The document discusses the exclusion of spent caustic solutions from petroleum refining from regulation as a solid waste under the Resource Conservation and Recovery Act (RCRA). Specifically: - Spent caustic solutions are used to treat liquid petroleum streams at petroleum refineries to remove compounds. - The spent caustic solutions are excluded from the definition of solid waste if used as a feedstock to produce naphthenic acid or cresylic acid. - The exclusion does not apply if the material is placed on the land or accumulated speculatively.