Page 66 - Statistics for Environmental Engineers

L1592_frame_C06 Page 58 Tuesday, December 18, 2001 1:43 PM

15
Percentage 10 All days

5

0
15

Percentage 10 Stable operation

5

0
0 5 10 15 20 25 30 35
5-day BOD (mg/L)

FIGURE 6.5 External reference distributions for effluent 5-day BOD (mg/L) for the complete record and for the stable
operating conditions.
0.20
15 7-day moving average
Percentage 10

5

0

15 30-day moving average
Percentage 10

5

0
3 5 7 9 11 13 15 17
Effluent BOD (mg/L)

FIGURE 6.6 External reference distributions for 7- and 30-day moving averages of effluent 5-day BOD during periods
of stable treatment plant operation.
the plant is capable over long stretches of time (Berthouex and Fan, 1986). This is the reference distribution
against which new daily effluent measurements should be compared when they become available, which
is five or six days after the event in the case of BOD data.
If the 7-day moving average is used to judge effluent quality, a reference distribution is required for this
statistic. The periods of stable operation were used to calculate the 7-day moving averages that produce
the reference distribution shown in Figure 6.6 (top). Figure 6.6 (bottom) is the reference distribution of
30-day moving averages for periods of stable operation. Plant performance can now be monitored by
comparing, as they become available, new 7- or 30-day averages against these reference distributions.
© 2002 By CRC Press LLC