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What is the perfect seed for the BOD test? That’s easy…one that results in the
glucose/glutamic acid (GGA) standard having an average BOD of 198 mg/L, and a
standard deviation of zero. Obviously, the perfect seed doesn’t exist. So what makes
a good BOD seed? Also an easy answer… one that gives an average close enough to 198
mg/L, with a low standard deviation indicating good precision. But that definition requires
interpretation, and if the lab is doing carbonaceous BOD (cBOD) rather than BOD, it may not
be the correct definition.
Standard Methods 5210B says that the GGA test is run to determine “seed effectiveness.”
In the 19th and earlier editions, it also said that the “DO uptake of seeded dilution water
should be between 0.6 and 1.0 mg/L.” That criterion was removed in the
20th edition, and the GGA test is the only test of seed quality.
Summarizing, the seed should result in an average GGA close
enough to 198 mg/L, with a standard deviation of <10 mg/L, these
values being derived from a nationwide study. What makes up such
a seed, and where is it most likely found?
The most important quality of a good seed is that it has a
sufficient population of healthy, hungry bacteria. Many equate
this to having a high BOD. If you are using a natural seed (e.g.,
one from wastewater), you want the seed material to have
enough organic material (BOD) present to serve as food for the
bacteria. If you are using a lyophilized (so called synthetic)
seed, the bacteria don’t need food (until reconstituted), so
don’t expect a high BOD for the seed control bottle.
Another quality of a good seed is that it be consistent. You
don’t want a seed that averages 198 mg/L for GGA, but
bounces around from 120 mg/L in one batch to 266 in the next.
So where is a consistent, healthy population of bacteria
likely to be found? Standard Methods correctly states that one
of the best sources is primary effluent from a domestic
wastewater treatment plant WWTP). Usually, such bacteria are
consistently healthy and numerous. Perhaps second best in a
domestic WWTP would be final effluent before disinfection
where bacteria should be healthy, consistent, but not so
numerous...or perhaps from an oxidation ditch, or some other
location in the plant. Many analysts opt for raw influent,
probably mistakenly equating “high BOD” with “good seed.”
Raw influent is usually very inconsistent
Commercial labs and industrial wastewater dischargers not
having a flow of domestic sewage running through their locations have the choice of either
using seed obtained from a local WWTP, or using a synthetic seed. If using the latter, one
wants to consider if BOD, or cBOD is being reported. If it is BOD, a seed containing some
nitrogenous bacteria (nitrifiers) should be used. If it is cBOD, it doesn’t matter if nitrifiers are
present in the seed or not since they would be inhibited. Some synthetic seeds contain
nitrifiers, and some purposely do not. Another caution when using synthetic
seed…Standard Methods says to use enough seed in seeded samples to get a depletion of
0.6 to 1.0 mg/L from the seed. With synthetic seeds, this may require using so much seed
that it becomes the major ingredient in the bottle. This would be counterproductive, and
some regulators would argue that if the GGA results are good (little bias, good precision), it
doesn’t matter how much depletion the seed contributed.
Unfortunately there is no magic perfect seed. For your BOD analysis, use a seed that most
consistently meets the Standard Methods criteria stated above for each test. This may vary
and will probably require some research.
Reprinted with permission from Environmental Express (Volume III; Number 1; March 2005).
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