Mixotrophic denitrification process driven by lime sulfur and butanediol: Denitrification performance and metagenomic analysis.

Heterotrophic sulfur-based autotrophic denitrification is a promising biological denitrification technology for low COD/TN (C/N) wastewater due to its high efficiency and low cost. Compared to the conventional autotrophic denitrification process driven by elemental sulfur, the presence of polysulfide in the system can promote high-speed nitrogen removal. However, autotrophic denitrification mediated by polysulfide has not been reported. This study investigated the denitrification performance and microbial metabolic mechanism of heterotrophic denitrification, sulfur-based autotrophic denitrification, and mixotrophic denitrification using lime sulfur and butanediol as electron donors. When the influent C/N was 1, the total nitrogen removal efficiency of the mixotrophic denitrification process was 1.67 and 1.14 times higher than that of the heterotrophic and sulfur-based autotrophic denitrification processes, respectively. Microbial community alpha diversity and principal component analysis indicated different electron donors lead to different evolutionary directions in microbial communities. Metagenomic analysis showed the enriched denitrifying bacteria (Thauera, Pseudomonas, and Pseudoxanthomonas), dissimilatory nitrate reduction to ammonia bacteria (Hydrogenophaga), and sulfur oxidizing bacteria (Thiobacillus) can stably support nitrate reduction. Analysis of metabolic pathways revealed that complete denitrification, dissimilatory nitrate reduction to ammonia, and sulfur disproportionation are the main pathways of the N and S cycle. This study demonstrates the feasibility of a mixotrophic denitrification process driven by a combination of lime sulfur and butanediol as a cost-effective solution for treating nitrogen pollution in low C/N wastewater and elucidates the N and S metabolic pathways involved.