Mechanisms insights into denitrification in activated sludge systems under different carbon source conditions: substrate metabolism, electron transfer, and microbial analysis

Carbon source affects nitrate reduction by acting as electron donor, which is a key factor for heterotrophic denitrification. However, there is limited information on the mechanism of denitrification under different carbon sources. In this study, the production of electrons by carbon metabolism, the transformation of electrons by the electron transport system, and the competition of electrons by denitrification enzymes (NAR, NIR, NOR, and N2OR) were compared in activated sludge systems fed by sodium citrate (CA), sodium acetate (Ac) and glucose (Glu) as carbon sources. It was obvious that the microbial community structures were different in CA, Ac, and Glu supported systems, and Azoarcus (48.2%), Thauera (36.6%), and Saccharimonadales (45.4%) were the dominant genus, respectively. These suggested the type of carbon sources affected microbial species to make carbon and nitrogen metabolism different, resulting in the various pollutants removal characteristics.