One of the biological processes of nitrogen utilization is denitrification. This process is mediated through the action of certain microorganisms. The denitrifying organisms can reduce nitrite to molecular nitrogen and even to ammonia. In some cases, nitrite can also be oxidized to nitrate. Many different types of bacteria can use nitrite in place of oxygen as a final hydrogen acceptor. Thus, denitrification occurs under anaerobic conditions (absence of oxygen) in which oxidation-reduction occurs utilizing the oxygen contained in the nitrite. Consequently, a source of combined hydrogen and limitation of the supply of oxygen in the system are required for denitrification. The following are conditions under which denitrification is likely to occur:
A supply of nitrites and oxidizable matter: Since most of the anaerobic bacteria which cause denitrification are heterotrophic, these processes are favored by the presence of oxidizable organic matter which not only provides food for energy but may also help to create the needed anaerobic conditions.
Absence of free oxygen: Anaerobic conditions, in the presence of organic matter, are conducive to denitrification. In the presence of free oxygen, the process of denitrification is inhibited or stopped completely.
Supply of Moisture: The microorganisms responsible for denitrification are able to grow and carry on their respiration in systems that contain from one fourth to one half of their water holding capacity, but under such circumstances anaerobic conditions are uncommon. Therefore, denitrification seldom occurs in dry environments.
Hydrogen Ion Concentration (pH): A few species of bacteria can cause denitrification in systems as acidic as 2.0 whereas other species are active in systems as alkaline as 10.0. Most denitrifiers prefer a near neutral solution.
Temperature: There are many microorganisms in water systems capable of growing in temperatures from just above the freezing point to as high as 70 degrees Celsius. Most denitrifying bacteria favor temperatures within 15-45 degrees Celsius.
For the denitrifying process to occur, the organisms need a carbon source (organic matter), an alkaline environment, and reduced oxygen. All of these conditions are common in closed water systems. Stay tuned for the final blog in this series which will address the control of denitrification.