Ontario Operator-in-Training (OIT) Practice Exam

Which two key variables affect the disinfection effectiveness of chlorine?

1. Chlorine residual and temperature

2. Chlorine dosage and pH

3. Chlorine residual and contact time

4. Contact time and temperature

The correct answer is: Chlorine residual and contact time

Chlorine disinfection is a critical process in water treatment, and two key variables that significantly influence its effectiveness are chlorine residual and contact time. Chlorine residual refers to the amount of chlorine that remains in the water after initial disinfection has occurred. This residual is essential because it ensures that any remaining pathogens are destroyed as water moves through the distribution system. If the chlorine residual is too low, there may not be sufficient disinfection power to eliminate harmful microorganisms effectively. Contact time is the duration that chlorine is in contact with the water being treated. When water flows past chlorine, the longer it remains in contact, the more effective the chlorine will be at inactivating pathogens. Adequate contact time allows the chlorine to penetrate and kill microorganisms, thus enhancing overall disinfection. Taking both of these factors into account allows for a more efficient disinfection process, ensuring safe and clean drinking water. Understanding the interplay between chlorine residual and contact time is vital for water operators to optimize disinfection strategies and achieve regulatory compliance.