Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 249

■ Example 16.23
Problem: How many pounds per day of chlorine are required to provide a dosage of
2.3 mg/L in 820,000 gpd?

Solution: Change gpd to MGD:

820,000 gpd = 0.82 MGD

Calculate the pounds per day:

Chlorine (lb/day) = 2.3 mg/L × 0.82 × 8.34 = 15.7 lb/day

h ypoChlorite d osage
At many wastewater facilities, sodium hypochlorite or calcium hypochlorite is used
instead of chlorine. The reasons for substituting hypochlorite for chlorine vary;
however, due to the passage of stricter hazardous chemicals regulations by the
Occupational Safety and Health Administration (OSHA) and the U.S. Environmental
Protection Agency (USEPA), many facilities are deciding to substitute nonhazardous
hypochlorite for the hazardous chemical chlorine. Obviously, the potential liability
involved with using deadly chlorine is also a factor involved in the decision to sub-
stitute it with a less toxic chemical substance.
For whatever reason, when a wastewater treatment plant decides to substitute
chlorine for hypochlorite, the wastewater operator needs to be aware of the differ-
ences between the two chemicals. Chlorine is a hazardous material. Chlorine gas
is used in wastewater treatment applications at 100% available chlorine. This is an
important consideration to keep in mind when making or setting chlorine feed rates.
For example, if the chlorine demand and residual require 100 lb/day chlorine, the
chlorinator setting would be just that—100 lb/24 hr. Hypochlorite is less hazardous
than chlorine; it 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 economical consideration for those facilities thinking about
substituting hypochlorite for chlorine. Some studies indicate that such a switch
can increase overall operating costs by up to 3 times the cost of using chlorine.

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

Chlorine (lb/day) = Chlorine (mg/L) × Flow (MGD) × 8.34 lb/gal (6.22)

Chlorine (lb/day)
%Av

Hypochlorite (lb/day) = ×100 (6.23)

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