Integration of methane removal in aerobic anammox-based granular sludge reactors

Combined partial nitritation-anaerobic ammonium oxidation (anammox) processes have been widely applied for nitrogen removal from anaerobic digestion reject water. However, such streams also contain dissolved methane that can escape to the atmosphere, hence contributing to global warming. This study investigates the possibility of integrating methane removal in aerobic anammox-based granular sludge reactors, through modelling and simulation. Methane removal could be established through aerobic methane-oxidizing bacteria (MOB), denitrifying anaerobic methane-oxidizing bacteria (damoB, NO₂^- + CH₄ → N₂ + CO₂), and/or archaea (damoA, NO₃^- + CH₄ → NO₂^- + CO₂). The simulation results demonstrated that the combined removal of nitrogen and methane was feasible at low dissolved oxygen conditions. Aerobic MOB were the main responsible microorganisms for removing methane. A sensitivity analysis of key kinetic parameters showed a shift in the methanotrophic populations depending on the most favourable parameters for each microbial group, while keeping high nitrogen and methane removal efficiencies. Possible methane stripping during aeration could be limited by increasing the depth within the reactor column at which aeration was supplied. Overall, the integration of methane removal in aerobic anammox-based granular sludge reactors seems to be a promising process option to reduce the carbon footprint from wastewater treatment.