Enhanced nitrogen removal from high-strength ammonium wastewater by improving heterotrophic nitrification-aerobic denitrification process: Insight into the influence of dissolved oxygen in the outer layer of the biofilm

This work aims to optimize the process of heterotrophic nitrification-aerobic denitrification (HN-AD) in the membrane-aerated biofilm reactor (MABR) by way of increasing the dissolved oxygen (DO) concentration in the outer layer biofilm to treat high-strength ammonium-nitrogen (NH4+-N) wastewater. The results show that the removal efficiencies of NH4+-N, total nitrogen (TN) and chemical oxygen demand (COD) increased by 42.4%, 65.7%, and 15.2%, respectively, in MABR-1 with high-DO concentrations than in MABR-2 with low-DO concentrations in the outer layer biofilm. High-DO concentrations in the outer layer biofilm increased the thickness of the aerobic layer to encourage HN-AD bacteria enrichment, which paved the way for nitrogen removal enhancement in MABR-1. Moreover, based on the analysis of functional gene expressions predicted by PICRUSt2, the relative abundances of seven key genes (hao, nirK, napA, napB, norB, norC and nosZ) relating to HN-AD bacteria in the MABR-1 significantly increased, indicating that DO increasing in the outer layer biofilm remarkably promoted the occurrence of HN-AD process. From the viewpoint of economic efficiency, the advantage of the novel strategy of increasing DO concentrations in the outer layer biofilm is that this optimized alternative for the MABR provides a superior nitrogen removal process in the high-strength NH4+-N wastewater treatment.