Enhancing nitrogen removal performance of anammox granular sludge with ascorbic acid at low temperatures

The application of anammox granular sludge in mainstream wastewater treatment under low-temperature conditions faces several challenges. This research investigates the effects of antioxidant ascorbic acid on the properties of anammox granular sludge subjected to low temperature stress. The results demonstrated that the different concentrations of ascorbic acid significantly improved the nitrogen removal efficiency of anammox granular sludge at low temperature (15 °C). In comparison to the control group without ascorbic acid, the average total nitrogen removal efficiency of anammox reactors increased by 14.97%, 26.35% and 41.03% at 15 °C with 5, 10, and 20 mg/L ascorbic acid, respectively. The strength, stability and mass transfer of granular sludge were promoted by ascorbic acid at 15 °C. Ascorbic acid enhanced the energy metabolism of anammox bacteria (AnAOB) by scavenging intracellular reactive oxygen species (ROS), thereby facilitating the adaptation of AnAOB to low-temperature stress. Notably, the abundance of Candidatus Kuenenia in anammox granular sludge at low temperatures increased significantly. Simultaneously, the hydrophobic amino acids synthesis genes of the bacteria in anammox system were upregulated to modulate the composition of extracellular polymeric substances (EPS) of anammox granular sludge, which enhanced the hydrophobic interactions of granular sludge and reduced the electrostatic barrier on the surface of the sludge. These phenomena facilitated the aggregation of smaller particles into larger ones, restoring the stability of the granular sludge and improving its nitrogen removal capacity in the anammox reactor under low-temperature conditions.