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I also question if an anaerobic zone is possible at all in this large of substraight. I guess if you're talking very large CC, then you may be getting denitrification from low oxygen cores in the CC, similiar to how live rock is able to process nitrates. Just thinking aloud. I think I'm going to regret this.
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This is what I was trying to get at with a faster flow-through rate of the bed. I understand denitrification is not happening IN the bed itself, but UNDER it. But allowing a larger grain in your bed opens up more water getting down there, and hence more oxygen. This is going to result in Less denitrification unless you can compensate for this in some fashion. Hence the deeper bed idea.
Remember, there are a lot of success AND failure stories when using plenums, just like any other style of filtration. The reason it fails usually is because the bed was improperly setup to allow too much diffussion down to the plenum area.
This subject has already been fine-tuned by Jaubert and the Monaco aquarium as far as grain sizes and bed depths are concerned. According to thier research, they already HAVE the optimal size substrate. Mojo, you yourself said we are trying to take what they did and make it work for us. This means compromise in order to achieve other benefits. The compromise in this case is more oxygen to the plenum area, and hence less denitrification than before.
On another note, these faster flow rates also raise pH levels slightly, meaning that the little elemental replenishment you were getting is now going to disappear. If the PO4 is being re-bound, why not the desirables too?
Doug,
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Mike,
On your last post regarding the building. Why two layers of screen? Would that not be more cloggable, { , dont ask me what that word is, }. Also I would use a minimum of 4 in. of gravel.
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I started out reading this thread thinking that 2 layers of screen would surely be beneficial. as it allows a safe barrier when siphoning or stirring the gravel on the surface. This top layer would be placed approximately where denitrification starts to take place, or even slightly above to ensure oxygen saturation doesn't occur. Sure it has a better chance of clogging, but at the same time, it helps keep the clogs up in the top where you could also vacum during your normal water changes. This in return would go to extending the life of the bed.
I'm not FULLY against trying to find a way to clean the plenum, but I'm not sure draining it is the best idea. I just can't possibly see how it's not going to affect the tank in some nasty way.
Another idea to achieve this, WITHOUT destroying the plenum's functionality might be to have a closed loop attached to it, with a micron filter or something in-line. Open the ball valves, turn on the pump (a magnum 350 would be ideal IMO), and push whatever water you suck out right back in, but with sediment removed. The only problem here is trying to make sure the water in the magnum is fully anaerobic before turning it on, right?
I'm still not even convinced that the Magnum closed loop will work, but I think it has a bit more plausability than the draining method, which would totally destroy your tanks filtration capabilities for some time. We need to find a less-destructful way of achieving the cleaning so that our occupants don't notice the "bump" in the system.
Just my 2 pennies,
Cos