Stoichiometry of combined aerobic and methanogenic COD transformation

Under oxygen-limited conditions, both aerobic respiration and methanogenesis were accomplished by a mixed culture developed from a municipal anaerobic digester sludge and maintained under batch conditions. Oxygen-limited systems were maintained for over 130 days, producing methane and biomass while consuming oxygen under stable conditions, with chemical oxygen demand (COD) removal averaging 92%. Typically, the dissolved oxygen in the culture medium was zero. However, short periods of greater-than-zero dissolved oxygen were evident. In this case, methanogens were not killed by dissolved oxygen, but may have been inhibited during oxic periods. Electron equivalent balances were employed to construct stoichiometric equations for propionate- and ethanol-fed cultures that received enough oxygen to oxidize approximately 0, 10, and 30% of the added substrate. Oxygen-limited cultures exhibited maximum yield coefficients (Y) of between 0.06 and 0.11 g volatile suspended solids per gram COD. These values are more typical of methanogenic, rather than aerobic systems. Under some conditions, it may be possible to achieve low COD values associated with aerobic systems and produce the minimum waste biomass associated with methanogenic systems in a single-sludge, oxygen-limited process. Other possible benefits of methanogenic, oxygen-limited systems include: degradation of specific organics and mixtures of organics that proceed through both reductive and oxidative biological reactions; degradation of specific organics through cometabolic, methanotrophic reactions; concurrent nitrification and denitrification; and description of natural systems under oxygen-limited conditions.