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Re: Aquatic Plants Digest V3 #657
Hello James,
You have the basics of BOD measurement right.
The "seed capsules" are supposed to supply the bacteria absolutely
necessary for the test to work, as well as nutrients, for the bacteria
to utilize the available carbon. I am NOT certain, but a little bit of
finely divided mulm, with a "bit" (match head size/liter) of a soluble
fertilizer *might* work just fine.
To run the test one needs at least two identical bottles, better four,
to average the results.
One bottle you fill to the very top with the water to be measured,to
which you added *little* of the seed & nutrient. Measure DO and add more
water, so there is no airspace above the liquid. Air = oxygen = error!
The second bottle you fill with distilled (RO, deionized) water and
treat it *EXACTLY* like the first -- same size seed, same amount of
fertilizer, same storage. This is your blank. You subtract its value
from the reading of the sample, to obtain the BOD of the sample.
The second DO reading must be done with minimum exposure to air. In the
BOD bottles the measuring electrode fits the bottle like a cork, so no
air can enter during measurement. One could probably build a "cork"
around the electrode that will do the job.
I trust that I did not forget a step -- but then somebody will catch it!
Best,
George
> A question for the Chemists....
>
> Tests for Biological (Biochemical?) Oxygen Demand, B.O.D., call for
> measuring and comparing the dissolved Oxygen levels before and after (5
> days) incubation of a sample of water. From the LaMotte Website I obtained
> the following:
>
> BIOCHEMICAL OXYGEN DEMAND (BOD) This is a determination of the amount of
> organic material in wastewater by measuring and comparing the dissolved
> oxygen content before and after incubating the sample for 5 days at 20*C.
> All reagents, including seed capsules and glassware needed to perform this
> test are included in the kit. Incubator and DO meter are not included. See
> p. 52 for BOD accessories.
>
> LaMotte sells test kits for both Dissolved Oxygen and B.O.D. - I have the
> Dissolved Oxygen Test Kit but not the one specifically designed to measure
> B.O.D.
>
> My question is: how close an approximation of the B.O.D. can I get without
> using LaMotte's special test kit (which contains "seed capsules") if I
> merely use their Dissolved Oxygen Test Kit and maintain a sample of water
> (in the dark) at 20C for 5 days, i.e. - If I measure the D.O. in an aquarium
> on Day 1 and then take another sample and keep it in the dark at 20C for 5
> days and then measure the D.O. in that sample on Day 5? Is there something
> special in the "seed capsules" that are used in the B.O.D. kit or would the
> organics and bacteria naturally in the aquarium water be sufficient to
> provide at least a ballpark figure for B.O.D. in the tank?
>
> Last Sunday, the D.O. level in one of my tanks was measured at 7.0 ppm O2. I
> took another water sample and kept it in the dark at 20C for 5 days. Today
> (Day 5), I measured the D.O. in that sample and recorded the D.O. at 6.2 ppm
> O2. Am I correct in assuming that the B.O.D. in the aquarium is 0.8 ppm O2?
> If this is an accurate measurement of the B.O.D., how would this rate as an
> indicator of the health of the aquarium (the 0.8 ppm reading)?
>
> Thanks,
>
> James Purchase
> Toronto, Ontario
> jpurch at interlog_com