Denitrification is the process in which nitrate is removed from the aquarium water. Removal is attained biologically using two basic processes; dissimilatory and assimilatory nitrate reduction. Dissimilatory nitrate reduction involves anaerobic bacteria using the nitrate for respiration, returning it to the water as nitrogen and is the mechanism in which most people are referring to when they talk about "denitrification". Assimilatory nitrate reduction involves the physical uptake of the nitrate by bacteria or algae.
Assimilatory Nitrate Reduction
Assimilatory nitrate reduction involves the nitrate being directly absorbed into the tissue mass of algae or bacteria. The algae may be cultured for exactly this purpose in a refugium (macro algae) or algae turf scrubber (turf / micro algae). The nitrogen (and phosphate, though that is unrelated to the nitrogen cycle) is then permanently removed from the system when the algae is manually harvested and discarded.
Dissimilatory Nitrate Reduction
Dissimilatory nitrate reduction involves anaerobic bacteria using the nitrate for respiration, rather than oxygen that aerobic bacteria utilises. It occurs within regions that are anoxic, without any oxygen present (with the oxidation reduction potential between +50 mV and -50mV). The bacteria that is typically associated with doing this are called Pseudomonas, which breakdown nitrates to obtain oxygen, with the by-product being nitrogen gas (N2) which is then lost to the atmosphate. There are a large number of heterotrophic bacteria which can function as denitrifiers. Importantly, this type of bacteria requires a separate food source. In the example reaction given below, the food provided is methanol. Note that denitrification liberates an hydroxide ion (OH−) which raises pH and alkalinity thereby offsetting the hydrogen ion liberated by nitrification.
NO3− + 5/6 CH3OH → 1/2N2 + 5/6 CO2 + 7/6 H2O + OH− nitrate + 5/6 methanol → 1/2 nitrogen gas + 5/6 carbon dioxide + 7/6 water + hydroxide
Denitrification occurs naturally within a system that has liverock and a deep sand bed (DSB). A DSB is a bed of sand at least 10cm (4") deep, in which the water is poorly oxygenated enough due to low water flow that anaerobic sections (areas without any oxygen present) can form. A variation on this method involves the use of a plenum, or small space of still water, underneath the sand bed - again, this water becomes so poorly oxygenated due to bad flow that it becomes anaerobic, allowing the bacteria which convert nitrate to nitrogen to grow. The process can also be assisted by the use of a denitrator or nitrate reactor, which generates this anaerobic region within a more compact filter.
- (Spotte 1992): Spotte, S., Captive Seawater Fishes: Science and Technology, Wiley Interscience, John Wiley and Sons: New York, 1992, p 942.