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The Nitrogen Cycle

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The nitrogen cycle. (Adapted from Manual of Instruction for Sewage Treatment Plant Operators, New York State Department of Health,  pg 11.)

The nitrogen cycle.
(Adapted from Manual of Instruction for Sewage Treatment Plant Operators, New York State Department of Health, pg 11.)

The nitrogen cycle is the process by which nitrogen is converted between it’s various chemical forms. Atmospheric nitrogen (78% by volume) is first “fixed” by two natural processes: lightning (a small amount) and biological fixing by bacteria known as diazotrophs. A third industrial process, the Haber-Bosch process, is used to generate the ammonia used in fertilizers. This industrial process accounts for about 30% of the total nitrogen fixation.

Once the nitrogen has been fixed, it is next assimilated by plant life in the form of nitrate or nitrite ions. From here, the nitrogen either becomes a component of organic matter via plant decay, or is eaten by animal life and converted to animal proteins, which are then converted to animal wastes (primarily the urea in urine) and other organic matter.

Decomposition converts the organic matter and urea into ammonia, via ammonification. Ammonia is then nitrified in a two-step bacterial process: first from ammonia to nitrite by bacteria such as Nitrosomonas and then to nitrate by bacteria such as Nitrobacter. This step is an aerobic process, as Nitrosomonas and Nitrobacter require oxygen to survive.

Finally, the nitrates are converted to atmospheric nitrogen via denitrification, a bacterial process carried out by Pseudomonas and Clostridium bacteria. This process occurs under anaerobic (oxygen-free) conditions.

Nitrogen in Wastewater

Relating to nitrogen removal from wastewater, we are mostly concerned with the steps of ammonification, nitrification and denitrification. Incoming urea is converted first to ammonia and then to nitrites.

Notes and Image Credits:

Header Image: By Cicle_del_nitrogen_de.svg: *Cicle_del_nitrogen_ca.svg: Johann Dréo (User:Nojhan), traduction de Joanjoc d'après Image:Cycle azote fr.svg.derivative work: Burkhard (talk)Nitrogen_Cycle.jpg: Environmental Protection Agencyderivative work: Raeky (Cicle_del_nitrogen_de.svgNitrogen_Cycle.jpg) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons