What is the effect of longer contact time on disinfection?

• A. It leads to more complete pathogen inactivation.
• B. It reduces water flow.
• C. It increases taste and odor.
• D. It has no effect on disinfection.

Answer: (A)

Longer contact time gives the disinfectant more opportunity to react with and inactivate microorganisms. In disinfection, we think in terms of CT (concentration × time); if the concentration stays the same, increasing the time raises CT and generally leads to more complete pathogen inactivation. That’s why treatment systems are designed to provide a minimum contact time in the disinfection basin—to ensure enough interaction between the disinfectant and microbes. Flow and taste/odor effects aren’t directly about how long water sits with the disinfectant; flow is a hydraulic factor, and taste/odor relate more to concentration and chemical byproducts. So the main effect of longer contact time is greater disinfection effectiveness.