In a disinfection context, what factors determine the actual CT value?

• A. Residual concentration and contact time in the system.
• B. Temperature and color.
• C. pH alone.
• D. Flow rate alone.

Answer: (A)

Disinfection effectiveness is captured by the CT concept: the actual CT value is determined by the product of the disinfectant’s residual concentration in the water (C) and the time the water stays in contact with it (T) in the system. The higher the residual concentration and the longer the contact time, the larger the CT value, and thus the greater the achieved disinfection.

Temperature and pH influence how effective a given CT is (they can make disinfection easier or harder for the same C and T), but the CT value itself is defined by C and T in the system. Flow affects contact time (through how long water spends in the contact zone), so it ties into CT indirectly, but the core determinant remains the measured residual concentration and the actual contact time.