Hach-Lange BODTRAK II: Respirometric Biochemical Oxygen Demand (BOD) - Manual (Page 33)

BODTrak™ II procedures
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5.7 Special considerations
5.7.1 Sample dilution
Unknown sample BOD effluent is typically in the 0 to 70 mg/L range. Unknown
sample BOD influent is typically in the 0 to 700 mg/L range. When the oxygen
requirement of a sample is more than 700 mg/L, dilute the sample with high-quality
distilled or deionized water.
Calculate the results to include the additional dilution factor. Example: If the BOD of
the sample is 1000 mg/L, dilute the sample 1:1 with distilled or deionized water. The
estimated BOD is now 500 mg/L. Use the sample volume specified in the table for
the 0 to 700 mg/L range of the chosen procedure. Multiply the instrument reading
result by 2. If using the Hach Standard Method procedure, continue with remaining
calculations.
5.7.2 Sample seeding
Some types of BOD samples do not contain sufficient bacteria to oxidize the
organic matter in the sample. Many industrial wastes are of this type. Some sewage
treatment plant effluents are chlorinated and essentially sterile. A BOD test cannot
be done in the absence of viable bacteria. To test such samples, seed each bottle
from a source known to contain a viable bacterial population.
Settled domestic wastewater plant influent or primary clarifier effluent are the
preferred sources of seed for most samples. Mixed liquor or undisinfected effluent
can be used for seed, but it is recommended to include a nitrification inhibitor.
Commercial seed sources are sometimes suitable. To prepare, see the instructions
from the manufacturer.
5.7.3 Sample temperature
Standard Methods for the Examination of Water and Wastewater, 21st Ed., 2005,
5210 D recommends an incubation temperature of 20 ±1 °C (68 °F) for the BOD
test. Put the BODTrak II instrument in an incubator that is adjusted to 20 ±1 °C. An
applicable BOD incubator is available from Hach ( section 8.1 on page 41 ). Warm or
cool samples to 20 ± 1 °C.
Note: Instrument performance has not been validated at temperatures other than 20 °C.
5.7.4 Toxic materials
Industrial and chlorinated samples often contain toxic substances and require
special considerations when running BOD tests. Toxic materials in the sample will
cause decreased BOD values. Dilute the sample to minimize the toxic materials or
their effects. Refer to Standard Methods for the Examination of Water and
Wastewater, 21st edition, 5210 D.