Chlorine Demand and Residual

demand = dose - residual, C T = residual \times t

Breakpoint chlorination: dose must pass the demand (NH3, organics, Fe²⁺) to leave a free residual. CT tables (handbook) set t for a given residual, pH, and inactivation credit.

$d ose$ (mg/L): Applied chlorine
$r es i d u a l$: Free (or total) remaining
$C T$ (mg·min/L): Concentration × time

Step-by-step solved example

Dose 6.0 mg/L, residual 1.2 mg/L after 30 min. Demand and CT?

Contact tank: residual × t = CT after demand is met.

1. Demand
6.0−1.2=4.8 mg/L.
2. CT
1.2×30=36 mg·min/L.

Answer: demand=4.8 mg/L, CT=36 mg·min/L

10 practice questions

0/10 correct

1.Free chlorine is

2.HOCl dominates at

3.Combined residual before breakpoint is mostly

4.If residual is 0, CT is

5.UV disinfection uses

6.Giardia typically needs a

7.Dose 8, demand 5: residual =

8.t in CT should be

9.Dechlorination with SO2/bisulfite

10.Chlorine gas vs hypochlorite: demand math is the same but