220
FromFigure1it iseasytoverifythat thezparameter for
Legionellapneumophila
is 5.29 °C. Therefore, if we want a D parameter that is one tenth of another
one, as for example, to pass from a value of 100 minutes to 10 minutes, we
must elevate the temperature of the water in 5.3 °C. According to Sanden
et al
.
(1989) the values D and z obtained for
Legionella
are not values that indicate
unusual heat resistance and therefore, from the point of view of thermo
bacteriology, there are no great difficulties to destroy this microorganism by
using heat.
However, fromapractical point of view, Schulze-Röbbecke andBuchholtz
(1992) state that the employment of the thermal treatment for the elimination
of
Legionella
in water distribution systems is not an easy measure to apply.
For example, with an initial population of 105 UFC∙L
-1
and expecting a final
population of 100 UFC∙L
-1
, we have to maintain the water at the temperature
of 60 °C per 6 minutes, assuming a D value equal to 2 minutes (average value
obtained from the data used to build Figure 1 for the temperature of 60 °C).
It is important to emphasize that the selected temperature to perform the
reduction of the population of
Legionella
must be maintained along the
entire system in order to make the treatment effective. According to Stout
et al
. (1986), a reduction in the population of
Legionella
by eight log cycles
(reduction of 99.999999%) was obtained while maintaining the water at
the temperature of 60 °C per 25 minutes, at 70 °C per 10 minutes, or at 80
°C per 5 minutes. Nevertheless, Plouffe
et al
. (1983) point out that after a
water distribution system is colonized by
Legionella pneumophila
, to apply
only a thermal treatment at 60 °C will hardly cause the eradication of this
microorganism from the pipes.
