A critical part of a municipality's task in developing or implementing a local pretreatment program is the development of technically based numerical effluent limitations (local limits) on the discharge of incompatible pollutants to a POTW. These limitations are often incorporated directly into a municipal ordinance or are applied through individual control mechanisms, such as permits, orders, etc., issued to nondomestic users of the POTW. Such limits are needed to enforce the prohibited discharge standards of the General Pretreatment Regulations and to implement three fundamental objectives of the National Pretreatment Program: to prevent the introduction of pollutants into the POTW which could interfere with its operation; to prevent pass-through of untreated pollutants which could violate applicable water quality standards or National Pollutant Discharge Elimination System (NPDES) effluent limitations; to prevent the contamination of POTW sludge which would limit the selected sludge uses or disposal practices. Local limits supplement National categorical standards, which are Federal technology-based limitations, imposed on all facilities in selected industrial categories. Local limits are necessary in cases where an industry is not covered by categorical standards, or where categorical standards are not adequate to protect the POTW or receiving stream or to prevent undue contamination of the sludge.