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FIVE-DAY BIOCHEMICAL
OXYGEN DEMAND |
7.2 |
Biochemical oxygen demand represents the
amount of oxygen consumed by bacteria and other microorganisms
while they decompose organic matter under aerobic conditions
at a specified temperature.
The presence of a sufficient concentration of dissolved oxygen is critical to
maintaining the aquatic life and aesthetic quality of streams and lakes.
Determining how organic matter affects the concentration of dissolved oxygen
(DO) in a stream or lake is integral to water-quality management. The decay of
organic matter in water is measured as biochemical or chemical oxygen demand.
Oxygen demand is a measure of the amount of oxidizable substances in a water
sample that can lower DO concentrations (Nemerow, 1974; Tchobanoglous and
Schroeder, 1985).
The test for biochemical oxygen demand (BOD) is a bioassay procedure that
measures the oxygen consumed by bacteria from the decomposition of organic
matter (Sawyer and McCarty, 1978). The change in DO concentration is measured
over a given period of time in water samples at a specified temperature.
Procedures used to determine DO concentration are described in NFM 6.2. It is
important to be familiar with the correct procedures for determining DO
concentrations before making BOD measurements. BOD is measured in a laboratory
environment, generally at a local or USGS District laboratory.
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Accurate
measurement of BOD requires an accurate determination of DO.
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There are two stages of decomposition in the BOD test: a carbonaceous stage
and a nitrogenous stage (fig. 7.2-1).
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The carbonaceous
stage, or first stage, represents that portion of oxygen demand involved
in the conversion of organic carbon to carbon dioxide.
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The nitrogenous
stage, or second stage, represents a combined carbonaceous plus
nitrogeneous demand, when organic nitrogen, ammonia, and nitrite are
converted to nitrate. Nitrogenous oxygen demand generally begins after
about 6 days. For some sewage, especially discharge from wastewater
treatment plants utilizing biological treatment processes, nitrification
can occur in less than 5 days if ammonia, nitrite, and nitrifying bacteria
are present. In this case, a chemical compound that prevents nitrification
should be added to the sample if the intent is to measure only the
carbonaceous demand. The results are reported as carbonaceous BOD (CBOD),
or as CBOD5 when a nitrification inhibitor is used.
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| Figure 7.2-1. Biochemical
oxygen demand curves: (A) typical carbonaceous-demand curve
showing the oxidation of organic matter, and (B) typical
carbonaceous- plus nitrogeneous-demand curve showing the oxidation of
ammonia and nitrite. (Modified from Sawyer and McCarty, 1978.) |
The standard oxidation (or incubation) test period for BOD is 5 days at 20
degrees Celsius (BOD5). The
BOD5 value has been used and reported for many
applications, most commonly to indicate the effects of sewage and other organic
wastes on dissolved oxygen in surface waters (see TECHNICAL NOTE). The 5-day
value, however, represents only a portion of the total biochemical oxygen
demand. Twenty days is considered, by convention, adequate time for a complete
biochemical oxidation of organic matter in a water sample, but a 20-day test
often is impractical when data are needed to address an immediate concern.
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The BOD5 and CBOD5 tests have
limited value by themselves in the assessment of stream pollution and do
not provide all of the relevant information to satisfy every study
objective (Nemerow, 1974; Stamer and others, 1983; Veltz, 1984).
Additional analyses of water samples for chemical oxygen demand, fecal
bacteria, and nutrients can aid in the interpretation of
BOD5.
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An ultimate
carbonaceous BOD (CBODu)
test is needed to obtain additional BOD information, and can be used for
modeling DO regimes in rivers and estuaries (Hines and others, 1978;
Stamer and others, 1983). Guidelines for the CBODu determination are
described in Stamer and others (1979, 1983).
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Note that BOD results represent
approximate stream oxygen demands because the laboratory environment does
not reproduce ambient stream conditions such as temperature, sunlight,
biological populations, and water movement.
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| TECHNICAL NOTE: A 5-day duration for BOD determination has no
theoretical grounding but is based on historical convention. Tchobanoglous
and Schroeder (1985) provide the following background: "In a report
prepared by the Royal Commission on Sewage Disposal in the United Kingdom
at the beginning of the century, it was recommended that a 5-day, 18.3°C,
BOD value be used as a reference in Great Britain. These values were
selected because British rivers do not have a flow time to the open sea
greater than 5 days and average long-term summer temperatures do not
exceed 18.3°C. The temperature has been rounded upward to 20°C, but the
5-day time period has become the universal scientific and legal reference."
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