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.