To demonstrate 4-log inactivation of viruses, adequate C x T must be provided.

• A. absence of coliforms in 95% of samples
• B. absence of coliforms in 95% of samples for that period
• C. adequate C x T
• D. adequate C x T for that sampling period

Answer: (C)

The main idea is that virus inactivation is driven by the product of disinfectant concentration and contact time, called C x T. To achieve a 4-log reduction, the process must deliver a C x T that meets or exceeds the validated CT value for the virus under the water’s temperature and pH conditions. When C x T is adequate, the disinfection system is expected to provide the required level of viral inactivation.

The other ideas hinge on bacterial indicators or sampling windows, which do not directly prove the necessary viral reduction. Absence of coliforms in samples relates to bacterial quality, not the specific CT needed for viruses, and talking about a period or sampling period does not address whether the actual C x T meets the required CT for viral inactivation.