Denitrifiers in Mainstream Anammox Processes: Competitors or Supporters?

The partial nitrification–anammox (anaerobic ammonium oxidation) process (PN/A) was proposed to remove nitrogen from lowstrength municipal wastewater. The huge economic incentives are driving researchers and practitioners to make tremendous efforts in order to implement PN/A processes in full-scale systems. However, the typical variations in temperature, nutrients, and organic concentrations create many challenges in PN/A processes treating municipal wastewater. Instead of producing nitrite through partial nitrification prior to the anammox process, much attention has been paid recently to the formation of nitrite from denitrification as a process alternative. Such process concept shown here appears to be a promising way forward for achieving energy-neutral or energy-positive wastewater treatment. Operating costs and energy recovery are comparable with PN-based anammox processes, while better effluent quality, lower greenhouse gas emissions and long-term stability are achievable with partial denitrification – anammox process. In contrast to the generally accepted viewpoint, the potentially positive role of denitrifiers should be recognized in order to better benefit the stability and robustness of sustainable wastewater treatment plant operation.