In calculating the C x T value, the contact time is extended to which point in the system?

• A. first customer
• B. influent of the first storage tank
• C. effluent of the first storage tank
• D. furthest point in the distribution system

Answer: (A)

The C × T value comes from multiplying the disinfectant concentration by the time water remains in contact with that disinfectant. When calculating this for distribution safety, you extend the contact time to the point where the water first reaches a consumer—the first customer. This ensures the disinfection effectiveness is still adequate as water begins to move from the treatment point into the distribution system and toward users.

Extending to this early point keeps the Ct calculation tied to the actual point of consumer exposure, rather than just the treatment facility itself. The influent or effluent of the first storage tank are still within the treatment/distribution interface, but using those points would not represent the time water has to remain disinfected as it travels to customers. Extending to the furthest point in the distribution system would incorporate the entire distribution travel time, which is not how the standard Ct requirement is defined for ensuring safety at the tap.

So, the contact time is extended to the first customer.