ANAMMOX & Partial Nitrification/denitrification - ways to remove wastewater nitrogen while lowering utility costs

Wastewater Nitrogen Cycle from Wikipedia.org.

With tighter effluent ammonia and total nitrogen limits on the horizon, many treatment plants are evaluating ways to improve biological nitrogen removal while keeping a cap on capital or operational costs.  Among the options is Anaerobic Ammonia Oxidation (ANAMMOX) and using partial nitrification followed by denitrification. 

ANAMMOX inovolves a unique biochemistry that existing in several slow growing genera that when in anaerobic conditions can utilize nitrite and ammonium as an energy source. 

• Step 1 (AOB)                            2NH4+ + 3O2 → 2NO2− + 4H+ + 2H2O
• Step 2 (ANAMMOX)                NH4+ + NO2− → N2 + 2 H2O
  
  

Partial Nitrification with denitrification relies on a more conventional process to yield nitrogen gas. Of course, there are intermediate steps not detailed below - but the final reaction products are what is important.

• Step 1 (AOB)                            2NH4+ + 3O2 → 2NO2− + 4H+ + 2H2O
• Step 2 (denitrification)           NO2− + CHO (organic) → N2 + CO2 + H2O

For ANAMMOX to work a system must have sufficient residence time and growth environment to favor building an ANAMMOX population. Systems capable of ANAMMOX include:

• Side stream ANAMMOX treatment reactor separate from the main biological treatment unit.
• Use of MBBR media in the existing system 
• Use of Aerobic Granular Sludge (AGS) technology
• MBR system with higher MLSS, longer MCRT

​The complexity of ANAMMOX including potential consruction costs, has led to interest in partial nitrification with immediate denitrification. Here you need to encourage AOB activity followed by suitable soluble BOD under anoxic conditions for cultures to convert NO2 into N2 gas. This can be done by closely monitoring redox potentials and using step-feed or other high soluble BOD source just prior to the anoxic zone.