Environmental Engineering FOURTH EDITION

Measurement of Water Quality 89

EXAMPLE 5.2. The BOD versus time data for the first 5 days of a BOD test are obtained
as follows:

Time (days) BOD (mfl)

2 10

4 16

6 20

Calculate k; and &.

The intercept (a) = 0.545 and the slope (b) = 0.021. Thus:

The (tlBOD(t))”3 values 0.585,0.630, and 0.669 are plotted as shown in Fig. 5-4.

k’, = 2.61 - = 0.10 day-’

(E)

1

2.3(0.10)( 1.5433

Lo = = 26.8 ma.

If, instead of stopping the BOD test after 5 days, we allowed the test to continue
and measured the dissolved oxygen each day, we might get a curve like that shown in
Fig. 5-5. Note that after about 5 days the curve turns sharply upward. This discontinuity
is due to the demand for oxygen by the microorganisms that decompose nitrogenous
organic compounds to inorganic nitrogen. In the following example, microorganisms
decompose a simple organic nitrogen compound, urea (NH2 CO. NH2), releasing

0.5~ 0 0 2 4 6

Time (days)

Figure 5-4. Plot of k; and Lo, for Example 5.2.