Wis. Admin. Code NR § 211.20
The department may require any POTW with a design flow greater than 5 MGD which receives indirect discharges which pass through or interfere with the POTW's operations or are subject to pretreatment standards to establish a POTW pretreatment program in accordance with the provisions of ss. NR 211.21 through 211.26. In addition, the department may require any POTW with a design flow of 5 MGD or less to establish such a program or part of such a program if the nature or volume of industrial user contributions, treatment process upsets, violations of POTW effluent limits, contamination of municipal sludge, or other circumstances warrant such a program in order to prevent interference with POTW treatment operations or the pass-through of untreated pollutants or in order to improve opportunities for disposal of municipal sludge. The pretreatment program shall be developed in accordance with a compliance schedule established by the department in the POTW's WPDES permit which calls for the completion by specific dates of major events leading to the development and implementation of the pretreatment program.
Wis. Admin. Code Department of Natural Resources § NR 211.20