Local Limits are established to protect the Publicly Owned Treatment Works (POTW) [Wastewater Treatment Plant] operations and to ensure that its discharges comply with State and Federal requirements, a POTW will design its local limits based on site-specific conditions. Among the factors a POTW should consider in developing local limits are the following: the POTW’s efficiency in treating wastes; its history of compliance with its NPDES permit limits; the condition of the water body that receives its treated effluent; any water quality standards that are applicable to the water body receiving its effluent; the POTW’s retention, use, and disposal of sewage sludge; and worker health and safety concerns. The General Pretreatment Regulations require the following:
- POTWs that are developing pretreatment programs must develop and enforce specific limits on prohibited discharges, or demonstrate that the limits are not necessary [40 CFR 403.8(f)(4)].
- POTWs that have approved pretreatment programs must continue to develop and revise local limits as necessary [40 CFR 403.5(c)(1)].
- POTWs that do not have approved pretreatment programs must develop specific local limits if pollutants from non-domestic sources result in interference or pass through and such occurrence is likely to recur [40 CFR 403.5(c)(2)].
EPA and the States have approved more than 1,400 POTW pretreatment programs. Each program must develop, implement, and enforce technically based local limits. Because most of the POTWs that require pretreatment programs now have them, only a few new programs are approved each year. Work on local limits continues, however, because POTWs with approved programs must periodically review these local limits. EPA regulations require that POTWs with approved programs must “provide a written technical evaluation of the need to revise local limits under 40 CFR 403.5(c)(1), following permit issuance or reissuance” [ 40 CFR 122.44(j)(2)(ii)]. Additionally, EPA recommends that Control Authorities review the adequacy of local limits if current wastewater treatment plant performance fails or will fail to attain applicable NPDES, State, or local permit requirements or other operational objectives, including water quality objectives of receiving waters; and if the performance shortcomings may be reasonably attributed to pass through or interference caused by a POC. Finally, Control Authorities may find it beneficial to reevaluate their local limits when a change in POTW operations results in a significant change in operational objectives; when the POTW experiences a significantly different influent flow or pollutant characteristics; or when a significant alteration of key environmental criteria occurs.