Effluent limitations are enforceable parameters that dictate the amount of pollution a facility may discharge. There are two basic types of effluent limits: technology-based effluent limits and water quality-based effluent limits. Each type may be numeric (e.g., 28 ug/L copper) or narrative (e.g., “no visible oil sheen”).
There are two types of technology-based effluent limits that apply to industrial stormwater discharges: best conventional pollutant control technology (BCT) and best available pollutant control technology (BAT). Facilities must implement BCT to control discharges of the “conventional” pollutants (BOD, TSS, pH, oil and grease and coliform) and BAT to control discharges of toxic and nonconventional pollutants. For process wastewater, EPA selects an available pollutant control technology based on performance and other factors, including affordability on an industry sector-wide basis, and sets a numeric effluent limit based on the performance of that technology. Unlike process wastewater, which is usually a continuous discharge with predictable pollutant concentrations, industrial stormwater runoff is highly variable. This variability prevents EPA from setting numeric technology-based effluent limits that apply across the board for industrial stormwater. Even so, industrial stormwater dischargers must still implement BCT and BAT. Industrial stormwater dischargers achieve BCT/BAT through combinations of BMPs, including, but not limited to, the basic good housekeeping BMPs mandated in their permits. As discussed above, many permits include benchmarks, which are non-enforceable numeric concentrations that facilities and regulators use to help assess whether an industrial site’s BMPs amount to BCT/BAT.