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The Nitrogen Cycle

Updated: May 4, 2022

Three graphics of the nitrogen cycle are shown in this short, simple post. The first graphic below is one of the most common and you will come across variations of this in different textbooks. This graphic represents my first "paying-attention" view of the nitrogen cycle. As you will see in the source below, this graphic comes from a 1977 EPA publication, an early purchase I made at the start of my wastewater career, a purchase made as part of my ongoing effort to develop my technical library. Though this paperback is slowly falling apart it is still a technical reference I use to this day. (Special note of thanks: Thank you Ron T for noticing the error in my reproduction of this EPA nitrogen graphic!! It is now correct, as of May 3, 2022, because of your sharp eye. Sending me an email letting me know is something I very much appreciate.)

Solid lines show important pathways in the biological treatment of wastewater. The broken line for nitrogen fixation is only added to complete the cycle. Approximately 60 to 80% of the nitrogen in raw domestic wastewater is in the form of ammonia nitrogen. The rest is primarily in the form of organic nitrogen.

Wastewater Nitrogen Cycle

Figure II-2

Source: United States Environmental Protection Agency. Municipal Operations Branch. Office of Water Program Operations. “Process Control Manual for Aerobic Biological Wastewater Treatment Facilities.” EPA-430/9-77-006. Washington, D.C., March 1977.

You can download a PDF version of this excellent reference (EPA, 1977) below. The Wastewater Nitrogen Cycle graphic is on page II-21. But you might also be interested in Table II-12, the "Allowable Concentrations of Heavy Metals" on page II-94.

Download PDF • 5.77MB




This post has been formatted and saved as a three-page PDF file which you can download here.

Nitrogen Cycle
Download PDF • 196KB


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