The Administrator shall, not later than 15 months after November 8, 1984, submit a report to the Congress concerning those substances identified or listed under section
6921 of this title which are not regulated under this subchapter by reason of the exclusion for mixtures of domestic sewage and other wastes that pass through a sewer system to a publicly owned treatment works. Such report shall include the types, size and number of generators which dispose of such substances in this manner, the types and quantities disposed of in this manner, and the identification of significant generators, wastes, and waste constituents not regulated under existing Federal law or regulated in a manner sufficient to protect human health and the environment.