Water Treatment Processes - Mathematics and Statistics for the Environment - page 660

23.8 CHEMICAL USE CALCULATIONS
In a typical plant operation, chemical use is recorded each day. Such data provide a record of daily
use from which the average daily use of the chemical or solution can be calculated. To calculate
average use in pounds per day (lb/day), we use Equation 23.94. To calculate average use in gallons
per day (gpd), we use Equation 23.95:

Average use (lb/day) = Total chemical used (lb)/Number of days (23.94)

Average use (gpd) = Total chemical used (gal)/Number of days (23.95)

To calculate the days supply in inventory, we use Equation 23.96 or Equation 23.97:

Days supply in inventory = Total chemical in inventory (lb)/Average use (lb/day) (23.96)

Days supply in inventory = Total chemical in inventory (gal)/Average use (gpd) (23.97)

■ EXAMPLE 23.121
Problem: Calcium hypochlorite usage for each day during a week is given below. Based on these
data, what was the average lb/day hypochlorite chemical use during the week?

Monday 50 lb/day

Tuesday 55 lb/day

Wednesday 51 lb/day

Thursday 46 lb/day

Friday 56 lb/day

Saturday 51 lb/day

Sunday 48 lb/day

Solution:

Average use (lb/day) = Total chemical used (lb)/Number of days

= 357 lb/7 days = 51 lb/day

■ EXAMPLE 23.122
Problem: The average calcium hypochlorite use at a plant is 40 lb/day. If the chemical inventory in
stock is 1100 lb, how many days' supply is this?

Days supply in inventory = Total chemical in inventory (lb)/Average use (lb/day)

= (1100 lb in inventory)/(40 lb/day average use) = 27.5 days

23.9 CHLORINATION CHEMISTRY
Chlorine is used in the form of free elemental chlorine or as hypochlorites. Temperature, pH, and
organic content in the water influence its chemical form in water. When chlorine gas is dissolved in
water, it rapidly hydrolyzes to hydrochloric acid (HCl) and hypochlorous acid (HOCl):

Cl 2 + H 2 O ↔ H + + Cl - + HOCl (23.98)