Effect of High Salinity on Elemental Sulfur Autotrophic Partial Denitrification and Feasibility Evaluation of an Enhanced Strategy with Glycine Betaine Addition: Performance, Microorganisms, and Mechanisms

Elemental sulfur autotrophic partial denitrification (S0-PDN) can supply nitrite for anaerobic ammonium oxidation without an organic carbon requirement. This study aimed to evaluate the impact of high salinity on S0-PDN and investigate the feasibility of an enhanced strategy involving the addition of glycine betaine (GB). The results showed that high salinity inhibited S0-PDN by over 60% as salinity increased from low (0 and 5 g/L) to high (10 and 15 g/L) levels, even though Thiobacillus (65.9%–84.0%) was always the dominant genus. The reason might be the suppression of high salinity on the bioactivity. Surprisingly, the addition of GB (1 mmol/L) effectively enhanced the performance of S0-PDN under high salinity conditions (10 g/L), resulting in a 5.1-fold increase in the NO3–-N conversion efficiency and a 3.1-fold increase in the effluent NO2–-N concentration. Metagenomics analysis revealed that high salinity did not alter the dominant microbial composition. GB facilitated the recovery of the nitrate conversion capacity of sulfur autotrophic denitrifying bacteria with the up-regulation of nitrate reductase genes (by 18.1%) and Sox genes (by 21.4%). However, GB also stimulated the growth of heterotrophic denitrifying bacteria. Part of the nitrite produced in S0-PDN was reduced through heterotrophic denitrification with GB as an electron donor. Overall, the addition of GB was demonstrated as an effective approach to mitigate the inhibitory effects of high salinity on S0-PDN, which offers valuable theoretical support for its application in high-salinity wastewater treatment.