FIGURE 4.
Traditional Nitrification/denitrification process (N/dN) (A) and anammox-mediated removal of ammonia coupled to partial nitritation (B). (A) Shows the input and byproducts of the N/dN process: aeration (blue bubbles) for the complete oxidation of ammonia by aerobic ammonia oxidizers (AOB, in yellow) and nitrite oxidizing bacteria (NOB, in purple including complete ammonia oxidizers), organic carbon as electron donor for denitrification, and the production of sludge, and potential release of GHG gases (N2O) represented in dark gray. Potential emissions of N2O during nitrification (yellow, broken line) could take place under oxygen limitation (Caranto et al., 2016; Thakur and Medhi, 2019). (B) The partial-nitritation/anammox process (PN/A), where nitrite from partial-nitrification by AOB (in yellow) requiring less aeration (blue bubbles), is used to oxidize ammonia directly by anammox bacteria (in red), without performing the complete nitrification-denitrification (broken lines, light gray). Aeration control is crucial to balance nitrite production and the prevention of N2O from oxygen-limited nitrification.