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The United States Environmental Protection Agency has proposed new categorical pretreatment effluent standards for the Metal Products and Machinery Industry, which are more stringent than current discharge limits in the automotive industry. Therefore, this study was conducted to evaluate metal-sulfide precipitation chemistry as an alternative to metal-hydroxide precipitation chemistry for removing Cu, Ni, Pb, and Zn. There were three aspects of this study: (1) theoretical analysis of both metal–hydroxide and metal–sulfide chemistry; (2) experimental evaluation of commercially available sulfur-containing precipitants using deionized water; and (3) experimental evaluation of the precipitants using wastewater samples from three automotive manufacturing plants (transmission, engine, and assembly plants). The primary findings are: (1) In theory, metal–hydroxide chemistry can achieve the proposed standards when no chelating agents are present. This is not true when as small as 1 mg/L of ethylenediaminetetra-ace...