Wastewater Calculations - Mathematics and Statistics for the Environment - page 704

Concentration (mg/L) × Flow (MGD) × 8.34 lb/gal = lb/day (24.88)

Concentration (mg/L) × Volume (MG) × 8.34 lb/gal = lb (24.89)

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

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.

24.10.1 C hemiCal F eed r ate
In chemical dosing, a measured amount of chemical is added to the wastewater (or water). The
amount of chemical required depends on the type of chemical used, the reason for dosing, and the
flow rate being treated. The two expressions most often used to describe the amount of chemical
added or required are

• Milligrams per liter (mg/L)
• Pounds per day (lb/day)

A milligram per liter is a measure of concentration. As shown below, if a concentration of 5 mg/L
is desired, then a total of 15 mg chemical would be required to treat 3 L:

5mg3
L3

× 15 mg

3L

=

×

The amount of chemical required therefore depends on two factors:

• The desired concentration (mg/L)
• The amount of wastewater to be treated (normally expressed as MGD)

To convert from mg/L to lb/day, Equation 24.88 is used.

■ EXAMPLE 24.60
Problem: Determine the chlorinator setting (lb/day) required to treat a flow of 5 MGD with a chemi-
cal dose of 3 mg/L.

Solution:

Chemical = Chemical × Flow × 8.34 lb/gal = 3 mg/L × 5 MGD × 8.34 lb/gal = 125 lb/day

■ EXAMPLE 24.61
Problem: The desired dosage for a dry polymer is 10 mg/L. If the flow to be treated is 2,100,000
gpd, how many lb/day polymer will be required?

Solution:

Polymer = Polymer × Flow × 8.34 lb/day = 10 mg/L × 2.10 MGD × 8.34 lb/day = 175 lb/day

Key Point: To calculate chemical dose for tanks or pipelines, a modified equation must be used.
Instead of MGD flow, MG volume is used: