[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: BOD
Paul Krombholz wrote:
> 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.
Thanks for the information Paul. For what it's worth, the tank in question
has been set up for just over a month, and has a layer of clay sub-soil, a
second layer of mineral topsoil + 10% peat + Micronized Iron + FTE, and a
top layer of plain aquarium gravel and the liquid fertiliser is a modified
form of PMDD (without the trace elements). The "substrate recipe" came from
Steve Pushak's Website - I am just trying to see if I can have good results
with his methodology. It was heavily planted with H. polysperma initially
and receives 2 Watts/Gal of Chroma 50 lighting and DIY CO2 (Yeast Method). I
am considering installing a second dual light fixture over the tank, which
would up the light intensity to 4 Watts/Gal but that might just lead to
driving the "engine" of the tank a tad too fast, too soon.
I've used soil and/or peat moss in substrates before but have never really
followed any of the tanks closely for water parameters nor been able to
compare it directly to another tank right beside it which uses a different
type of substrate and fertiliser scheme. I am performing a 25% water change
every week and feed the fish in the tank lightly with dry food and live baby
brine shrimp.
My interest in measuring B.O.D. in this tank was mainly spurned by the
presence of organics in the substrate (peat moss and topsoil) and the
possibility of them "leaking" into the water column or otherwise causing
problems. If my suggested method of measuring B.O.D. (using the regular
LaMotte Dissolved Oxygen Test Kit) will work, even if only in a comparative
way in this particular tank, it just gives me one more parameter to look at
when evaluating the methodology of the tank's set-up and maintenance. All
other measured parameters are fine and the plants in the tank are growing
well - Nitrate-N was (I thought) a bit high at 3 ppm N03-N a few weeks into
the process but is starting to drop off following a reduction of my weekly
PMDD additions. There is no visible algae growing on any of the plants, even
the black beard algae which was growing on a few leaves of the initial
cuttings has disappeared.
The O2 concentration last Sunday morning was 7 ppm. A water sample was taken
and held in darkness at 20C for 5 days. When measured on Friday morning, the
O2 reading of that sample was 6.2 ppm, leading me to assume a B.O.D. of 0.8
ppm in that tank, at that time. I am assuming of course that there are
bacteria present in the water which can use dissolved organics and O2 in
their metabolism - in an aquarium, even one just set up for a month, I think
that is a pretty safe assumption - ergo I'm not really worried about not
having "inoculated" the sample with any special bacterial culture as is
called for in the actual LaMotte BOD Test Kit.
It is far too early to deduce much about the methodology or my maintenance
of this particular tank - one right beside it using a different substrate
and fertiliser scheme is just as healthy looking (from a look at the growth
of the plants anyway). But by following a number of tanks closely for a year
I hope to be able to see if there are any qualitative differences in the
various "roads to Rome" that we all use to have nicely planted aquaria.
James Purchase
Toronto, Ontario
jpurch at interlog_com