Mixotrophic Denitrification Using Thiocyanate and Phenol As Electron Donors: Kinetics and Microbial Mechanisms

Thiocyanate (SCN-) and phenol, as reducible inorganic and organic toxic compounds, coexist in coal and petrochemical wastewater. However, the kinetics and microbial mechanisms of them as electron donors for mixotrophic denitrification have not been investigated. In this study, thiocyanate autotrophic denitrification (TAD) sludge was used to explore the kinetics and microbial mechanisms of mixotrophic denitrification using phenol and SCN- as electron donors. The batch experimental results showed that TAD sludge can efficiently use SCN- as electron donor for denitrification. Low-concentration phenol to TAD sludge presented no significantly affect SCN- degradation and only slightly inhibited denitrification. Higher 300 mg/L phenol significantly hampered both processes. However, the sludge gradually acclimated to phenol in continuous-flow reactor, achieving stable mixotrophic denitrification. Metagenomic analysis revealed Thiobacillus (sole SCNase-harboring genus) and Ottowia as dominant players. Thiobacillus, responsible for SCN- degradation, had complete sulfur oxidation and denitrification enzyme system. Ottowia, responsible for phenol degradation, also had some sulfur oxidation and denitrification genes. Under the leadership of Thiobacillus and Ottowia, various microorganisms collaborated to utilize SCN- and phenol for mixotrophic denitrification, which result in SCN-, phenol, and NO3- N removed simultaneously. This study provides new insights into the denitrification in toxic wastewater and establishes a metabolic model for it.