To ensure adequate contact time before chlorine addition, PAC should have at least ______ minutes.

• A. 10 min
• B. 15 min
• C. 20 min
• D. 25 min

Answer: (B)

Adequate contact time before disinfection lets the PAC interact with the water long enough for adsorption to occur and for the mixture to develop uniform conditions. When PAC is in contact with the water, it captures dissolved organic matter and other compounds that would otherwise consume chlorine or form disinfection byproducts. Giving PAC about 15 minutes of residence time before adding chlorine ensures most of this adsorption can take place and the mixture is well blended, so chlorine is used efficiently and a stable residual can be maintained downstream.

If contact time is too short, PAC may not remove enough organics, leaving higher chlorine demand and the potential for higher byproduct formation. While longer times (like 20 or 25 minutes) can be implemented in some plants, 15 minutes is the commonly cited minimum that balances effectiveness with practical plant design and operation.