Explain the concept of CT in water disinfection and how it informs chlorine dosing.

• A. CT equals disinfectant concentration (C) times contact time (T); higher CT values indicate greater disinfection; dosing should achieve the required CT for target pathogens.
• B. CT stands for Chlorine Trial and determines taste.
• C. CT is chemical temperature; higher temperature increases disinfection.
• D. CT is contact toxicity; higher CT means more hazardous.

Answer: (A)

The essential idea is that disinfection effectiveness is determined by the product of the disinfectant concentration and the time water stays in contact with it. For chlorine, this means CT equals the free chlorine concentration (C, in mg/L) times the contact time (T, in minutes), giving CT in mg·min/L. For a given target microorganism, at a specific temperature and pH, there is a CT value that achieves the desired inactivation. So, chlorine dosing is planned to produce a residual concentration and maintain it for enough time in the system to reach that required CT. Temperature and pH influence how fast inactivation occurs (HOCl is more active than OCl−, and higher temperatures generally speed the process), so the CT value you aim for depends on these conditions. In practice, you balance achieving the needed CT with practical limits on taste, corrosion, and by-product formation, ensuring the residual is maintained long enough to meet the target disinfection.