Can membrane aerated biofilm reactor (MABR) realize more efficient simultaneous partial nitrification and denitrification than conventional biofilm under low COD/N?

Simultaneous nitrification and denitrification (SND) is an efficient nitrogen-removal process, yet its application in low COD/N wastewater is limited in conventional co-diffusion biofilms due to carbon source waste and microbial competition. Membrane aerated biofilm reactors (MABR) could theoretically realize more efficient SND by employing counter-diffusion biofilms to separate oxygen and carbon sources. However, the SND efficiencies were not satisfactory in previous publications, possibly because excessive aeration and uncontrolled nitrite-oxidizing bacteria (NOB) growth increased the demand for carbon sources. Herein, three MABRs using oxygen-affinity PP-PDMS membranes were operated with an extremely low aeration pressure (0.6 kPa) to initiate SND in high-strength nitrogen wastewater (200 mg/L NH₄⁺-N) with low COD/N (R1: 2; R2: 3; R3: 4). A stable simultaneous partial nitrification and denitrification (SPND) was established in R2 and R3 (TN removal of 73.8% and 79.8%, respectively), which was higher than the reported co-diffusion biofilms. Increased COD/N did not significantly enhance nitrogen removal once SPND was established. Microbiological analysis indicated that ammonia-oxidizing bacteria (AOB) and archaea (AOA) both contributed to nitrification, and NOB were incompletely suppressed. Fluorescence in situ hybridization (FISH) and mass transfer calculation revealed that carbon sources inhibited NOB and enhanced oxygen transfer by increasing the oxygen concentration gradient. However, nitrate accumulation and COD waste still occurred and led to a gap between MABR and single denitrification because of the unavoidably high oxygen at the membrane-biofilm interface. Overall, counter-diffusion biofilms outperformed co-diffusion biofilms in TN removal, but entire SPND was still hard to be attained.