Aerobic, Anaerobic and Anoxic… How to Differentiate Between These Terms?

Within a wastewater system, a biological treatment can be essential to achieve water that meets quality standards. However, it can only be effective if the preferred conditions for bacteria are met.  Bacterial metabolism is complex, and each bacterial species can only perform its essential functions optimally if it is provided with the right nutrients. In the field of water treatment, one key parameter differentiates biological processes: the availability of oxygen.

Terms such as aerobic, anaerobic and anoxic are widely used in wastewater treatment lingo. That is why it is important to understand their meaning correctly. The complexity of their use resides in the fact that they can refer to both environmental conditions and types of bacterial metabolism. Once this distinction between the two uses is clear, applying them appropriately in context becomes much easier.

Aerobic conditions: Under these conditions, free oxygen (O₂) is present. The bacteria that perform aerobic respiration (i.e., those that use molecular oxygen as a terminal electron acceptor during the respiration process) proliferate in this environment.¹ In wastewater treatment, once this condition is met, both nitrification and the aerobic oxidation of organic matter can occur.¹ It is also interesting to note that it is under these aerobic conditions that sludge formation reaches its maximum yield.²

Anoxic conditions: Under these conditions, free molecular oxygen is absent. However, oxygen remains present in bound form, either in nitrogen compounds (NOx) or sulfur compounds (SOx).³ Bacteria that thrive at this level oxygenation can use bound oxygen as an energy source during anaerobic respiration. In the field of wastewater treatment, the bacteria that proliferate under these conditions are responsible for denitrification. Nitrites and nitrates generated by nitrification are then reduced by these microorganisms into inert gas, specifically nitrogen gas (N₂).¹

Anaerobic conditions: Under these conditions, the environment is completely devoid of oxygen. The bacteria that proliferate there are unable to tolerate the presence of oxygen. This level of oxygenation is maintained in water treatment systems to promote the biological processes of hydrolysis, fermentation (acidogenesis), and methanogenesis. This is how organic matter is broken down and biogas, mainly composed of methane and carbon dioxide, is produced.¹

It is important to note that a natural hierarchy exists among bacteria based on the type of metabolism they use. Microorganisms capable of growing in aerobic conditions generally dominate those that develop in anoxic or anaerobic environments. Similarly, anoxic bacteria tend to outcompete strictly anaerobic bacteria. This predominance can be explained by the energy yield specific to each metabolic pathway, measured notably by the ΔG of the reactions.⁵ Oxygen, as the most efficient terminal electron acceptor, enables significantly higher energy production than that obtained with alternative acceptors such as NOx⁶. Thus, aerobic bacteria obtain the energy necessary for their vital functions and growth more quickly, giving them a clear competitive advantage over anoxic and anaerobic bacteria. The same principle explains why anoxic bacteria, which also have better energy efficiency than anaerobic bacteria, tend to outperform them in a shared environment.

When integrated into a water treatment system, these three levels of oxygenation each offer distinct advantages for achieving specific reduction targets. If desired, it is also possible to combine these conditions within a single biological process. Some facilities use hybrid-media technology, in which a single biological support simultaneously holds aerobic, anoxic, and anaerobic bacteria. The oxygen gradient, which decreases from the surface toward the centre of the medium, naturally creates zones conducive to each metabolic pathway. There is also the sequencing batch reactor (SBR), a technology that is widely used throughout the world, in which these three oxygenation conditions coexist, in sequence, within the same tank.⁷ If you would like to learn more about the characteristics of this biological reactor, we invite you to continue your reading with this article :

https://aquasan.ca/articles/operation-of-an-sbr-which-parameters-influence-performance/

At Aquasan, our experts understand the distinction between these complex concepts and know how to make them simple and concrete. Thanks to their expertise, our customers benefit from tailored support to help them optimize their wastewater treatment system.

1. Metcalf & Eddy, Wastewater Engineering – Treatment and Reuse, 2003
2. Sumeet Sharma, Team one biotech, Anoxic vs. Anaerobic vs. Aerobic Wastewater Treatment, Avril 2025, https://www.teamonebiotech.com/blog/anoxic-vs-anaerobic-vs-aerobic-wastewater-treatment/?srsltid=AfmBOoqx9LzDjzWbApcLuZbe7j3Ye7eG4FirvXYRYetSgagLG6WE1XwF
3. Suez, Water and cell metabolism, https://www.suezwaterhandbook.com/water-and-generalities/water-a-fundamental-element/biology-of-water/water-and-cell-metabolism
4. Khan Academy, Fermentation and anaerobic respiration https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/a/fermentation-and-anaerobic-respiration
5. Cooper GM. The Cell: A Molecular Approach. 2nd edition. 2000, https://www.ncbi.nlm.nih.gov/books/NBK9903/
6. Staicu LC, Cosmidis J, Mansor M, Paquete CM, Kappler A. Microbial respiration – a biomineral perspective. Septembre 2025, https://pubmed.ncbi.nlm.nih.gov/40996469/
7. MRA, Exploring Key Dynamics of Sequencing Batch Reactor (SBR) Activated Sludge Process Industry, Mars 2025 https://www.marketreportanalytics.com/reports/sequencing-batch-reactor-sbr-activated-sludge-process-41649?utm_source=chatgpt.com#summary