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This is an old thread, but just for the heck of it I looked up the basic process since it's in my wheelhouse (chemical engineering). Here is an academic article that describes the basic process, in this case as applied to septic tank effluent.
Of specific interest to this particular thread, note the stoichiometric reaction shown as equation (1) on page 349. Approximately speaking, one proton is produced for every nitrate ion that is reduced to nitrogen gas. This, of course, acidifies the effluent, which then dissolves the limestone layer. The net effect is nitrate reduction, sulfate production, and dissolution of calcium carbonate (limestone). The overall reaction scheme is exactly as Randy has explained in the thread - in a seawater system, the net effect would be nitrate reduction, sulfate production, and, in contrast to the freshwater systems described in the literature, consumption of alkalinity.
The reason for this is straightforward. In the freshwater sewage treatment schemes, the pH in the reactor is quite low, which allows for dissolution of the limestone bed included as an inorganic carbon source for the heterotrophic bacteria. Depending on how the reactor is configured, the limestone bed also allows for restoration of a more neutral pH for the overall treatment effluent to avoid the highly negative effects of acidic discharge to the soil.
However, in our systems, the inorganic carbon required by the bacteria for growth would come from the seawater (lowering the overall alkalinity). The low pH required to dissolve calcium carbonate sufficiently to restore the alkalinity consumed by the bacteria in the reduction of nitrate would be highly undesirable.
I'm speculating here, but I would think a somewhat complicated reactor configuration would have to be put together to 1) provide effective denitrification, 2) dissolve calcium carbonate to restore the alkalinity, and finally 3) raise the pH back to tank-appropriate values. Either that, or the overall alkalinity dosing to the tank would have to increase by quite a lot (as Randy notes) to compensate for a much simpler reactor design. If this was done by 2-part dosing, I'd think the amount of extra sodium going into the water would be significant, and as the reactor effluent would be enriched in sulfate, the sulfate/chloride balance of the water would change from seawater considerably faster.
Overall, and considering the relative simplicity of nitrate reduction using the already-available anoxic zones in a reef tank by simply adding an inorganic carbon source, the system described in this thread doesn't seem to offer any advantages, and adds considerable complexity to a tank.
Of specific interest to this particular thread, note the stoichiometric reaction shown as equation (1) on page 349. Approximately speaking, one proton is produced for every nitrate ion that is reduced to nitrogen gas. This, of course, acidifies the effluent, which then dissolves the limestone layer. The net effect is nitrate reduction, sulfate production, and dissolution of calcium carbonate (limestone). The overall reaction scheme is exactly as Randy has explained in the thread - in a seawater system, the net effect would be nitrate reduction, sulfate production, and, in contrast to the freshwater systems described in the literature, consumption of alkalinity.
The reason for this is straightforward. In the freshwater sewage treatment schemes, the pH in the reactor is quite low, which allows for dissolution of the limestone bed included as an inorganic carbon source for the heterotrophic bacteria. Depending on how the reactor is configured, the limestone bed also allows for restoration of a more neutral pH for the overall treatment effluent to avoid the highly negative effects of acidic discharge to the soil.
However, in our systems, the inorganic carbon required by the bacteria for growth would come from the seawater (lowering the overall alkalinity). The low pH required to dissolve calcium carbonate sufficiently to restore the alkalinity consumed by the bacteria in the reduction of nitrate would be highly undesirable.
I'm speculating here, but I would think a somewhat complicated reactor configuration would have to be put together to 1) provide effective denitrification, 2) dissolve calcium carbonate to restore the alkalinity, and finally 3) raise the pH back to tank-appropriate values. Either that, or the overall alkalinity dosing to the tank would have to increase by quite a lot (as Randy notes) to compensate for a much simpler reactor design. If this was done by 2-part dosing, I'd think the amount of extra sodium going into the water would be significant, and as the reactor effluent would be enriched in sulfate, the sulfate/chloride balance of the water would change from seawater considerably faster.
Overall, and considering the relative simplicity of nitrate reduction using the already-available anoxic zones in a reef tank by simply adding an inorganic carbon source, the system described in this thread doesn't seem to offer any advantages, and adds considerable complexity to a tank.