Selective Enrichment of Partial Denitrifying Bacteria Enabling High Nitrite Production under Stepwise Elevated Salinity: Sludge Granulation and Metagenomic Mechanisms

The innovative partial denitrification/anammox (PD/A) process holds significant promise for treating industrial wastewater containing nitrate, yet it may encounter the challenge of high salinity’s inhibitory impact. In this study, a nitrite-producing PD process, crucial for stable and efficient anammox, was developed. The effect of salt on performance was thoroughly assessed, and metabolic mechanisms were dissected. Results showed that increasing salinity to 20 g/L enhanced PD performance, raising the nitrate-to-nitrite transformation ratio from 63.1 to 70.1% and the nitrite production rate from 27.9 to 39.1 mg of N/g VSS/h. Salinity also promoted sludge granulation by stimulating extracellular polymeric substance (EPS) secretion to resist stress. Microbial analysis revealed the enrichment of PD functional bacterium (Thauera phenylacetica). Gene expression analysis showed upregulation of narGHI (encoding NO3–-N reductase) and downregulation of nirS (encoding NO2–-N reductase), which contributed to improved PD performance. Salinity stress had a limited impact on carbon metabolism and electron transport processes. The secretion of EPS acted as a compensatory mechanism, alleviating the inhibitory effects of salinity on these processes and providing microorganisms with an extended period for self-repair. Overall, these findings offer new insights into the salt-effect PD process, showing the potential for its application in saline wastewater treatment.