Statistics for Environmental Engineers

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A long operating record was used to generate the top histogram in Figure 6.5. From the operator’s log it was learned that many of the days with high BOD had some kind of assignable problem. These days were defined as unstable performance, the kind of performance that good operation could eliminate. Eliminating these poor days from the histogram produces the target stable performance shown by the reference distribution in the bottom panel of Figure 6.5. “Stable” is the kind of performance of which





All days





FIGURE 6.5 External reference distributions for effluent 5-day BOD (mg/L) for the complete record and for the stable operating conditions.

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.

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