In-situ treatment of an end of rivers by autotrophic/ heterotrophic nitrification and aerobic denitrification (HN-AD) membrane aerated biofilm reactors (MABRs)

This is the first to investigate the in-situ treatment of an end of rivers by autotrophic/ heterotrophic nitrification and aerobic denitrification MABRs. After treatment, the removal efficiency of NH4 + -N and total phosphorus (TP) was 97.1% and 76.4% respectively. NH4 + -N, total nitrogen (TN), and TP could respectively reach 0.12 mg/L, 2.94 mg/L, and 0.05 mg/L in December (temperature 1 ℃−10 ℃, total rainfall 1.4 mm, salinity 17,420 mg/L-27,500 mg/L). The extracellular polymers (EPSs) secreted by microorganisms may protect the MABR biofilms at low temperatures and high salinity conditions. During the rainy season (temperature 19 ℃−29 ℃, rainfall 10.4–194.4 mm, June to September), downstream NH4 + -N and COD can reach 0.34 mg/L and 8 mg/L, respectively with upstream NH4 + -N 1.57 mg/L and COD 21 mg/L. And the MABR modules were fixed at 2–3 m depth, which may mitigate rainwater impact on MABR biofilms. Additionally, the microbial population structures in the MABR biofilms and the sediment surfaces of the river were analyzed based on 16 S rRNA identification technology. It was found that Acinetobacter (0.05%−2.18%), Paracoccus (0.05%−0.75%), Pseudomonas (0.08%−0.22%), and Thiobacillus (0.04%−1.21%) collaboratively contributed to nitrogen removal from the river through different types of denitrification processes in this study. Among the denitrification processes, aerobic denitrification was dominant in the MABR biofilms, while autotrophic denitrifying desulphurization (ADD) and HN-AD were dominant in the river sediment surfaces. The study is of great significance for river water purification, aquatic life protection, and coastal ecosystem security.