Figure 9.3 Wastewater nitrogen cycle. In the wastewater nitrogen cycle, molecular
nitrogen is removed from the atmosphere and returned to the atmosphere.
The movement of nitrogen through the cycle involves a number of conversions
(oxidation and reduction reactions) and a variety of bacteria. Molecular nitrogen
(N2 ) is removed from the atmosphere by nitrogen-fixing bacteria. These bacteria
produce ammonium ions (NH þ
4 ) from molecular nitrogen. The ammonium ions
produced by nitrogen-fixing bacteria are incorporated into amino acids and proteins
within green, leafy plants. The plants (fruits, leaves, roots, seeds, and stems)
are consumed by humans, and some of the nitrogen within the plants is released
in human bodily wastes (urine and fecal material) into the sewer system in the
form of urea, amino acids, and proteins. Some of these organic-nitrogen wastes
undergo hydrolysis and deamination in the sewer system and release ammonium
ions. Some of these organic wastes undergo deamination in the aeration tank and
release ammonium ions. There are several fates for the ammonium ions within
the aeration tank. If the pH of the tank increases to 9.4 or greater, some of the
ammonium ions are converted to ammonia (NH3 ) and are lost to the atmospher