Anoxic conditions involve environments in which molecular or free oxygen (O2) is absent, though bound oxygen may be present. “Anoxic” refers to the state of the environment rather than microorganisms and their processes.
In wastewater treatment, anoxic conditions differ from anaerobic conditions based on the presence or absence of nitrogen. Though anoxic conditions do not contain molecular oxygen, they may contain nitrates or nitrites. Nitrates (NO3) and nitrites (NO2) both contain bound oxygen because they include oxygen molecules as part of their structures. An anoxic environment may include these compounds as long as it does not include free oxygen.