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"Depends on whether using chemical analysis or a probe for measuring dissolved oxygen.

^Corrective measures are possible for most interferences. Uncorrected positive interferences lead to overestimation; negative ones lead to underestimation.

^Using some TOC methods, not all organic carbon is reacted and measured.

"Depends on whether using chemical analysis or a probe for measuring dissolved oxygen.

^Corrective measures are possible for most interferences. Uncorrected positive interferences lead to overestimation; negative ones lead to underestimation.

^Using some TOC methods, not all organic carbon is reacted and measured.

The ultimate BOD (BODu) is the carbonaceous BOD that results from prolonged incubation, usually taken as 20 days. For domestic sewage, this value is often about 70% of the COD. This is because even for highly biodegradable compounds, some of the substrate is converted to cellular material, not respired for energy. Since the BOD5 is often about 70% of the BODu, the standard BOD5 test typically gives a value that is about 50% of the COD. However, for various compounds and mixtures, this ratio can vary from 0 (nonbiodegradable under the BOD test conditions) to more than 1.0 (biodegradable, but not chemically oxidized in the COD test). Table 13.3 compares BOD, COD, and TOC, showing their relative values for domestic sewage and some of the advantages and disadvantages of each test.

Because the BOD procedure relies on oxidation by microorganisms, it is inherently prone to variability despite efforts at standardization. Even slight variations in inoculation or other test conditions can lead to changes in oxidation rate and hence different oxygen depletions at the close of the test period. Despite its weaknesses, however, the BOD is still highly useful and widely used.

ThOD, COD, and BOD all give a good indication of the amount of energy available from oxidation of a compound or mixture. This is because the amount of oxygen consumed in an oxidation is linked very closely to the amount of energy released during that reaction [typically, 14,000 J/g O2; see the discussion of equation (16.5) in Section 16.2.3].

Biodégradation Biodegradation can be defined as the transformation of a compound through biological activity. It was first included in some dictionaries in the 1960s. Degradation is a broader term that would also include transformation as a result of other chemical or even physical changes. This might include photolysis (a chemical reaction in which light provides the activation energy), for example, or abiotic catalysis at the surface of a clay mineral.

It is interesting to note that some of the early tests of biodegradability were for plastics, and degradation was considered undesirable. Thus, even changes in color or opacity were considered degradation and might lead to rejection of a new polymer. On the other hand, some plastic bags now labeled "biodegradable" degrade only very slowly, and thus are

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