AUTHORS: Yahya MT, Landeen LK, Mesina MC, Kutz SM, Schultze R, & Gerba CP

PUBLICATION REF: Canadian Journal of Microbiology 36: 109-116, 1990

PURPOSE OF STUDY:
The recommended minimum level of free chlorine for disinfection of public swimming pools is 1 mg/liter. This level is difficult to maintain due to the chlorine-demanding organic material introduced by bathers themselves as well as the environment. Eye and skin irritation may also occur at the minimum chlorine level needed for effective disinfection. Electrolytically generated copper/silver ions are also microbiocidal and are much less subject to degradation but are slower acting than chlorine. Therefore, the authors tested the hypothesis that using the two methods together would accomplish effective disinfection while reducing the level of free chlorine required.

MATERIALS AND METHODS:
Two 32-gallon plastic containers, one indoors (temperature range 22 to 25) and the second outdoors exposed to sunlight (temperature range 18 to 36) were filled with tap water. After chemical analysis and adjustment of pH and test levels of disinfectants, bath water and urine were added to stimulate typical swimming conditions. Four treatment regimens were tested: (1) No added disinfectants (2) Free chlorine alone at the generally recommended level of 1 mg/liter (3) Free chlorine at 0.3 mg/liter combined with copper and silver ions at a ration of 400 ug/liter of copper to 40 ug/liter of silver (4) Copper and silver ions alone at the same ratio as above. An isolate of Staphylococcus sp was employed for bacterial challenge testing since previous work had shown that staphylococci are more resistant to disinfection than are coli form bacteria. The experiment was continued for 12 weeks.

RESULTS:
In the test of free chlorine alone, location proved to be critical. In the outdoor setting subject to strong sunlight and high temperatures, no residual chlorine could be detected 3 to 4 hours after optimization. Indoor, where environmental factors were much less extreme, a residual level of 0.1 to 0.3 mg/liter was found after 24 hours.
Bacterial counts were kept within drinking water standards (as recommended for swimming pools) by either high levels of chlorine alone or by the combination regimen of copper and silver ions with low levels of chlorine: the difference in total bacterial numbers was not significant. Hen challenged with Staphylococcus sp isolate, the combination of copper and silver ions with low levels of chlorine achieved a 2.4 log 10 reduction in bacterial numbers within 2 minutes, while the single-agent regimes (free chlorine alone, or copper/silver alone) showed only 1.5 & 0.03 log 10 reductions respectively. Under Staphylococcus sp challenge, the combined copper/silver and free chlorine had a faster log 10 reduction of microbial numbers than did treatment with a high level of chlorine alone.

CONCLUSIONS:
The addition of electrolytically generated copper/silver ions in the radio tested (400 ug/liter copper to 40 ug/liter silver) allowed reduction in the concentration of free chlorine to one third of the level customarily recommended. The use of copper/silver may provide resisting protection in swimming pools after chlorine has been rendered ineffective due to contamination from swimmers and the natural environment.