Effect of N/S ratio on anoxic thiosulfate oxidation in a fluidized bed reactor: Experimental and artificial neural network model analysis

Abstract Anoxic thiosulfate (S 2 O 3 2− ) oxidation using autotrophic denitrification by a mixed culture of nitrate reducing, sulfur oxidizing bacteria (NR-SOB) was studied in a fluidized bed reactor (FBR). The long-term performance of the FBR was evaluated for 306 days at three nitrogen-to-sulfur (N/S) molar ratios (0.5, 0.3 and 0.1) and a hydraulic retention time (HRT) of 5 h. S 2 O 3 2− removal efficiencies >99% were obtained at a N/S ratio of 0.5 and a S 2 O 3 2− and nitrate (NO 3 − ) loading rate of 820 (±84) mg S-S 2 O 3 2− L −1 d −1 and 173 (±10) mg N-NO 3 − L −1 d −1 , respectively. The S 2 O 3 2− removal efficiency decreased to 76% and 26% at N/S ratios of 0.3 and 0.1, respectively, and recovered to 80% within 3 days after increasing the N/S ratio from 0.1 back to 0.5. The highest observed half-saturation ( K s ) and inhibition ( K I ) constants of the biofilm-grown NR-SOB obtained from batch cultivations were 172 and 800 mg S-S 2 O 3 2− L −1 , respectively. Thiobacilus denitrificans was the dominant microorganism in the FBR. Artificial neural network modeling successfully predicted S 2 O 3 2− and NO 3 − removal efficiencies and SO 4 2− production in the FBR. Additionally, results from the sensitivity analysis showed that the effluent pH was the most influential parameter affecting the S 2 O 3 2− removal efficiency.