During chlorine-based disinfection of wastewater, what is the purpose of maintaining a residual chlorine, and how does contact time affect disinfection?

• A. To Maintain Ongoing Disinfection
• B. A residual is maintained to provide ongoing disinfection; longer contact time increases CT and improves inactivation.
• C. To Improve Taste and Odor
• D. To Control Sludge Growth

Answer: (B)

Maintaining a chlorine residual keeps disinfection going as the water moves through the contact process and into the receiving environment. The effectiveness of chlorination is described by CT, the product of the chlorine concentration (the residual) and the contact time. A residual provides continuous exposure, so pathogens keep being inactivated even if conditions change downstream. Longer contact time increases CT, which enhances microbial inactivation. The other options don’t address how disinfection actually works: residual chlorine isn’t about taste or odor, nor is it primarily about sludge growth.