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Often, even higher concentrations of free chlorine are required to
control the contamination of
Legionella
. In this case the process is called
hyperchlorination
. To avoid corrosion problems, the most used practice
is to inject a chlorine compound to generate HOCl in amount enough to
reach the concentration between 20 and 50 mg∙L
-1
. After some time, this
solution is drained and the pipes are rinsed with water with a concentration
of free chlorine between 0.5 and 1.0 mg∙L
-1
(Lin
et al
., 1998, AWT, 2003).
One of the main problems concerning the use of hypochlorous acid
in the disinfection is the fact that it decomposes in hot water. Another
problem associated to the chlorination is its corrosivity, but this problem
can be reduced by coating the internal part of the piping with sodium
silicate (Muraca
et al
., 1987, Kilvington e Price, 1990, WHO, 2002, AWT,
2003). Due to the corrosivity, this process has been replaced by copper/
silver ionization, which will be addressed later. Another important problem
related to the use of hyperchlorination in the attempt to reduce the presence
of
Legionella
is the possibility to accelerate the formationof trihalomethanes.
In addition to these inconveniences, there are two other aspects: not always
the chlorination can efficiently eradicate the presence of
Legionella
from
the water distribution system, because the microorganisms can be housed
in biofilms or in cysts of protozoan, which complicates a lot the action of
the hypochlorous acid. And also, hyperchlorination gives only temporary
results.
Studies show that if shock hyperchlorination is employed, the
recolonization of the surfaces by
Legionella pneumophila
can occur when
there is reduction in the levels of chlorine (Kilvington e Price, 1990) and
Saby
et al
., 2005). According to these authors, this recolonization can occur
because it is not possible to destroy the cells of
Legionella
, but inhibit its
