The federal Clean Water Act requires industrial plants to obtain National Pollutant Discharge Elimination System (NPDES) permits for discharging wastewater. These permits specify numerical limits on the mass or concentration of contaminants. Historically, as permits are renewed, these limits are made more restrictive. Permit limits can be “technology-based;” i.e., established on the basis of removal technology capability. For the petroleum refining industry and each other industrial category, the U.S. Environmental Protection Agency (U.S. EPA) determines the concentrations or mass of contaminants expected from a facility employing “best available technology economically achievable,” or BAT. U.S. EPA publishes these values as effluent guidelines for use by local permitting authorities.