Mitigating Nitrous Oxide Emission from a Lab-Scale Membrane-Aerated Biofilm Reactor

The membrane-aerated biofilm reactor (MABR) is an emerging technology for the biological treatment of wastewaters. It can achieve simultaneous nitrification and denitrification due to anoxic liquid conditions. The counter diffusion of oxygen and nutrients in the biofilm allows for aerobic and anoxic layers, providing conditions where the formation, accumulation and consumption of nitrous oxide can all occur. The microbial processes involved in the production and consumption of N2O are complex, and, due to the innovative nature of the MABR, understanding the influence of operational factors helps to minimise N2O emission. Using a lab-scale 20L MABR system, an investigation was carried out to determine the influence of operational factors on the emission of nitrous oxide from the reactor. A direct link between the nitrous oxide emissions and bulk liquid conditions could not be established with only limited statistical correlation between them. It was found that under both steady loading rates and transient conditions, the emission of nitrous oxide was most influenced by the air flow rate through the membranes. The majority of N2O emissions occurred via the membrane off-gas and not through the liquid. N2O flux through the membrane was influenced not only by the accumulation of N2O in the biofilm side but also by the gas residence time on the lumen side. Therefore, minimising the air flow rate is an effective strategy to mitigate nitrous oxide emissions from the MABR.