Treatment of Anaerobic Digester Liquids via Membrane Biofilm Reactors: Simultaneous Aerobic Methanotrophy and...

Anaerobic digestion (AD) produces useful biogas and waste streams with high levels of dissolved methane (CH₄) and ammonium (NH₄⁺), among other nutrients. Membrane biofilm reactors (MBfRs), which support dissolved methane oxidation in the same reactor as simultaneous nitrification and denitrification (ME-SND), are a potential bubble-less treatment method. Here, we demonstrate ME-SND taking place in single-stage, AD digestate liquid-fed MBfRs, where oxygen (O₂) and supplemental CH₄ were delivered via pressurized membranes. The effects of two O₂ pressures, leading to different O₂ fluxes, on CH₄ and N removal were examined. MBfRs achieved up to 98% and 67% CH₄ and N removal efficiencies, respectively. The maximum N removal rates ranged from 57 to 94 mg N L^-1 d^-1, with higher overall rates observed in reactors with lower O₂ pressures. The higher-O₂-flux condition showed NO₂^- as a partial nitrification endpoint, with a lower total N removal rate due to low N₂ gas production compared to lower-O₂-pressure reactors, which favored complete nitrification and denitrification. Membrane biofilm 16S rRNA amplicon sequencing showed an abundance of aerobic methanotrophs (especially Methylobacter, Methylomonas, and Methylotenera) and enrichment of nitrifiers (especially Nitrosomonas and Nitrospira) and anammox bacteria (especially Ca. Annamoxoglobus and Ca. Brocadia) in high-O₂ and low-O₂ reactors, respectively. Supplementation of the influent with nitrite supported evidence that anammox bacteria in the low-O₂ condition were nitrite-limited. This work highlights coupling of aerobic methanotrophy and nitrogen removal in AD digestate-fed reactors, demonstrating the potential application of ME-SND in MBfRs for the treatment of AD's residual liquids and wastewater. Sensor-based tuning of membrane O₂ pressure holds promise for the optimization of bubble-less treatment of excess CH₄ and NH₄⁺ in wastewater.