The usage of ozone is only senseful in cold water systems. But the decisive question is whether ozone is used for a short-term (1-2 hours) or for a long term (> 6 hours) prevention of microbial growth. In the first case, > 50 ppb ozone is generally sufficient whereas in the second case at least 20 ppb are required.
One should keep in mind that WFI cold systems have basically a higher risk of microbial contamination. The need for ozone in large ring systems or in areas difficult to access may be higher. The ozone levels mentioned should thus be achieved in the return flow. Setting the correct ozone concentration for the system must be done within the scope of the PQ - i.e. validation of the water system.
In contrast, ozonisation of hot-stored WFI systems doesn't make sense. Indeed, the half-life of ozone considerably decreases at temperatures over 40° Celsius. Moreover, the heat in hot WFI system causes sanitisation itself; the usage of additional ozone wouldn't be meaningful. The risk of biofilm formation in hot-stored WFI systems is considerably lower.