It can occur under conditions of no oxygen or conditions of extremely low oxygen concentration. Denitrifying bacteria are generally found in stagnant water. Low flow or no flow areas within a closed recirculating water system would be ideal areas for denitrifying bacteria to flourish. Molecular nitrogen can be converted to ammonium ions through a process called Nitrogen Fixation. In nitrogen fixation the ammonium ions are used to assimilate amino acids and proteins.
Nitrogen fixation is a aerobic process.
However, aerobic nitrogen-fixing bacteria can withstand extremely low levels of oxygen. The use of Sodium Nitrite as a corrosion inhibitor at levels of 800-1500 ppm of nitrite offers an excellent source of nutrition for the three processes described above. Most closed recirculating water systems will have areas of anaerobic and aerobic conditions. Therefore it is possible to have more than one process operating within the same system. At the present time there are no field tests available to determine either qualitatively or quantitatively if these processes are occurring.