Averages (Arithmetic Mean) and Median - Basic Mathematics for Water and Wastewater Operators - page 111

Notice that no tank in this example has a capacity anywhere close to the mean.
Determining the median capacity requires us to take the mean of the two middle
values; because they are both 100,000 gal, the median is 100,000 gal. Although three
of the tanks have the same capacity as the median, this result offers no indication
that one of these tanks holds a million gallons, information that could be important
for the operator to know.
■ Example 7.5
Problem: Effluent BOD test results for a treatment plant during the month of August
are shown below:
Test 1 22 mg/L
Test 2 33 mg/L
Test 3 21 mg/L
Test 4 13 mg/L
What is the average effluent BOD for the month of August?
Solution:
(22 mg/L + 33 mg/L + 21 mg/L + 13 mg/L) ÷ 4 = 22.3 mg/L
■ Example 7.6
Problem : For the primary influent flow, the following composite-sampled solids con-
centrations were recorded for the week:
Monday 310 mg/L SS
Tuesday 322 mg/L SS
Wednesday 305 mg/L SS
Thursday 326 mg/L SS
Friday 313 mg/L SS
Saturday 310 mg/L SS
Sunday 320 mg/L SS
Total 2206 mg/L SS
What is the average SS?
Solution:
Average SS = Sum of all measurements ÷ Number of measurements used
= 2206 mg/L SS ÷ 7
= 315.1 mg/L SS