Hach BODTrak, Instrument Manual, Page: 38 / 63

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3.2.2 Time Lag
Tests that begin with insufficient bacteria during the incubation
period produce data similar to that of Curve H in Figure 4. For
sample processing when insufficient bacteria are present, seed the
sample as described in section 4.3 Sample Seeding.
Bacteria acclimation also produces conditions that could generate
Curve H in Figure 4. This sometimes occurs when running
standards, even though seed has been added.
3.2.3 High Oxygen Demand
Samples that are over range (for example, a BOD over 350 mg/L
when a 160-mL sample is taken) will produce results as shown in
Curve K in Figure 4. Dilute the sample according to section 4.2
Sample Dilutions, or use a higher BOD range and a different
sample volume as directed in Table 5.
When the BOD range of a sample is unknown, use the results
from the Chemical Oxygen Demand (COD) test, or the results
from a series of BOD tests using the same sample but different
volumes, or dilution ratios to select an appropriate BOD range.
Generally, effluent normally 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 greater than 700 mg/L, prepare a sample dilution (see section
4.2 Sample Dilutions).
3.2.4 Nitrification
The condition shown by Curve A
N
in Figure 4 is an example of
nitrification. Biological oxidation of organic nitrogen usually
occurs after five days with normal domestic waste because it
takes that long for the nitrifying bacteria to develop; however, an
abnormally high uptake of oxygen (especially when testing final
effluent) is evidence of nitrifying bacteria adding appreciably to
the oxygen demand.
Control nitrification problems with Hach Nitrification Inhibitor
(Cat. No. 2533-35). Dispense the inhibitor powder into an empty
sample bottle and then add the sample. When using the Hach
Dispenser cap (Cat No. 459-01), dispense two shots
(approximately 0.16 grams) into the empty bottle.
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