Water Treatment Processes - Mathematics and Statistics for the Environment - page 655

Free Residual
Formed

Chloramines
Formed

Chloramines
Destroyed

Demand

Free
Available
Chlorine
Residual

Combined Chlorine Residual

Chlorine Added to Water

FIGURE 23.4 Breakpoint chlorination curve.

■ EXAMPLE 23.110
Problem: A chlorinator setting is increased by 2 lb/day. The chlorine residual before the increased
dosage was 0.2 mg/L. After the increased chlorine dose, the chlorine residual was 0.5 mg/L.
The average flow rate being chlorinated is 1.25 MGD. Is the water being chlorinated beyond the
breakpoint?

Solution: Calculate the expected increase in chlorine residual using the mg/L to lb/day equation:

Increase in chlorine (lb/day) = Expected increase (mg/L) × Flow (MGD) × 8.34 lb/gal

2 lb/day = x mg/L × 1.25 MGD × 8.34 lb/gal

x = (2 lb/day)/(1.25 MGD × 8.34 lb/gal) = 0.19 mg/L

The actual increase in residual chlorine is

0.5 mg/L - 0.19 mg/L = 0.31 mg/L

■ EXAMPLE 23.111
Problem: A chlorinator setting of 18 lb chlorine per 24 hr results in a chlorine residual of 0.3 mg/L.
The chlorinator setting is increased to 22 lb per 24 hr. The chlorine residual increased to 0.4 mg/L
at this new dosage rate. The average flow being treated is 1.4 MGD. On the basis of these data, is the
water being chlorinated past the breakpoint?

Solution: Calculate the expected increase in chlorine residual:

Increase in chlorine (lb/day) = Expected increase (mg/L) × Flow (MGD) × 8.34 lb/gal

4 lb/day = x mg/L × 1.4 MGD × 8.34 lb/gal

x = (4 lb/day)/(1.4 MGD × 8.34 lb/gal) = 0.34 mg/L

The actual increase in residual chlorine is

0.4 mg/L - 0.3 mg/L = 0.1 mg/L