Spent sulfidic caustic (SSC), produced from petroleum plants, contains high levels of H2S and alkalinity. It can be used to denitrify nitrate-nitrogen via a biological nitrogen removal process, as both the electron donor and buffering agent for sulfur-based autotrophic denitrification. However, SSC also contains some recalcitrant organic compounds such as BTEX, so it has to be refined. To remove BTEX, air stripping was conducted in a laboratory scale, and as a result, over 93% of the BTEX were removed within 30min. For the reformation of the refined SSC, Na2S2O3·5H2O, methanol and organic material, produced from a biodiesel production plant, were supplemented, and referred to as new sulfidic caustic I (NSCI), II (NSCII), III (NSCIII), respectively. Thereafter, these products were applied to a modified Ludzack-Ettinger (MLE) process to evaluate their effects on the effluent COD and TN concentrations. As a result, there was no increase in the COD level on the injection of NSC due to the removal of BTEX via air stripping. In addition, compared to no NSC injection, 44.0% more TN was removed with an injection of NSC III, which were the most effective conditions. Thus, the application of NSC to the biological nitrogen removal process was successfully performed. These results may contribute to the development of resource recovery technology.