Start-up mechanism of simultaneous nitrification-endogenous denitrification process for treatment of low C/N wastewater: Insights into reactor performance and microbial community dynamics

The aim of this study is to propose a start-up strategy by reducing DO concentration, optimizing the duration of the operation mode and gradually decreasing influent C/N for simultaneous nitrification-endogenous denitrification (SNED) process to economically and effectively treat low carbon/nitrogen (C/N) wastewater, and to explore the start-up mechanism from the microbiome. After start-up, the efficiency of simultaneous nitrification-denitrification, nitrogen loading rate, and nitrogen removing rate increased from 8.3%, 60 mg N/L⋅d, and 55.2 mg N/L⋅d to 60.9%, 160 mg N/L⋅d, and 143.6 mg N/L⋅d, respectively. Meanwhile, the total nitrogen removal efficiency and the conversion of intracellular carbon were maintained at 90.2% and 89.1%. Correspondingly, microbial community diversity was significantly inhibited, however, the key functional microbes remarkably increased such as Defluviicoccus (denitrifying glycogen accumulating organisms), Paracoccus (heterotrophic nitrification and aerobic denitrification bacteria), etc. The reduction of heterotrophs led to the streamlining of network scale and complexity. But, the emergence of more positive interaction in functional subnetworks, stable module numbers, and the redundancy of keystone taxa contributes to the targeted regulation of functional microorganisms, as further confirmed by increased community function. Deterministic process dominated the community assembly during start-up period, where homogenizing selection is the main driving force for key functional microbes, e.g., Defluviicoccus and Paracoccus. This study provided key information for practical application of SNED process and laid the foundation for cost-effective treatment of low C/N wastewater.