Caustics are used in petroleum refining to remove hydrogen sulfide from various hydrocarbon streams. It was previously demonstrated that spent sulfidic caustics from two Conoco refineries could be successfully biotreated at the bench scale, resulting in neutralization and removal of active sulfides. Sulfides were completely oxidized to sulfate to Thiobacillus denitrificans. Microbial oxidation of sulfide produced acid, which at least partially neutralized the caustic. Biotreatment of a Conoco spent sulfidic caustic has now been demonstrated at pilot scale (1000 gal or 3875 L). Results were comparable to those obtained at the bench scale. The economics and design of a commercial system to treat 1 gpm (3.8 L/min) of spent caustic are presented.