Solid-Phase Fe(II)-Mediated Autotrophic Denitrification for Nitrate Removal from Wastewater with a Low Carbon-to-Nitrogen Ratio

In this study, three solid-phase iron sources—FeS, FeCO3, and sponge iron—were used to investigate the autotrophic denitrification efficiency in low-carbon-to-nitrogen (C/N) wastewater. Among these iron sources, FeS obtained the best efficiency, with a nitrate removal of 96% at 120 h under C/N=1 and NO3−-N=40 mg·L−1. Then, batch experiments with FeS as an electron donor were conducted to evaluate the effect of different influent C/N ratios and NO3−-N concentrations. According to the results of the experiments, the nitrate removal efficiency of the mixotrophic system was greater than that of the pure autotrophic system under low-C/N ratios, and the TN (total nitrogen) removal efficiency was also enhanced. In the mixotrophic system, the nitrate removal reached 98% in 48 h under a C/N ratio of 1, and NO3−-N was in the range of 20 mg·L−1 to 40 mg·L−1. Additionally, some accumulation of ammonia was observed in the nitrogen conversion process. According to microbial analysis, the synergistic action of heterotrophic and autotrophic denitrifiers improved nitrate removal.