Autotrophic denitrification and inhibitory effect caused by the injection of spent sulfidic caustic in a modified Ludzack-Ettinger process

As spent sulfidic caustic (SSC) from petroleum plants contains a high concentration of alkalinity and sulfur compounds, SSC can be applied in sewage treatment system as an electron donor for autotrophic denitrification. In our previous study, the reuse of SSC in the biological nitrogen process was successful, and some neutralization may be required for stable treatment performance. In this study, the pH of SSC was neutralized to 12.0 from 13.3, and the modified Ludzack-Ettinger process was conducted for 90 days with the municipal wastewater. Some toxic effects of SSC on microorganisms were tested via a specific oxygen uptake rate (SOUR) assay. According to the SOUR assay, as compared with no SSC injection condition, SOUR was reduced by approximately 5.4% when 4 mL SSC/L was injected and the effective concentration of a toxicant causing 50% inhibition of the microorganism’s activity (EC₅₀) was 22.6 mL/L. During the days of operation, the COD removal and nitrification efficiency were over 53.0 and 98.2%, respectively. The TN removal efficiency was 56.6% and the nitrogen removal rate (NRR) was 0.15 kg/m³·d when the hydraulic retention time (HRT) in the anoxic tank was 3 h. The ratio of nitrifying bacteria was unaffected by the HRT, and Nitrobacter spp. and Nitrospira genus existed at similar ratios. The ratio of T. denitrificans increased after the injection of SSC and was approximately 6.5%.