Dual-layer elemental sulfur-packed denitrification reactor to control sulfate generation and sulfide discharge by two-point inlet and internal recirculation

In this study, an elemental sulfur (S⁰) autotrophic denitrification reactor (SADR) and a wood chunk and S⁰ mixotrophic denitrification reactor (WSMDR) were constructed with dual-layers to effectively remove nitrate from water using two-inlets and internal recirculation. The denitrification rates were 66-114 and 70-104 g-N/(m³·d) for the SADR and WSMDR, respectively. Sulfate production was 5.5-5.9 and 3.2-4.5 mg SO₄^2-/mg reduced N in the SADR and WSMDR, respectively, being lower than theoretical value. In addition, there was no sulfide emission from either reactor. Chlorobium, Chlorobaculum, Ignavibacterium, Sulfuritalea, and Thiobacillus were involved in nitrate reduction in both reactors. Chlorobiaceae had the highest abundance and played an essential role in maintaining the integrity of the co-occurrence pattern. The abundance of functional genes positively correlated with the denitrification performance. This study demonstrates that the operation of two-inlets and internal recirculation can effectively reduce byproduct generation, thereby promoting the practical application of the SADR and WSMDR.