Various "flavors" of wastewater ammonia oxidation and nitrogen removal

Ammonia, nitrite, and nitrate removal across biological wastewater treatment systems gives many operators fits. Why? Unlike organic compound degradation, ammonia nitrogen goes through several distinct steps to detoxify and finally remove the nitrogen from the water. Today I am going to cover the various groups of microbes involved in the wastewater nitrogen cycle.

• AOB (Ammonia Oxidizing Bacteria) - the first and possibly the most critical step is the conversion of ammonia into nitrite. Multiple organisms can do this step including: Nitrosomonas, Nitrosospira, and Nitrosococcus. 
• NOB (Nitrite Oxidizing Bacteria) - these organisms convert the nitrite produced by the AOB into nitrate. Organisms performing this step include: Nitrobacter, Nitrococcus, Nitrospina, and Nitrospira.
• Denitrifiers - many wastewater bacteria under anoxic conditions can use nitrate/nitrite as an alternative to oxygen in respiration. Some organisms even use nitrite and nitrate under aerobic conditions. The most active genera of denitrifiers include Pseudomonas, Paracoccus, and Hyphomicrobium.
• COMAMMOX (Complete Ammonia Oxidation) - these microbes can do both ammonia and nitrite oxidation inside one cell. In this group we have a subset Nitrospira. 
• ANAMMOX (Anaerobic Ammonium Oxidation) - an interesting way to save aeration energy by harnessing an interesting microbial biochemistry. Anammox systems use AOB to convert ammonia into nitrite. Then under anoxic conditions, a group of microbes can take nitrite (NO2) + ammonium (NH4) into nitrogen gas and water. These microbes have several interesting properties and have an extremely slow growth rate. Implementing Anammox technology often includes running a long sludge age and incorporation of media which allows for Anammox cultures to remain fixed in biofilm. 
• Heterotrophic Nitrification - organisms including Pseudomonas and Paracoccus species have demonstrated ammonia oxidation pathways while growing on organic compounds. In addition to being denitrifiers, these strains also oxidized ammonium. Factors and environments that encourage this process merit further study and may result in new ways to meet ammonia/nitrite treatment goals.