The generation of highly reactive oxidizing agents such as peroxides and superoxides in the liquid medium causes not only functional inhibition but also rapid cell lysis (Gottschalk and Peinemann, 1992) of obligatory anaerobic species. However, as demonstrated by other researchers oxygen inhibition is less severe as generally expected. One major reason for that is that mixed anaerobic communities comprise also many facultative anaerobic strains. Especially among those species involved in acidogenesis. These microorganisms rapidly eliminate dissolved oxygen and thereby protect obligate anaerobic strains. (Gerritse, 1990 and Kumar et al., 2015). Botheju and Bakke (2010) found that rigorous initial aeration of an anaerobic inoculum resulted in a three times longer lag period before gas generation started but also noted a certain reversibility of the inhibition. While details of the reversibility of oxygen inhibition of methanogens remains unknown such an effect was also observed by Gerritse (1990). Zitomer (1998), who conducted experiments under limited oxygenation conditions, reported that the activity was inhibited for a short period (30 min) but methanogens were not irretrievable damaged.