In this work, successful treatment of refinery spent-sulfidic caustic was achieved in activated sludge system bioaugmented with three specific bacterial strains (including Bacillus thuringiensis, Bacillus cereus, and Acidovorax ebreus) at both laboratory and pilot scale. After 18-fold dilution by volume, the diluted wastewater was treated aerobically. The volumetric loading rate of chemical oxygen demand (COD) was studied by treating variously diluted samples of high-strength wastewater (COD 3,100–3,600 mg/L). The results obtained with the raw wastewater were remarkably good, with an average COD removal of around 80%. The addition of peptone and trace elements improved the performance of the process. The mixed liquor suspended solids (MLSS) concentration in the reactors could be increased from the 2,500–3,000 mg/L MLSS level of general activated sludge processes to 6,000–8,000 mg/L after installation of draft tubes. These results showed that activated sludge process may be a powerful tool for treating refinery spent caustic by bioaugmentation and nutrient amendment.