Intermittent aeration to regulate microbial activities in membrane-aerated biofilm reactors: energy-efficient nitrogen removal and low nitrous oxide emission

The membrane-aerated biofilm reactor (MABR) is applied for autotrophic nitrogen removal. However, the proper and flexible control of nitrogen turnover is challenging in the counter-diffusion biofilms. In this study, we regulated microbial activities by providing aeration continuously versus intermittently in two lab-scale MABRs. Nitrogen consumption rates by individual functional microbes were estimated from bulk measurements using a mass balance approach. Nitrite-oxidizing bacteria (NOB) proliferated under continuous aeration while were significantly suppressed under intermittent aeration, and NOB suppression activated anaerobic ammonium-oxidizing bacteria (AMX). Nitritation performance in the MABR was studied with long-term bulk measurements and in situ biofilm microprofiles of dissolved oxygen (DO) and pH. During intermittent aeration the pH-related effects rather than the DO limitation effect determined nitritation success, including free ammonia speciation as substrates and inhibitors. Biofilm transition phases were monitored upon aeration switches. Canonical correspondence analysis suggested that the relative transition after anoxia and aeration intermittency were less decisive for biofilm performance compared to the relative aeration duration. Heterotrophic bacteria showed minor denitrification rates with aeration control, but contributed to the mitigated emissions of nitrous oxide (N₂O). N₂O production hotspots were identified at the top anoxic biofilm zone under continuous aeration, instead, under intermittent aeration an anoxic reduction zone was established by denitrifying heterotrophs. Our observations support intermittent aeration control as a simple strategy for the energy-efficient nitrogen removal with less N₂O emissions in MABRs.