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
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.
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.
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.