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SECTION 4 SPECIAL CONSIDERATIONS
4.1 Range and Volume Selection
Because the BODTrak instrument can measure various waste
substances, a variety of total oxygen demand test results can be
obtained. The instrument provides four direct-reading ranges
(0-35, 0-70, 0-350, and 0-700 mg/L). Use Table 5 on page 20 to
select the volume and range corresponding to the expected
sample BOD. For example, when using a sample with an
expected BOD of 35 mg/L or less, use a 420-mL volume.
For best results, analyze samples immediately after collection. If
this is not possible, preserve samples at low temperature (4 °C)
for no longer than 24 hours.
4.2 Sample Dilutions
If the sample's BOD is unknown, you can generally assume that
effluent is normally in the 0-70 mg/L range while influent is
usually in the 0-700 mg/L range.
If a sample does not contain sufficient nutrients for optimum
bacteria growth, add the contents of one BOD Nutrient Buffer
Pillow to each bottle. Do not add the BOD Nutrient Buffer Pillow
if close simulation of original sample characteristics is required.
4.2.1 When Oxygen Demand Exceeds 700 mg/L
When the O
demand of a sample exceeds 700 mg/L, dilute the
2
sample with high-quality dilution water. Make the dilution water
with distilled water that does not contain organic matter or traces
of toxic substances such as chlorine, copper, and mercury.
Demineralizers can release undetected organic matter that will
create an objectionable oxygen demand. The most practical way
to consistently produce water of low organic content is by
distillation from alkaline permanganate. (For example, add 2 g
KMnO
and 4 g NaOH for every liter of water.)
4
After distillation, place 3 L of distilled water in a jug and bring
the water temperature to 20 °C. Add the contents of one BOD
Nutrient Buffer Pillow for 3 L to ensure sufficient nutrient
concentration for diluted samples. Cap the jug and shake it
vigorously for one minute to saturate the water with oxygen. Do
not store the solution.
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