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[APD] RE: Aquatic-Plants Digest, Vol 14, Issue 31

> Tom Barr commented on the possibility of H2S generation with NNR, but as
> that requires lower redox than NNR I was hoping that there would be none,
> and in the three of the five test tanks broken down after five year's
> there were no telltales of H2S generation.  But that is what plenums are
> supposed to avoid.  I see no biologic reason for lower redox areas higher
> the substrate than the NNR levels - why should that occur?
> Two other plenum tanks are still operating, but one of those -the original
> test tank - is going to be reset this fall.  The last setup will if
> continue indefinitely, but I saw what I wanted from the tests, that NNR
> work in FW planted or not, and I don't plan to dig any deeper in that
> Robert Ricketts

My point was that H2S will be produced before you see any NO3=>NH4

Plenums don't avoid anything(H2S etc), it's just dead space that allows
some better circultation in lower reaches(hence slowed O2 availabilty, not
too much, but not too little-this O2 is mediated __mainly__ by bacteria,
not simply layering or spaces).
 This keeps the flow of O2 better, but the rate of reduction is determined
more by the loading rate of OM into the substrate and thus the bacteria
breaking this material down.
If you add a lot of reduced compounds such as soil, NH4, labile OM etc then
you can achieve highly reduced conditions if there is no/very little flow
out of the substrate.

If you place a dead/dying Aponogeton bulb into the substrate(very rich in
carbs/OM) and bacteria attack and break it down, what happens then? The
aerobes remove all the O2, then all that's left is anaerobes to feed on
this material, then the amount of OM and flux through the substrate play a
After some time, the H2S bacteria go after it.

After they have slowely removed all the OM, the substrates returns to
normal unless you keep adding/loading the substrate with more labile OM(the
loading rate).
No OM, no reduction/low O2 levels. 

Flourite etc, any highly porous substrate will have each grain having an
aerobic exterior and anaerobic interior, screw the plenum layers(wasted
valuable space), this works better and has better characters for plant
growth and stability(you can move the gravel around since each grain is a
protected unit. 

Silica sand etc, vs a highly porous substrate material has much more
surface area and internal space for denitrification, coils also work okay.
But anyone worth their salt will use plants to remove their NO3(add to the
filter or floating water sprite etc) from their tap/tank:-)

Tom Barr



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