Science of Water : Concepts and Applications

Environmental Biomonitoring, Sampling, and Testing 259
Some experimentation is needed to determine the appropriate dilution factor for a particular
sampling site. The result is the difference in DO between the fi rst 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.
BOD Sampling, Analysis, and Testing
The approved BOD sampling and analysis procedure measures the DO depletion (biological oxidation
of organic matter in the sample) over a 5-day period under controlled conditions (20°C in the
dark). The test is performed using a specifi ed incubation time and temperature. Test results are used
to determine plant loadings, plant effi ciency, and compliance with NPDES effl uent limitations. The
duration of the test (5 days) makes it diffi cult to use the data effectively for process control.
The standard BOD test does not differentiate between oxygen used to oxidize organic matter
and that used to oxidize organic and ammonia nitrogen to more stable forms. Because many
biological treatment plants now control treatment processes to achieve oxidation of the nitrogen
compounds, there is a possibility that BOD test results for plant effl uent and some process
samples may produce BOD test results based on both carbon and nitrogen oxidation. To avoid this
situation, a nitrifi cation inhibitor can be added. When this is done, the test results are known as
carbonaceous BOD (CBOD). A second uninhibited BOD should also be run whenever CBOD is
determined.
When taking a BOD sample, no special sampling container is required. Either a grab or composite
sample can be used. BOD 5 samples can be preserved by refrigeration at or below 4°C (not
frozen)—composite samples must be refrigerated during collection. Maximum holding time for
preserved samples is 48 h.
Using the incubation approved test method, a sample is mixed in several different concentrations
(dilutions) with dilution water. The dilution water contains nutrients and materials to provide
optimum environment. Chemicals used include dissolved oxygen, ferric chloride, magnesium sulfate,
calcium chloride, phosphate buffer, and ammonium chloride.
√ Note: Remember, all chemicals can be dangerous if not used properly and in accordance with
the recommended procedures. Review appropriate sections of the individual chemical materials
safety data sheet (MSDS) to determine proper methods for handling and for safety precautions
that should be taken.
Sometimes it is necessary to add (seed) healthy organisms to the sample. The DO of the dilution
and the dilution water is determined. If seed material is used, a series of dilutions of seed material
must also be prepared. The dilutions and dilution blanks are incubated in the dark for 5 days at 20°C ±
1°C. At the end of 5 days, the DO of each dilution and the dilution blanks are determined.
For the test results to be valid, certain criteria must be achieved. These test criteria are listed
as follows:
1. Dilution water blank DO change must be ≤0.2 mg/L.
2. Initial DO must be >7.0 mg/L but ≤9.0 mg/L (or saturation at 20°C and test elevation).
3. Sample dilution DO depletion must be ≥2.0 mg/L.
4. Sample dilution residual DO must be ≥1.0 mg/L.
5. Sample dilution initial DO must be ≥7.0 mg/L.
6. Seed correction should be ≥0.6 but ≤1.0 mg/L.
The BOD 5 test procedure consists of 10 steps (for unchlorinated water) as shown in Table 8.8.
√ Note: BOD 5 is calculated individually for all sample dilutions that meet the criteria. Reported
result is the average of the BOD 5 of each valid sample dilution.