Spent sulfidic caustics (SSCs) produced from petrochemical plants contain a high concentration of hydrogen sulfide and alkalinity, and some organic matter. Most of the SSCs are incinerated with the auxiliary fuel causing secondary pollution problems. The reuse of this waste is becoming increasingly important in terms of economical and environmental viewpoints. To denitrify wastewater with a low COD/N ratio, additional carbon sources are required. Therefore, autotrophic denitrification has received increasing attention. In this research, SSCs were injected as electron donors for sulfur-based autotrophic denitrification in a modified Ludzack-Ettinger (MLE) process. According to the variations in the SSCs dosage, the efficiencies of COD, nitrification and TN removal were evaluated. Heterotrophic denitrification by organic matter and autotrophic denitrification by SSCs were also investigated. As a result, adequate injection of SSCs showed stable autotrophic denitrification. To investigate some of the harmful effects of SSCs, fluorescence in situ hybridization (FISH) for nitrifying bacteria and Thiobacillus denitrificans was performed. Ammoniaoxidizing bacteria (AOB) and Nitrospira genus showed a similar pattern. Excessive injection of SSCs made nitrifying bacteria decrease and nitrification failure occur because of the high pH caused by the SSCs. The distribution of T. denitrificans was relatively uniform as SSCs were injected. This result means that T. denitrificans are available at high pH.