Hach BODTrak, Инструкция по эксплуатации инструмента, Страница: 44 / 63

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4.3.2 Determining BOD of Sample
After determining the BOD of the seed, apply the following
formula to determine the sample’s BOD.
Example:
A seeded sample is 10% seed and 90% sample (by volume).
The observed BOD is 60 mg/L, and the pure seed BOD is
150 mg/L.
4.3.3 Variations in Initial Bacterial Populations
Microbic counts of domestic sewage fluctuate by the hour, so
bacteria counts should be reported within a specified range.
Domestic sewage typically contains between 10
4
and 10
6
cells
per mL. Variations in initial bacterial populations will have a
minimal effect on the BOD test if the bacteria count in the sample
exceeds 10
3
cells per mL.
Low seed concentrations are more critical than those that are too
high. They delay the start of oxidation and cause low BOD
results. Use the trial and error method to determine the optimum
concentration of seed for a specific waste material. Adding 10%
of Polyseed Solution or raw domestic sewage seed to each BOD
sample is usually sufficient for the manometric method.
Try various concentrations of seed and determine the respective
BODs of the waste sample as well as of the seed itself. Choose
the seed concentration yielding the highest corrected waste
sample BOD. This seed percentage can range from 2 to 30%,
depending on the waste material tested.
BOD sample
BOD observed
( )
Decimal fraction of seed used x BOD seed
( )
–
Decimal fraction of sample used
------------------------------------------------------------------------------------------------------------------------------------------------------------------------ =
BOD sample
60 mg/L
( )
0.10 x 150 mg/L
( )
–
0.90
--------------------------------------------------------------------------------= 50 mg/L =
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