Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 364

Note: In previous years, it was normal practice to use the expression parts per million (ppm) as an
expression of concentration, as 1 mg/L = 1 ppm; however, current practice is to use mg/L as
the preferred expression of concentration.

■ EXAMPLE 14.16
Problem: Determine the chlorinator setting (lb/day) required to treat a flow of 8 MGD with a chlo-
rine dose of 6 mg/L.

Solution:

mg/L × MGD × 8.34 lb/gal = 6 mg/L × 8 MGD × 8.34 lb/gal = 400 lb/day

■ EXAMPLE 14.17
Problem: What should the chlorinator setting be (lb/day) to treat a flow of 3 MGD if the chlorine
demand is 12 mg/L and a chlorine residual of 2 mg/L is desired?

Note: The chlorine demand is the amount of chlorine used in reacting with various components
of the wastewater such as harmful organisms and other organic and inorganic substances.
When the chlorine demand has been satisfied, these reactions stop.

To find the unknown value of lb/day, we must first determine chlorine dose. To do this we must use
Equation 14.15:

Chlorine dose (mg/L) = Chlorine demand (mg/L) + Chlorine residual (mg/L) (14.15)

Chlorine dose = 12 mg/L + 2 mg/L = 14 mg/L

Then we can make the mg/L to lb/day calculation:

mg/L × MGD × 8.34 lb/gal = lb/day

12 mg/L × 3 MGD × 8.34 lb/gal = 300 lb/day

14.1.2.10.2 Hypochlorite Dosage
At many wastewater facilities, sodium hypochlorite or calcium hypochlorite is used instead of chlo-
rine. 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 the hazardous chemical chlorine with nonhazardous hypochlorite. Obviously, the poten-
tial liability involved with using deadly chlorine is also a factor involved in the decision to substitute
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
differences between the two chemicals. Chlorine is a hazardous material. Chlorine gas is used in
wastewater treatment applications as 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).