Environmental Engineering Reference
In-Depth Information
Sampling Considerations
Biochemical oxygen demand is affected by the same factors that affect
dissolved oxygen (see above). Aeration of stream water—by rapids and
waterfalls, for example—will accelerate the decomposition of organic and
inorganic material. BOD levels at a sampling site with slower, deeper waters
might be higher for a given column of organic and inorganic material than
the levels for a similar site in high aerated waters. Chlorine can also affect
BOD measurement by inhibiting or killing the microorganisms that decom-
pose the organic and inorganic matter in a sample. When sampling in chlo-
rinated waters (such as those below the effluent from a sewage treatment
plant), it is necessary to neutralize the chlorine with sodium thiosulfate.
Biochemical oxygen demand measurement requires taking two samples
at each site. One is tested immediately for dissolved oxygen, and the sec-
ond is incubated in the dark at 20°C for 5 days and then tested for the dis-
solved oxygen remaining. The difference in oxygen levels between the first
test and the second test (in milligrams per liter) is the amount of BOD. This
represents the amount of oxygen consumed by microorganisms and used
to break down the organic matter present in the sample bottle during the
incubation period. Because of the 5-day incubation, the tests are conducted
in a laboratory.
Sometimes by the end of the 5-day incubation period, the dissolved oxy-
gen level is zero. This is especially true for rivers and streams with a lot of
organic pollution. Because it is not possible to know when the zero point was
reached, determining the BOD level is also impossible. In this case, diluting
the original sample by a factor that results in a final dissolved oxygen level
of at least 2 mg/L is necessary. Special dilution water should be used for the
dilutions. Some experimentation is necessary to determine the appropriate
dilution factor for a particular sampling site. The final result is the differ-
ence in dissolved oxygen between the first measurement and the second,
after multiplying the second result by the dilution factor.
Standard Methods
prescribes all phases of procedures and calculations for BOD determination.
A BOD test is not required for monitoring water supplies. (For more informa-
tion, see Method 5210 in
Standard Methods
.)
Temperature
An ideal water supply should have, at all times, an almost constant temperature
or one with minimum variation. Knowing the temperature of the water supply
is important because the rates of biological and chemical processes depend on
it. Temperature affects the oxygen content of the water (oxygen levels become
lower as temperature increases), the rate of photosynthesis by aquatic plants,
