BOD Test Requirements
by Tim Loftus
Legend has it that the 5-day BOD (Biological Oxygen Demand) test was
developed in England. Sewage was dumped in a river and it took five days for it
to reach the ocean, hence the five-day incubation requirement in the BOD
method.
Obviously, the BOD test is not the most scientifically based test we
perform. As such, it has many problems associated with it. The most significant
problem is that the results come five days after the fact. By that time you’ve
already discharged any problem wastewater. Then the variability of the seed must
be constantly monitored. Sometimes the bacteria wants to work, sometimes they
don’t. Again, by the time you find out, it is five days too late. Some analysts
support the use of respirometry to give more timely results, but regulatory
agencies have not fully accepted the methods yet so NPDES permits continue to
specify that we perform the standard 5-day BOD test.
There is no absolute BOD value of a sample as there would be for say copper
or lead. BOD results are test defined. In other words, BOD values are based on
the parameters of the test method, not on any “true” BOD value. Below are the
requirements that must be met for a BOD analysis to be valid (i.e. be in a form
that can be used to compare BOD values with other sources). By meeting these
requirements, it means that you have the method correct. It does not cover the
additional problems of sample toxicity or errant dilutions. For information on
how to set up and calculate a BOD analysis, refer to Standard Methods for the
Examination of Water and Wastewater, 18th edition.
The BOD blank (a BOD bottle full of dilution water containing only the
required nutrients, but not any seed) must not show a DO, or dissolved oxygen,
depletion of more than 0.2 mg/L after the five day incubation period. A drop of
more than 0.2 mg/L indicates some type of contamination or calibration error.
The seed, or the microorganisms added to industrial
wastes or disinfected wastewater effluent samples to break down the organic
compounds, should contribute 0.6 to 1.0 mg/L DO uptake per BOD bottle.
A glucose-glutamic acid standard, made according to Standard Methods,
should produce a BOD result of 198 mg/L +/- 30.5. However, it’s best to
determine your own average and standard deviation so you can develop a more
accurate and useful range and control chart.
Ideally, sample dilutions should show about a 50% DO decrease after the 5-day
incubation period. At a minimum, there should be at least a 2.0 mg/L DO change
between the initial and the final reading. There should also be a residual DO of
at least 1.0 mg/L.
The blank, seed determination, and glucose-glutamic acid standard should be run
every time a BOD analysis is performed. If any of these basic BOD requirements
are not met, then the test is considered invalid and remedial action is needed.
It’s important to keep accurate records of these measurements so that you can
monitor trends. You may find, for example, that seed viability changes with the
seasons. Knowing this, adjustments in the amount of seed added to each bottle
can be made to meet the 0.6 to 1.0 mg/L seed uptake requirement.
For all its shortcomings, the 5-day BOD test is here for a while. It has a lot
of variability and must meet a number of requirements to be valid. However, it
is possible to consistently meet these requirements, but it takes work and
careful monitoring of everything that goes into the analyses.
The information in this article is based on an EPA accepted test method for
NPDES monitoring. 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 at (207) 782-0917 (rcrand@gwi.net)
or Tim Loftus at (508) 949-3865 (timloftus@email.msn.com).
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