Chlorine demand, mg/L = chlorine dosage, mg/L − chlorine residual, mg/L

Chlorine dosage, mg/L = chlorine demand, mg/L + chlorine residual, mg/L

Chlorine residual, mg/L = chlorine dosage, mg/L − chlorine demand, mg/L

Ct calculation, time = (Disinfectant residual concentration, mg/L) × (Time), units must be compatible