Environmental Engineering FOURTH EDITION

86 ENVIRONMENTALENGINEERING

I I I I I I I I

0 1 2 3 4 5

Time (days)

Figure 5-3. Typical oxygen uptake curves in a BOD test.

Sample B also had an initial dissolved oxygen concentration of 8 mg/L, but the

oxygen was used so fast that it dropped to 0 by the second day. Since there is no

measurable dissolved oxygen left after 5 days, the BOD of sample B must be more

than 8 - 0 = 8 mg/L, but we do not know how much more because the organisms in

the sample might have used more dissolved oxygen if it had been available. Samples

like this require diluting the sample. mically, five $ dilutions are recommended for

wastewaters of unknown origin. Suppose sample C in Fig. 5-3 is sample B diluted by

&. The BOD5 for sample B would be

8-4

0.1

-- - 40 mgL.

It is possible to measure the BOD of any organic material (e.g., sugar) and thus

estimate its influence on a stream, even though the material in its original state might

not contain the microorganisms necessary to break down organic matter. Seeding is

a process in which the microorganisms that oxidize organic matter are added to the

BOD bottle. Seeding also facilitates measurement of very low BOD concentrations.

The seed source can be obtained from unchlorinated domestic wastewater or surface

water that receives degradable wastewater effluents.

Suppose we use the water previously described in curve A as seed water since

it obviously contains microorganisms (it has a 5-day BOD of 6mg/L). We now put

100 mL of an unknown solution into a bottle and add 200 mL of seed water, thus filling

the 300-mL bottle. Assuming that the initial dissolved oxygen of this mixture is 8 mg/L

and the final dissolved oxygen is 1 mg/L, the total oxygen consumed is 7 ma. Some

of this is due to the seed water, because it also has a BOD, and only a portion is due

to the decomposition of the unknown material. The oxygen consumed due to the seed

water is

2

3

6 x - = 4mg/L,