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Figure 4 Incorrect curves
1 High oxygen demand
2 Nitrification
3 Excessive time lag
4 Initial sample temperature
below 20 ºC or supersaturated
with oxygen
5 Bottle leak
8.2 High oxygen demand
Refer to
Figure 4
on page 11. Samples that are above range (for
example, a BOD over 350 mg/L when a 160-mL sample is taken) will
cause results as shown in Curve 1. Dilute the sample or use a higher
BOD range and a different sample volume. Refer to the Sample dilution,
Simplified procedure or the Hach Standard method procedure for more
information.
When the BOD range of a sample is unknown:
• Use the results from the Chemical Oxygen Demand (COD test).
Multiply the COD by 0.68 to get an estimated BOD value.
• Use the results from a series of BOD tests that use the same sample
but different volumes.
• Use dilution ratios to select an applicable BOD range.
Typically, effluent is in the 0–70 mg/L range while influent is in the
0-700 mg/L range. when the BOD of the sample is more than 700 mg/L,
prepare a sample dilution. Refer to the Sample dilution section in the
expanded version of this manual for more information.
8.3 Nitrification
Refer to
Figure 4
on page 11. The condition shown by Curve 2 is an
example of nitrification. Deviation from the typical curve (shown as the
dashed line) is apparent by the concave increase near the end of the
test period.
Biological oxidation of organic nitrogen usually occurs after 5 days with
typical domestic waste. Nitrifying bacteria develop more slowly than
other types of bacteria.
Some samples contain a high concentration of nitrifying bacteria and
nitrification results can occur sooner. Control nitrification problems with
Hach Nitrification Inhibitor. Dispense the inhibitor powder into an empty
sample bottle and then add the sample. With the Hach Dispenser cap,
dispense 6 shots (approximately 0.48 grams) into the empty bottle. Refer
to
Optional reagents
on page 12.
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