“Anaerobic” describes the respiration processes of microorganisms. Anaerobic respiration occurs in the absence of oxygen.
During anaerobic processes, both free and bound oxygen are typically absent. Nitrates and nitrites do not usually exist in wastewater processes that use anaerobic bacteria — bacteria that do not require oxygen — for digestion. These conditions differ from anoxic environments, in which free oxygen is also absent but bound oxygen like that found in nitrates and nitrites may be present.