230
Another compound with an elevated oxidizing power is ozone which is,
therefore, an excellent disinfectant agent even at low temperatures (AWT, 2003).
Studies pointed out by the WHO indicate that it is possible to reduce in 4 to
5 log cycles the population of
Legionella
by using a concentration of 0.3 mg∙L
-
1
and contact time of 20 minutes (WHO, 2002). In turn, Muraca
et al
. (1987)
demonstrated that to reduce 5 log cycles of the population of
Legionella
at 25 °C,
the time needed is between 4 and 5 hours using an ozone concentration between
1 and 2 mg∙L
-1
. However, it is not easy to maintain the concentration of ozone at
the desired value in water distribution systems due to its great volatility (WHO,
2002). The Association of Water Technologies (AWT) notes that it is almost
impossible to maintain a suitable residual of ozone in a dynamic system of water
disinfection, i.e., with flowing water, so they recommend ozone to be injected as
close as possible to the point of use of the water, thus ensuring a concentration
between 1 and 2 mg∙L
-1
(AWT, 2003). Other objections made by the AWT are:
ozone is not efficient in the destruction of
Legionella
in biofilms; implementation
and maintenance costs of the ozone generator are relatively high; and uniform
concentration is hardly achieved through all the interior of the pipes.
Domingue
et al
. (1988) comparing many biocide agents that can be used
in the water disinfection, showed that ozone is the most efficient biocide in
the control of
Legionella pneumophila
, achieving a reduction of two log cycles
(reduction of 99%) in the population of this microorganism in 5 minutes using
an ozone concentration between 0.10 and 0.30 µg∙mL
-1
. On the other hand, by
using a free chlorine concentration of 0.30 µg∙mL-
1
and 0.40 µg∙mL-
1
the same
level of destruction was obtained after 30 minutes and 5 minutes, respectively.
These authors also indicated that ozone is more efficient in pH values above
neutrality, unlike the disinfection with chlorine, which is more efficient in pH
values below neutrality.
