Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 246

8.34 lb, 1 million gallons of water weighs 8.34 million lb and would require 8.34 lb
of chemical to obtain a dosage of 1 ppm. Milligrams per liter (mg/L) is the metric
term for a dosage equal to ppm.

1 gallon = 8.34 lb

1 ppm = 1 mg/L

The number of pounds of chemical needed to achieve a certain dosage can be
determined by multiplying the ppm by the number of millions of gallons treated and
then by 8.34 lb/gal. The amount of water to be treated must always be in terms of
millions of gallons (MGD).

lb/day = mg/L × MGD × 8.34 lb/gal

One of the most frequently used calculations in wastewater mathematics is the
conversion of milligrams per liter (mg/L) concentration to pounds per day (lb/day)
or pounds (lb) dosage or loading. The general types of mg/L to lb/day or lb calcula-
tions are for chemical dosage, BOD, chemical oxygen demand (COD), suspended
solids (SS) loading/removal, pounds of solids under aeration, and waste activated
sludge (WAS) pumping rate. These calculations are usually made using either of the
following equations:

Dosage (lb/day) = Concentration (mg/L) × Flow (MGD) × 8.34 lb/gal (16.18)

Dosage (lb) = Concentration (mg/L) × Volume (MG) × 8.34 lb/gal (16.19)

Note: If the mg/L concentration represents concentration in a flow, then million
gallons per day (MGD) flow is used as the second factor; however, if the concen-
tration pertains to a tank or pipeline volume, then million gallons (MG) volume is
used as the second factor.

d osage f orMula p ie C hart
When converting pounds (lb) or mg/L, million gallons (MG) and 8.34 are key
parameters. The pie chart shown in Figure 16.4 and the steps listed below can be
helpful in finding lb or mg/L.

lb

8.34
lb/gal

mg/L

MG

FIGURE 16.4 Dosage formula pie chart.