Water Treatment Processes - Mathematics and Statistics for the Environment - page 654

Solution:

Chlorine dose = Chlorine demand (mg/L) + Chlorine residual (mg/L)
= 1.7 mg/L + 0.9 mg/L = 2.6 mg/L

■ EXAMPLE 23.108
Problem: The chlorine dosage for water is 2.7 mg/L. If the chlorine residual after 30 min of contact
time is found to be 0.7 mg/L, what is the chlorine demand expressed in mg/L?

Solution:

Chlorine dose (mg/L) = Chlorine demand (mg/L) + Chlorine residual (mg/L)

2.7 mg/L = x mg/L + 0.6 mg/L

x mg/L = 2.7 mg/L - 0.7 mg/L = 2.0 mg/L

■ EXAMPLE 23.109
Problem: What should the chlorinator setting (lb/day) be to treat a flow of 2.35 MGD if the chlorine
demand is 3.2 mg/L and a chlorine residual of 0.9 mg/L is desired?

Solution: Determine the chlorine dosage in mg/L:

Chlorine dose = Chlorine demand (mg/L) + Chlorine residual (mg/L)
= 3.2 mg/L + 0.9 mg/L = 4.1 mg/L

Calculate the chlorine dosage (feed rate) in lb/day:

Chlorine (lb/day) = Chlorine (mg/L) × Flow (MGD) × 8.34 lb/gal

= 4.1 mg/L × 2.35 MGD × 8.34 lb/gal = 80.4 lb/day

23.7.4 b reaKpoint C hlorination C alCulations
To produce a free chlorine residual, enough chlorine must be added to the water to produce what
is referred to as breakpoint chlorination , the point at which near complete oxidation of nitrogen
compounds is reached. Any residual beyond breakpoint is mostly free chlorine (see Figure 23.4).
When chlorine is added to natural waters, the chlorine begins combining with and oxidizing the
chemicals in the water before it begins disinfecting. Although residual chlorine will be detectable in
the water, the chlorine will be in the combined form with a weak disinfecting power. As we see in
Figure 23.4, adding more chlorine to the water at this point actually decreases the chlorine residual
as the additional chlorine destroys the combined chlorine compounds. At this stage, water may have
a strong swimming pool or medicinal taste and odor. To avoid this taste and odor, add still more
chlorine to produce a free residual chlorine. Free chlorine has the highest disinfecting power. The
point at which most of the combined chlorine compounds have been destroyed and the free chlorine
starts to form is the breakpoint .

Note: The actual chlorine breakpoint of water can only be determined by experimentation.

To calculate the actual increase in chlorine residual that would result from an increase in chlorine
dose, we use the mg/L to lb/day equation as shown below:

Increase in chlorine (lb/day) = Expected increase (mg/L) × Flow (MGD) × 8.34 lb/gal (23.89)

Note: The actual increase in residual is simply a comparison of new and old residual data.