Enhanced nitrogen removal from low strength anaerobic membrane bioreactor (AnMBR) permeate using complete nitrification and partial denitrification-anammox processes

In this study, an anaerobic membrane bioreactor coupled with a complete nitrification and partial denitrification-anammox process (AnMBR-NF-PDA) was developed to efficiently remove both chemical oxygen demand (COD) and nitrogen. Precise control of raw water ratios was utilized to adjust the ratio of COD/NO3--N, resulting in maximum nitrogen removal efficiency of 90.14% at a ratio of 3.44. Initially, specific anammox activity (SAA) increased with the proportion of raw water, peaking at 17.83 mg-N/(g-VSS<middle dot>d) in stage II before decreasing. This variation was attributed to the significant presence of filamentous bacteria, especially "Acinetobacter" (13.58%-31.59%), which facilitated nitrite generation, supporting the nitrous oxide hypothesis in partial denitrification processes and enabling cross-feeding with AnAOB. Additionally, the average particle size of granular sludge increased from 300 to 528 mu m under the influence of filamentous bacteria. Metagenomic analysis revealed an upsurge in genes associated with partial denitrification (NarG and NapA) as the COD/NO3-N ratio rose. The abundance of genes closely correlated with anammox (Hzs and Hdh) peaked during stage II, indicating the beneficial role of filamentous bacteria in the stable conversion of nitrite in PDA system. This study offers valuable insights into optimizing the synergistic metabolism and granulation processes involving filamentous bacteria and AnAOB, thereby laying the groundwork for the practical application of AnMBR coupled with anammox processes in wastewater treatment.