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

Anaerobic digestion (AD) produces useful biogas and waste streams with high levels of dissolved methane (CH ) and ammonium (NH ), 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 (O ) and supplemental CH were delivered via pressurized membranes. The effects of two O pressures, leading to different O fluxes, on CH and N removal were examined. MBfRs achieved up to 98% and 67% CH and N removal efficiencies, respectively. The maximum N removal rates ranged from 57 to 94 mg N L d , with higher overall rates observed in reactors with lower O pressures. The higher-O -flux condition showed NO as a partial nitrification endpoint, with a lower total N removal rate due to low N gas production compared to lower-O -pressure reactors, which favored complete nitrification and denitrification. Membrane biofilm 16S rRNA amplicon sequencing showed an abundance of aerobic methanotrophs (especially , and ) and enrichment of nitrifiers (especially and ) and anammox bacteria (especially Ca. and Ca. ) in high-O and low-O reactors, respectively. Supplementation of the influent with nitrite supported evidence that anammox bacteria in the low-O 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 O pressure holds promise for the optimization of bubble-less treatment of excess CH and NH in wastewater.