Adaptation of freshwater partial Denitrification/Anammox (PD/A) granules to low-salt conditions: Process performance and metabolic insights

The suppressive influence of salinity on biological wastewater treatment processes is well acknowledged; nevertheless, its ramifications on emerging Partial Denitrification/Anammox (PD/A) processes have received comparatively less attention. This research endeavors to address this disparity by examining the effects of saline exposure on the freshwater PD/A process. The findings revealed freshwater PD/A process can adapt to salt exposure, initially experiencing reduced nitrogen removal performance, followed by a gradual recovery to the initial level. Shorter resilience times at 5 g/L salt compared to 10 g/L suggest a higher tolerance of the PD/A system to low-salt conditions. Additionally, anammox functional bacteria exhibits greater sensitivity to salinity stress compared to denitrification bacteria, potentially leading to reduced proportions of anammox (from 94.2 % on day 70 to 60.9 % on day 77) in overall nitrogen removal during the recovery phase. Increased secretion of extracellular polymeric substances (EPS), particularly proteinaceous substances like tyrosine and tryptophan, is believed to confer resistance to salinity stress. Microbial analysis revealed substantial alterations in bacterial community structures within the PD/A system in response to salinity stress, with notable increases of PD functional microorganism (e.g., Thauera) and anammox bacteria (Candidatus Brocadia). Furthermore, relevant functional genes associated with anammox and denitrification processes showed increased expression levels, indicating the survival of functional bacteria. This likely contributed to the high EPS secretion and energy production, as evidenced by the heightened expression of genes associated with oxidative phosphorylation and the tricarboxylic acid (TCA) cycle. Overall, this study provides valuable insights into how the PD/A process adapts to salt exposure by modulating gene expression, regulating nitrogen and energy metabolism. These findings could help manage the influence of salinity on PD/A process operation, ensuring efficient nitrogen removal in salt-containing wastewater treatment systems.