- AOB/NOB - this process is the well-known chemoautotrophic bacteria. This is the familiar oxidation of ammonia to nitrite by Nitrosomonas. And the most common NOB is actually Nitrospira.
- COMAMMOX - Complete Ammonia Oxidation - A subset of Nitrospira can oxidize ammonia to nitrate. This reduces the number of single step AOB seen in the system.
- ANAMMOX - Anaerobic Ammonia Oxidation - organisms from the phylum Planctomycetes utilize NO2 + NH4 --> N2 + H2O. This requires AOB for nitrite, but the short cut to N2 gas results in substantial energy savings. The challenge here is ANAMMOX cultures have an extremely slow growth rate and require anaerobic conditions to have a metabolic advantage.
- Heterotrophic Nitrification - some heterotrophic organisms can oxidize ammonia and nitrite. These organisms grow various organic substrates but also have the ability to oxidize ammonia and nitrite.
- Simultaneous Nitrification Denitrification - building on heterotrophic nitrification with organisms such as Paracoccus denitrificans, Pseudomonas sp, and Alcaligenes sp having pathways for ammonia oxidation and denitrification even under aerobic conditions. Factors impacting SND pathways include carbon source, C/N ratio, pH, temperature, and Dissolved Oxygen.
- Simultaneous Removal of Nitrogen and Phosphorus - SNDPR - this is often seen in low COD systems and aquaculture, but some PAO also have the ability to take a role in removal of inorganic nitrogen in wastewater.
Challenge of using the above pathways in wastewater treatment
- Just because an organism has metabolic capabilities or genes, it does not mean that they will be turned on or used. For example, Paracoccus denitrificans can oxidize ammonia and grow on organic acids. If you have sufficient organics (COD), it will obtain more energy using standard heterotrophic pathways and you may see little ammonia oxidation. This is why C/N ratios can become very important for heterotrophic nitrification - in WW, this means lower F/M may trigger other energy yielding pathways.
- Do you need separate zones with various D.O. levels, or can we have multiple zones inside biofilms or granules. This is the mechanism seen in newer granular sludge systems where the individual granule has aerobic/anoxic/anaerobic zones. This also happens to a lesser extent in conventional floc/biofilm.
- Perhaps a mix of all of the above will be more robust and less susceptible to upset from changing conditions. We actually see this in working systems where organisms that can exploit a ecological niche will grow and populations do change in response to loading and environmental/operational conditions.
Good articles on heterotrophic nitrification, SND, and SNDPR
Pertti J. Martikainen, “Heterotrophic nitrification – An eternal mystery in the nitrogen cycle” Soil Biology and Biochemistry Volume 168, 2022. https://www.sciencedirect.com/science/article/pii/S0038071722000682?via%3Dihub
Shivani Shukla, et al. “Simultaneous Nitrification–Denitrification by Phosphate Accumulating Microorganisms” World Journal of Microbiology and Biotechnology (2020) 36:151
Zhao W, Bi X, Peng Y, Bai M. Research advances of the phosphorus-accumulating organisms of Candidatus Accumulibacter, Dechloromonas and Tetrasphaera: Metabolic mechanisms, applications and influencing factors. Chemosphere. 2022 Nov;307(Pt 1):135675. doi: 10.1016/j.chemosphere.2022.135675. Epub 2022 Jul 13. PMID: 35842039