Impact of tetracycline on mixotrophic denitrification process under different sulfur to nitrogen ratios

The sulfur-based autotrophic-heterotrophic denitrification, i.e., mixotrophic denitrification, is suitable for the nitrate and antibiotics removal in aquaculture tailwater at a low COD to nitrogen (C/N) ratio. This study focused on the effect of tetracycline (TC) on mixotrophic denitrification under different S/N ratios. Two bioreactors were simultaneously operated with or without dosing tetracycline under different sulfur to nitrogen (S/N) ratios of 3.94, 4.64 and 5.94. The results showed that the removal rate of total inorganic nitrogen (TIN) increased from 0.25 to 0.69 mg N L⁻¹ min⁻¹ with the rise of S/N ratio, while TC dosage significantly declined the removal efficiency of TIN. Dissimilatory nitrogen reduction to ammonia (DNRA) bacteria was detected when exposing to TC, indicating that DNRA presented more resistance to TC. The removal efficiency of TC in the denitrification system reached the maximum of 22.87 % at S/N of 4.64. Meanwhile the genus Marinicella was detected at this phase, which was conducive to the degradation of organic pollutants. This study found that TC promoted the accumulation of ammonia nitrogen, and had a great effect on sulfur autotrophic bacteria at S/N of 5.94. The removal of TC mainly depended on microbial co-metabolism, and there was a significant correlation between the reduction of TC concentration and the decrease of sulfur compounds (p < 0.05). 4.64 is the best S/N ratio for the mixotrophic denitrification process, which revealed maximum nitrate and TC removal rates.