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Re: B.O.D. and green water
A few more things about BOD:
(1) The BOD determination does not indicate directly the amount of organic
matter in the water. It merely gives you the rate of oxygen consumption.
There may be a lot of organic matter that is very refractory to
decomposition, and, therefore, you may only record a small BOD. On the
other hand, there may be a smaller amount that can be decomposed by
bacteria easily, and that may give you a larger BOD.
(2) If you suspect, or if you find out, that there is such a large BOD that
the oxygen content drops down to zero during the five days, you must dilute
your sample with deionized or distilled water in order to get an accurate
determination. You can try a 1/2 dilution and a 1/4 dilution, for example.
Suppose your 1/4 dilution drops from 7 ppm to 4ppm over the five day
period. The BOD would then be (7-4)*4, or 12 ppm.
I didn't know they were providing bacteria these days to seed the sample.
We always just used the bacteria that came along with the local waters when
I was doing BOD's. Maybe you need these seed bacteria when you are not
using natural stream, lake or aquarium waters, but making up various
mixtures of water and organic matter.
Steve Pushak wrote:
>Paul Krombholz did a lot of experiments with green water and adding
>nutrients including nitrate and these are written up in TAG 4:2 and 5:2.
>According to Paul's data adding nitrate doesn't do anything but make the
>green water greener. I'm afraid I can't go along with the suggestion
>that green water is _caused_ by a shortage of nitrate. It results when
>conditions are favourable for the algae to grow.
After a number of the nutrient additions, the green water did go away in
that experiment, but I think it went away because the plant (Eigeria densa)
got large. I don't know why the green water went away when the plant got
large. It could have happened because of allelochemicals or because (my
favorite hypothesis) the large surface area of the plant provided
attachment sites for organisms feeding on green water, or some other reason
I can't think of. I don't think the plant was superior at competing for
nutrients, because it didn't show any signs of being superior when it was
smaller. I had enough light so that I don't think that the plant shaded
out the green water algae.
Paul Krombholz, in cool, sunny, central Mississippi