Report No DWI0743

Nov 1993

1. SUMMARY

1.1 In a previous test programme an electrolytic cell was developed which could reduce the bacteriological count in water with low chloride concentrations by as much as 6 orders of magnitude. Some tests showed that bacteria reductions of up to 3 orders of magnitude could be achieved with no chlorides present. The purpose of the present experimental programme is to establish the mechanism of bacterial kill in the electrolytic cell so that the most appropriate electrode material and cell design can be specified. Additionally, this work is to assess the effects of this electrolysis process on viruses.

1.2 A bipolar rod cell with Ebonex electrodes was used for most of the test programme. The electrolyte was a standard solution made up of distilled water with the additions of NaCl and Na2SO4 to provide the necessary ions for chloride content and electrical conductivity.

1.3 At first techniques using microscopy were developed to see whether or not changes could be observed between untreated bacteria and those treated with chlorine or by the electrolytic process.

1.4 Examination of the treated bacteria under a light microscope using various stains has not revealed any signs of damage to the bacteria.

1.5 A special purpose built cell was used so that bacteria could be observed on the surface of the electrode during electrolysis. No significant changes to the bacteria were observed although there was a movement towards the +ve Ebonex electrode.

1.6 Scanning Electron Microscopy has shown the following: (a) The seed E.coli cells are generally approximately 1.5 µm long.

(b) There is no sign of damage to the E.coli cells when treated with chlorine.

(c) A small number of damaged cells were observed in both the inlet and outlet samples taken from the electrolytic cell tests. It is thought that this is due to natural causes rather than the effects of electrolysis.

1.7 It has not been possible to carry out examination with Transmission Electron Microscopy because of the low concentration of bacteria and not being able to produce a large enough pellet of the bacteria to process. This method of microscopy could have revealed internal changes to the bacteria cells.

1.8 Electron microscopy showed no conclusive evidence that bacteria were being damaged by the effects of direct oxidation (rather than by the effects of chlorine).

1.9 Experiments were carried out to assess how important chlorides are for effective disinfection, to investigate the significance of contact time between test samples and chlorine, to study the effects of organic substances on cell performance, and to investigate whether or not there is any recovery of the bacteria after treatment.

1.10 These experiments suggest that the main mechanism which is destroying the bacteria is that of chlorine generated from chlorides in the water.

1.11 There is no evidence from this test programme which suggests an alternative mechanism to which the kill can be attributed. It was noted that the addition of the bacterial culture to the water increased the chloride concentration by between 2 - 5ppm. This is due to chlorides in the nutrient broth. This had not been taken into account in previous work and could go some way to explain why there was apparently some kill when no NaCl was added (ref. 1).

1.12 A number of tests were carried out to assess the effect of the electrolytic cell on bacteriophage. These showed that at a Cl- concentration of 20ppm almost a 10^6 reduction in the number of infective bacteriophages could be expected with the standard electrolyte. It was also noted that there was no inactivation of the phages when the chloride concentration was as low as 2ppm.

1.13 The addition of 1ppm Cl2 for a contact time of 30 seconds had devastating effects on the bacteriophage with a reduction of nearly 6 orders of magnitude.

1.14 In general these results are encouraging and it is recommended that tests on a human virus be carried out.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.