Biochar enhances nitrogen removal capacity in sulfur-driven autotrophic denitrification at low temperatures:...

Sulfur-driven autotrophic denitrification (SAD) is a low carbon-footprint wastewater treatment process, but its stable operation is hindered by low temperatures. This study found that biochar amendment enhanced the denitrification performance in the SAD process at low temperatures: at 15 °C, the biochar-amended reactor achieved 94.57% nitrogen removal efficiency (NRE); at 10 °C, the control reactor lost denitrification capacity, while the biochar reactor maintained 28.39% NRE. Mechanistic investigations revealed that biochar enhances microbial energy utilization and bioelectrochemical capacity, widens the ecological niche of sulfur-oxidizing bacteria, and enriches Thiobacillus at low temperatures. Metabolic pathway reconstruction reveals that Thiobacillus contains more genes related to nitrogen (nar, nir, nor, nos, nap, nrf, gln, glt, and gdh) and sulfur (dsr, sox, fcc, asr, soe, sat, apr) cycles. Therefore, Thiobacillus is more potent in nitrogen removal and sulfur utilization. However, Sulfurimonas possesses only one pathway for denitrification and sulfur oxidation mediated by sox enzymes. In carbon metabolism, Sulfurimonas contains genes for Calvin-Benson cycle, resulting in its potential for carbon fixation and low-temperature adaptability. Overall, this study proposes a low-carbon strategy to enhance denitrification performance at low temperatures.