There are two classes of denitrators, one using anoxic conditions to convert nitrates to nitrogen and the other uses sulphur to remove nitrates and phosphates, and add calcium. For the latter, see sulphur denitrator, as this page just deals with the former.
The denitrator converts nitrates to nitrogen using the process of denitrification. For this process to occur, the region has to be anoxic, without oxygen present. This is achieved by flowing water slowly through a long pipe, with aerobic bacteria removing the oxygen in the initial section. Once all the oxygen has been removed, the bacteria starts using the nitrate for their respiration and converting it to nitrogen.
It takes a significant length of time to establish a working denitrator. 4 to 6 weeks are required for the bacteria population to correctly establish itself to the point at which nitrate is being processed. Care must be taken to ensure that the flowrate is not too slow, which will cause the filter to go anaerobic, generating hydrogen sulphide. And if the filter is disturbed, such as flowrate slowed or increased, then it can take several weeks for it to stablise again.
Effect of a nitrate reactor on the nitrate concentration, left is the effluent from the nitrate reactor, right is the aquarium water
- The Autotrophic Denitrator on Sulfur: Whats the status? by Marc Langouet
- FAQs on Marine Water Quality involving Chemical Filtration of Nitrate - WetWebMedia
- Nitrate in the Reef Aquarium by Randy Holmes-Farley - Advanced Aquarist's Online Magazine