Wastewater Calculations - Mathematics and Statistics for the Environment - page 706

24.10.2.3 Chlorine Residual
Chlorine residual is the amount of chlorine remaining after the demand has been satisfied.

■ EXAMPLE 24.65
Problem: What should the chlorinator setting (lb/day) be to treat a flow of 3.9 MGD if the chlorine
demand is 8 mg/L and a chlorine residual of 2 mg/L is desired?

Solution: First calculate the chlorine dosage in mg/L:

Chlorine dose = Chlorine demand + Chlorine residual = 8 mg/L + 2 mg/L = 10 mg/L

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

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

10 mg/L × 3.9 MGD × 8.34 lb/gal = 325 lb/day chlorine

24.10.3 h ypoChlorite d osage
Hypochlorite is less hazardous than chlorine; therefore, it is often used as a substitute chemical for
elemental chlorine. Hypochlorite is similar to strong bleach and comes in two forms: dry calcium
hypochlorite (often referred to as HTH) and liquid sodium hypochlorite. Calcium hypochlorite
contains about 65% available chlorine; sodium hypochlorite contains about 12 to 15% available
chlorine (in industrial strengths).

Note: Because neither type of hypochlorite is 100% pure chlorine, more lb/day must be fed into the
system to obtain the same amount of chlorine for disinfection. This is an important economi-
cal consideration for those facilities thinking about substituting hypochlorite for chlorine.
Some studies indicate that such a substitution can increase operating costs, overall, by up to
three times the cost of using chlorine.

To calculate the lb/day hypochlorite required, a two-step calculation is required:

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

Chlorine (lb/day)
%Available
100

= Hypochlorit ee(lb/day) (24.93)

■ EXAMPLE 24.66
Problem: A total chlorine dosage of 10 mg/L is required to treat a particular wastewater. If the flow
is 1.4 MGD and the hypochlorite has 65% available chlorine, how many lb/day of hypochlorite will
be required?

Solution: First calculate the lb/day chlorine required using the mg/L to lb/day equation:

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

10 mg/L × 1.4 MGD × 8.34 lb/gal = 117 lb/day

Then calculate the lb/day hypochlorite required. Because only 65% of the hypochlorite is chlorine,
more than 117 lb/day will be required: