Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 25

p opulation e quivalent or u nit l loading f aCtor
Although population equivalent (PE), or unit loading factor, is a parameter usually
associated with wastewater treatment, keep in mind that water (a water supply for
flushing, etc.) is also involved. When a wastewater characterization study is required,
pertinent data are often unavailable. When this is the case, population equivalent or
unit per capita loading factors are used to estimate the total waste loadings to be
treated. If we know the biochemical oxygen demand (BOD) contribution of a dis-
charger, we can determine the loading placed on the wastewater treatment system in
terms of equivalent number of people. The BOD contribution of a person is normally
assumed to be 0.17 lb/day BOD. To determine the population equivalent of a waste-
water flow, divide the lb/day BOD content by the lb/day BOD contributed per person
(e.g., 0.17 lb/day BOD).

BODconcentration(l b/day)

0.17 lb/day BODper person

5

Populationequivalent = (2.2)

■ Example 2.41
Problem: A new industry wishes to connect to the city's collection system. The
industrial discharge will contain an average BOD concentration of 389 mg/L, and
the average daily flow will be 72,000 gpd. What is the population equivalent of the
industrial discharge?

Solution: First, convert flow rate to million gallons per day:

72,000 gpd
1,000,000 gal/MG

Flow = = 0.072 MGD

Next, calculate the population equivalent:

389 mg/L × × 34 lb/mg/L/MG

0.17 lb/day BODper person

0.072 MGD8.

Populationequivalent =

= 13 74 people/day

■ Example 2.42
Problem: An industry proposes to discharge 3455 lb of BOD to the town sewer sys-
tem. What is the population equivalent of the proposed discharge?

Solution :

3455 lb/day
0.17 lb/da

Populationequivalent =

yyBOD perperson
people = 20 324 ,