by Tim Loftus
The first article of this series reviewed what quality control measures are
needed to validate a Biological Oxygen Demand, or BOD, analysis. The
second article covered pH adjustment, dechlorinating samples, calibrating
dissolved oxygen (DO) probes to the barometric pressure, and sources of
bubbles in the BOD bottles. This third and final article in the series will
focus on the results of BOD analyses.
You’ve set up the perfect BOD analysis. The sample was pH adjusted properly,
there were no bubbles in the bottle after five days, your blank, standard, and
seed all came out within acceptable limits. Then why are there three different
results for the three dilutions of the same sample?
There are a several reasons that these types of errors happen. The first is
incorrect dilution choices. The second is the fault of the sample (yes, we can
blame the sample sometimes!). However, both types of errors can be reduced or
even eliminated with a little bit of planning.
Dilution errors:
Choose dilutions that produce a DO decrease in the BOD bottle of at least 2
mg/L, but retains an oxygen residual of more than 1 mg/L after a five-day
incubation period. If the DO in a bottle did not decrease by more than 2 mg/L,
drop that measurement and calculate the remaining dilutions. Likewise, if the
oxygen residual in a bottle is less than 1 mg/L, drop that measurement and
calculate the remaining dilutions. For example: the initial DO for four
dilutions of a single sample is 8.5 mg/L. After a five-day incubation the oxygen
residual is 7.9 mg/L, 5.7 mg/L, 4.3 mg/L, and 0.5 mg/L (representing a dissolved
oxygen change of 0.6 mg/L, 2.8 mg/L, 4.2 mg/L, and 8.0 mg/L respectively). Since
the first dilution did not decrease the required 2 mg/L, do not use this
dilution in the calculation of the sample’s average BOD value. Also, the fourth
dilution left an oxygen residual less than 1 mg/L and should not be used in the
calculation of the sample’s average BOD value.
Occasionally a series of dilutions will all be bad. Either none of the dilutions
will give a DO decrease of 2 mg/L or all dilutions will use up almost all the
dissolved oxygen in the bottle. If the sample is “weak” in BOD and all the
dilutions use very little oxygen (a change of less than 2 mg/L), take the lowest
dilution and calculate it out as if it had dropped 2 mg/L. For example, the
initial DO of a BOD sample is 8.5 mg/L for three dilutions of 3%, 4%, and 5%.
After a five-day incubation, the residual oxygen level is 8.0 mg/L, 7.5 mg/L and
7.0 mg/L (for a DO change of 0.5 mg/L, 1.0 mg/L, and 1.5 mg/L respectively).
Since none of the sample dilutions meet the 2 mg/L DO change, calculate the
lowest diluted sample (the 5% diluted sample) as if the change was 2 mg/L rather
than the 1.5 mg/L measured value. Then report the result as “less than (<) this
calculated value.”
Calculations where all dilutions have an oxygen residual of less than 1 mg/L are
done similarly. But in these cases, calculate the result using the highest
diluted sample as if the oxygen residual is 1 mg/L and report as “greater than
(>) this calculated value.”
While the greater than (>) and less than (<) designations on results are the
best you can do under the circumstances, it is best to avoid it all in the first
place by developing a series of dilutions where at least one sample will fall
within the proper range. Even if you know, for example, that the BOD result for
the final effluent is less than the permit limit, you should still strive for
accuracy. You cannot add, subtract, multiply, or divide “less than” or “greater
than” signs. Especially when these sample results must be used in calculations
to determine plant loadings or removal rates.
Blame the sample:
Sometimes the sample may be toxic to the bacteria, or seed, that break down the
wastes. This is often seen as decreasing BOD results on a sample coinciding with
decreasing dilution rates. For example, three dilutions (1%, 2%, 3%) of an
industrial wastewater sample gives results of 450 mg/L, 375 mg/L, and 250 mg/L
respectively. This indicates a level of toxicity in the sample. In these cases,
calculate the BOD value using the most diluted sample (450 mg/L) since this
shows the least effect of toxicity.
This article concludes the series on BOD analyses. It takes lots of knowledge,
time, and experience to get the test to work properly. And then there will be
times that the bugs in the BOD bottle will do as they want anyway. Hopefully,
the information presented in these articles will help you in your laboratory to
achieve greater BOD accuracy. As usual, check your federal, state, and local
regulations. You may have additional regulations or reporting requirements that
you must meet.
If you have any questions, suggestions, or comments, please contact NEWEA
Lab Practices Committee Chair Phyllis Arnold Rand 207-782-0917 (prand@gwi.net)
or Tim Loftus at (508) 949-3865 (timloftus@msn.com).
