicals. This technology is purported to present the following advantages over present disinfection methods:

• The efficiency of the oxidants produced is, by mass, higher than that of chlorine;
• Oxidant mixtures can be produced on-site using only electricity and sodium chloride;
• Oxidant concentration and composition may be adjusted according to specific needs; and
• A residual may be maintained in product water.

Although there is no record of the large-scale application of this technique in water disinfection, it could be considered as a means of primary disinfection on small water treatment plants and to augment depleted disinfectant concentrations in distribution systems. Chlorination in various forms is regarded as a reliable, cost-effective method for disinfecting water for drinking purposes. It is used on small, remote plants and on large-scale sophisticated drinking water treatment plants. However in remote areas the application of chlorine presents problems where difficulties such as the following, may exist. These and other reasons may give impetus into investigating other disinfectants.

• Distance from the place of chlorine manufacture
• Transport and delivery schedules are unreliable or non-existent;
• Lack of sufficient expertise on the proper dosing of chlorine (especially of gaseous chlorine);
• The quality of chlorine gas obtained causes problems at the point of dosing.

Definitive research is required into the evaluation of the application of mixed oxidant technology for the treatment of drinking water, especially for use in remote areas as a primary disinfectant.

The objectives of this research project were as follows:

• To evaluate a relatively new concept that could be used for disinfection of water supplied to small and rural communities; and
• To make recommendations on the application of mixed oxidants and the possible benefits and shortcomings of such a system.

Conclusions

The results obtained in this study 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. The results and affects claimed to be achieved by the application of ECA technology sometimes borders on the metaphysical and not enough conclusive evidence is available to either support or discard the claims made. Since only two different Electrochemical Activation (ECA) devices were examined, and both posed or developed operational problems during the tests, it would unfair to make comments on any other ECA devices that may be available commercially or on the functioning of improved models as those that were tested. Results obtained however indicate that despite the operational problems experienced 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. This aspect was not only important for the execution of the tests which were done, but would also be of significant importance if commercial use of a mixed oxidant generator had to be considered. 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.

Recommendations for future research

The following aspects of the generation and application of mixed oxidants warrant further investigation:

• Examine and understand the principle of operation of the mixed oxidant generator.
• Develop methods to analyse for the different oxidative compounds present in the anolyte.
• Develop methods to analyse for the different compounds present in the catholyte.
• Design of a system in which the mixed oxidant solution can be applied such that full oxidative potential of all the compounds present in the fluid as well as the vapours can be utilised.
• Examine potential non-conventional applications and benefits of mixed oxidants.

A member of a previously disadvantaged population group participated in the research for this project.