Water Treatment Processes - Mathematics and Statistics for the Environment - page 621

Solution: The solution feed rate is 100 gal per day, or 100 gpd. Expressed as MGD, this is 0.000100
MGD. Use the mg/L to lb/day equation to calculate the actual feed rate:

Chemical feed rate = Chemical (mg/L) × Flow (MGD) × 8.34 lb/day

Chemical feed rate = 12,000 mg/L × 0.000100 MGD × 8.34 lb/day = 10 lb/day

The actual pumping rates can be determined by calculating the volume pumped during a speci-
fied time frame; for example, if 60 gal are pumped during a 10-min test, the average pumping rate
during the test is 6 gpm. Actual volume pumped is indicated by a drop in tank level. By using the
following equation, we can determine the flow rate in gpm:

2 3

0.785 ×× D × 7.48 gal/ft

Durationoftest(min)

Drop in level (ft)

Flow rate (gpm) = (23.37)

■ EXAMPLE 23.41
Problem: A pumping rate calibration test is conducted for a 15-min period. The liquid level in the
4-ft-diameter solution tank is measured before and after the test. If the level drops 0.5 ft during the
15-min test, what is the pumping rate in gpm?

Solution:

2 3

0.785 ×× D 7.48 gal/ft

Durationoftest(min)

Drop in level (ft) ×

Flow rate (gpm) =

3

0.785 × ( 4ft4ft ×× × ) 0.5 ft 7.48 gal/ft

= = 3.1 gpm

15 min

23.4.7 d etermining C hemiCal u sage
One of the primary functions performed by water operators is the recording of data. The lb/day or
gpd chemical use is part of the data from which the average daily use of chemicals and solutions
can be determined. This information is important in forecasting expected chemical use, compar-
ing it with chemicals in inventory, and determining when additional chemicals will be required. To
determine average chemical use, we use Equation 23.38 (lb/day) or Equation 23.39 (gpd):

Totalchemical used (l b)

Averageuse (lb/day) = (23.38)

Number of days

Totalchemical used (gal)

Averageuse (gpd) = (23.39)

Number of days

Then we can calculate the days supply in inventory:

Totalchemical in inventory(lb)

Averageuse (lb/day)

Days supplyininventory = (23.40)

Totalchemical in inventory(gal)

Averageuse (gpd)

Days supplyininventory = (23.41)