The Nitrogen Removal Performance and Functional Bacteria in Heterotrophic Denitrification and Mixotrophic Denitrification Process

The heterotrophic and autotrophic synergistic denitrification (HAD) system can effectively remove sulfide, nitrate, and organic carbon pollutants from municipal wastewater. However, the effect of sulfide on the functional bacteria in the denitrification system is still unclear. To better understand the mechanism of sulfide affected on bacteria in the system, the up-flow anaerobic sludge blanket (UASB) reactor was operated continuously under heterotrophic (no sulfide added) and mixotrophic conditions (with increased sulfide contents) for 120 days. The contents of protein (PN) in extracellular polymeric substances (EPS) were significantly increased with the addition of sulfide, which enhanced the flocculation of sludge and was beneficial to the formation of high-density microorganism communities. The dominant bacteria showed large differences under different nutrient conditions. The abundances of Thauera increased from 4.13% to over 12.94%, and that of Dechloromonas and Thiobacillus were 2.61–3.01% and 1.04–2.66% respectively after added sulfide. And the efficient performance of the system in mixotrophic conditions was accomplished with the interaction of heterotrophic sulfide-oxidizing, nitrate-reducing bacteria (Thauera, Dechloromonas), autotrophic sulfide-oxidizing, nitrate-reducing bacteria (Thiobacillus) and heterotrophic nitrate-reducing bacteria (Rubrivivax, Acidovorax, Simplicispira, Alicycliphilus). Moreover, the abundances of Nar G, Nap A, Nir S, Nor B, and Nos Z were significantly enhanced in mixotrophic conditions, indicating that the nitrogen metabolism potential of the system was also improved after added sulfide. These results elucidated the reasons for the enhanced denitrifying capacity of the system by adding S2− from the microbiological point of view and provided a theoretical basis for the establishment of an efficient denitrification system.