Ideally, the basis of a local limit is the establishment of individual percent removal efficiencies through monitoring of influent and effluent quality at the WWTP. The allowable headworks loading (AHL) is calculated using the pollutant loading from either NPDES limits or derived from the most restrictive water quality criteria in R323.1057 (Rule 57), divided by the fraction of the pollutant not removed by the WWTP. However, some pollutants may not be detected in the influent and others may not be sampled and analyzed. When monitoring does not identify WWTP removal efficiencies, the values in Appendix R of EPA's Local Limits Development Guidance, July 2004, may be used for specific treatment processes to identify the fraction of the pollutant not removed by the WWTP. Finally, dilution values with no credit for treatment may be utilized to determine the "removal" efficiency from the actual flows at the WWTP. Local limits are derived from the most restrictive AHL and take into account measured background loadings of pollutants (domestic, commercial, and non-SIU wastes), loadings from hauled wastes, and a safety factor. A minimum 10% safety factor should be applied to the allowable headworks loading for toxic pollutants.
There are other media considerations such as biosolids land application standards when developing maximum allowable headworks loading (MAHL) to account for all environmental standards. The most restrictive of these will the basis for the local limit. This can be different for each pollutant.