Explain how disinfection CT (concentration x time) governs the design of a disinfection system.

• A. CT has no relation to disinfection performance.
• B. A target CT value ensures sufficient microbial inactivation; actual CT depends on residual concentration and contact time in the system.
• C. CT is controlled only by temperature.
• D. CT is the ratio of pH to chlorine residual.

Answer: (B)

Disinfection effectiveness is governed by the product of how much disinfectant is present and how long the water stays in contact with it. This CT concept is used to set a target value that corresponds to the level of microbial inactivation required by regulations. In design, the system is arranged so that, under expected operating conditions, the water experiences at least that CT value. The actual CT realized is the measured residual disinfectant concentration (C) at the point of interest multiplied by the contact time (t) the water spends in the contact zone. Temperature and pH influence disinfection rates, so they affect both the attainable residuals and the required CT to achieve the same level of inactivation. To reach the target CT, you can increase the chlorine residual by dosing more disinfectant or increase the contact time by enlarging the contact basin or reducing flow. The key idea is that CT—concentration times time—links how much disinfectant remains with how long the organism exposure lasts to achieve the desired inactivation.