A facility is required to log what for virus control?

In the context of virus control within water treatment facilities, the concept of "log reduction" is crucial. A log reduction measures the effectiveness of the treatment process in removing or inactivating microorganisms, including viruses. When a facility is required to log a certain number of logs, this indicates the level of assurance that pathogenic viruses are effectively controlled.

In this case, the requirement to log 4 logs signifies that the facility must be capable of demonstrating a 99.99% reduction in viruses present in the treated water. This level of reduction is critical for ensuring the safety of the water supply and protecting public health, particularly because certain viruses can be resilient and pose significant health risks.

Logging 4 logs is a standard reference point in regulations aimed at virus control in surface water treatment processes, ensuring adequate disinfection. The other options indicating 2, 3, or 5 logs do not align with the established standards for comprehensive virus control in this context. Therefore, the requirement for 4 logs provides a necessary margin of safety and is aligned with best practices in water treatment.