Treated water flows through a chlorine contact basin with a hydraulic detention time of 45.0 minutes. If the free chlorine residual measured at the outlet is 1.60 mg/L, what is the calculated CT value in mg·min/L?

Formula Used: CT Value (mg·min/L) = Free Disinfectant Residual (mg/L) × Effective Contact Time (minutes)

Step-by-Step Mathematical Derivation:

1. Multiply free chlorine concentration by contact time: 1.60 mg/L × 45.0 min = 72.0 mg·min/L.

⚠️ Common Exam Trap: EPA Surface Water Treatment Rules mandate specific minimum CT values based on water pH, temperature, and target log removal.