Water Treatment Processes - Mathematics and Statistics for the Environment - page 658

■ EXAMPLE 23.117
Problem: A total of 50 lb/day sodium hypochlorite is required for disinfection of a 1.5-MGD flow.
How many gallons per day hypochlorite is this?

Solution: Because lb/day hypochlorite has already has been calculated, we simply convert lb/day to
gpd hypochlorite required:

Hypochlorite (lb/day)
8.34 lb/gal

50 lb/day
8.34 lb/gal

Hypochlorite (gpd) = = = 6.0 gpd

■ EXAMPLE 23.118
Problem: A hypochlorinator is used to disinfect the water pumped from a well. The hypochlorite
solution contains 3% available chlorine. A chlorine dose of 1.3 mg/L is required for adequate dis-
infection throughout the system. If the flow being treated is 0.5 MGD, how many gpd of the hypo-
chlorite solution will be required?

Solution: Calculate the lb/day chlorine required:

Chlorine (lb/day) = 1.3 mg/L × 0.5 MGD × 8.34 lb/gal = 5.4 lb/day

Calculate the lb/day hypochlorite solution required:

Hypochlorite (lb/day) = 5.4 lb/day chlorine/0.03 = 180 lb/day

Calculate the gpd hypochlorite solution required:

Hypochlorite (gpd) = 180 lb/day/8.34 lb/gal = 21.6 gpd

23.7.7 p erCent s trength oF s olutions
If a teaspoon of salt is dropped into a glass of water it gradually disappears. The salt dissolves in
the water, but a microscopic examination of the water would not show the salt. Only examination at
the molecular level, which is not easily done, would show the salt and water molecules intimately
mixed. If we taste the liquid, we would know that the salt is there. We can recover the salt by evapo-
rating the water. In a solution, the molecules of the salt, the solute , are homogeneously dispersed
among the molecules of water, the solvent . This mixture of salt and water is homogeneous on a
molecular level. Such a homogeneous mixture is called a solution . The composition of a solution
can be varied within certain limits. The three common states of matter are gas, liquid, and solid.
In this discussion, we are only concerned, at the moment, with the solid (calcium hypochlorite) and
liquid (sodium hypochlorite) states.

23.7.8 C alCulating p erCent s trength u sing d ry h ypoChlorite
To calculate the percent strength of a chlorine solution, we use Equation 23.92:

%Avai lablechlorine

100

Hypochlorite (lb)

%Chlorinestrength = ×100 (23.92)

%Availablechlorine
100

Water (lb) + Hypochlo rite (lb)