Hach-Lange BODTRAK II: Respirometric Biochemical Oxygen Demand (BOD) - Manual (Page 38)

Troubleshooting
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7.1 High oxygen demand
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 ( Figure 5 on page 37 ).
Dilute the sample ( section 5.7 on page 33 ) or use a higher BOD range and a
different sample volume ( Table 4 on page 20 , Table 5 on page 23 or Table 6 on
page 24 ).
When the BOD range of a sample is unknown:
• Use the results from the Chemical Oxygen Demand (COD) test. An estimated
BOD value can be obtained by multiplying the COD by 0.68.
• Use the results from a series of BOD tests using the same sample but different
volumes.
• Or use dilution ratios to choose 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 ( section 5.7 on page 33 ).
7.2 Nitrification
The condition shown by Curve 2 is an example of nitrification ( Figure 5 on page
37 ). 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.
However, 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 replacement parts and
accessories ( Section 8 on page 41 ).
7.3 Excessive time lag
Curve 3 ( Figure 5 on page 37 ) shows a test that did not start with sufficient bacteria
during the incubation period. To do a test on a sample without sufficient bacteria,
seed the sample ( section 5.7.2 on page 33 ).
Bacteria acclimation also causes conditions that could cause Curve 3. This
sometimes occurs with standards and added seed. Add more seed or choose a
different seed source.
7.4 Sample temperature
The initial negative results of Curve 4 ( Figure 5 on page 37 ) show that the initial
sample temperature was below the specified range of 20 ± 1 ºC. A sample
supersaturated with oxygen will also display this type of curve ( section 5.7.3 on
page 33 and section 5.7.7 on page 34 ).