In the US EPA surface water rule, CT is defined as the product of disinfectant concentration and contact time.

• A. Concentration of disinfectant multiplied by contact time
• B. Disinfectant dose per contact target
• C. Time required to reach disinfection
• D. Concentration of disinfectant times contact time

Answer: (D)

The main idea is that CT represents how effective disinfection is by combining how much disinfectant remains in the water with how long the water is in contact with it. In the EPA surface water rule, CT is defined as the product of the disinfectant concentration (C) and the contact time (T), giving CT = C × T. This value, with units of mg·min/L, shows that disinfection depends on both a sufficient residual level and enough exposure time; increasing either C or T increases CT and the disinfection achieved. An example helps: if you have 2 mg/L of chlorine and 3 minutes of contact, CT is 6 mg·min/L, which must meet the regulatory CT target for the pathogen of concern. The stated option describes this exact relationship using the conventional phrasing: concentration of disinfectant times contact time. The other ideas—dose per contact target or time alone—do not capture the defined CT concept, which hinges on multiplying concentration by contact time.