Hi all,
For anaerobic bacteria I don't believe the flow rate and the oxygenation are that critical. Flow will happen at the lump surface, but the anaerobic bacteria would be residing deep within the lump where the flow would be very slow and the oxygen would already be consumed by the aerobic bacteria. growing closer to the grain surface...........My reasoning for that is a little complicated. I'm next planning to do a large hybrid high-tech/low-tech system with a low maintenance approach while trying to achieve optimal water conditions. So if the plants are using the nitrates, and then if I'm not pruning and am allowing the leaves to decompose naturally as I'm already mostly doing happily now, then that nitrogen will just be perpetually reintroduced back into the water. I'm intending to keep the water as lean as possible while using a soil substrate continuously replenished through perforated substrate manifolds via a peristaltic dosing pump drip. So nitrates should be in non-limiting in the substrate continuously, while any that make it into the water would rapidly be removed by a large amount of anaerobic bacteria.
I understand your reasoning, but I think the concept is basically flawed. In a closed system, like a canister filter, it is almost impossible to get the balance between aerobic/anaerobic. You could however do something similar in a system open to atmospheric oxygen. This is a schematic from a trickle filter dealing with waste water (landfill leachate in this case), and using an alfagrog type media.
(from <
http://plecoplanet.com/?page_id=829>)
But even this system would need occasional cleaning to stop the bacterial layer from becoming too deep. The efficiency of the nitrifying bacterial metabolism depends on a large water surfaces exposed to oxygen (the “Gas Exchange Capacity”), this is because the nitrifiers compete poorly for oxygen with the community of bacteria that are breaking down the organic matter, the ones responsible for much of the biochemical oxygen demand (BOD). You can make use of your sediment to some degree, because the uppermost surfaces of the substrate are a good location for nitrifying bacteria, mainly because the nitrification process uses a lot of oxygen. However only a few centimetres below the substrates’ surface, the diffusion of oxygen can't supply enough oxygen, and as oxygen levels fall anaerobic bacteria become more frequent (in exactly the same way that is shown in the schematic drawing of a cross section of a trickle filter). Many of these bacteria are in fact “facultative anaerobes”; when oxygen is in short supply, they are able to switch to a metabolism that doesn't require oxygen, instead, they use nitrate, stripping the oxygen and leaving nitrogen (N2) gas. The nitrifying bacteria provide the nitrate, and their high oxygen demands also tend to exhaust the limited supply of oxygen. These two types of bacteria will occur across a fluctuating boundary lying not far beneath the surface of the substrate. The same processes will also occur in the “rhizosphere” the aerated zone lying around aquatic plants roots. I agree with you that these processes are both a good reason for both having a substrate, and leaving it relatively undisturbed.
In a canister filter, inevitably as the flow speed drops, the oxygen in the water won't meet the oxygen demand from the bacteria inside the filter. There are 2 components of this biological oxygen demand(BOD), "ordinary" decomposition:
Oxidizable material + bacteria + nutrient + O2 ? CO2 + H2O + oxidized inorganic such as nitrate (NO3) or (Sulf(ph)ate) SO4.
and "nitrification": NH3 + CO2 + 1.5 O2 ? NO2- + CO2 + 0.5 O2 ? NO3-
Simply stated, a heavy load of organic materials being degraded in your system inhibits the nitrifiers by competing with them for oxygen. Dissolved oxygen concentrations above 1 mg/l are essential for nitrification to occur. If DO levels in the filter drop below this level nitrifications slows, or ceases altogether, the NH3 passes through the filter without entering nitrification and the fish experience high NH3, low O2 conditions in the tank, with potentially catastrophic effects.
I think you can have a system where you do very maintenance (I do very little), but you need to keep the filter media aerobic.
cheers Darrel
