Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 26

■ Example 2.43
Problem: A 0.5-MGD wastewater flow has a BOD concentration of 1600 mg/L BOD.
Using an average of 0.17 lb/day BOD per person, what is the population equivalent
of this wastewater flow?

Solution :

Hint: Don't forget to convert mg/L BOD to lb/day BOD, then divide by 0.17 lb/day
BOD per person.

/day)
0.17 lb/day BODper person
1600 mg/L

BODconcentration(lb

Populationequivalent =

× 0.5 MGD 8.34 lb/gal
0.17 lb/day BODper per

×

=

son

= 39,247 people

s peCifiC g ravity and d ensity
Specific gravity is the ratio of the density of a substance to that of a standard mate-
rial under standard conditions of temperature and pressure. The specific gravity
of water is 1.0. Any substance with a density greater than that of water will have
a specific gravity greater than 1.0, and any substance with a density less than that
of water will have a specific gravity less than 1.0. The difference between specific
gravity and density can often be confusing. To avoid such confusion and to ensure
clarity, the following definitions and key points regarding specific gravity and den-
sity are provided:

• Specific gravity

• Compares the density of a substance to a standard density

• Does not have units for solids and liquids

Note: Specific gravity can be used to calculate the weight of a gallon of liquid
chemical.

• Density

• Is the weight per volume

• Is expressed in lb/ft 3 for solids and gases

• Is measured in lb/gal or lb/ft 3 for liquids

• Of water varies slightly with temperature and pressure

• Of gases changes significantly with changes in temperature and pressure

• Of water is 62.4 lb/ft 3 or 8.34 lb/gal

Density (lb/gal) = Specific gravity of chemical × Water (lb/gal) (2.3)