(a) All existing SPDES permittees that are not POTWs must notify the regional water engineer as soon as they know or have reason to believe that any activity has occurred or will occur that would result in the discharge of any pollutant that is not a discharge authorized by a SPDES permit as defined in section 750-1.2 of this Part.

(b) Facility expansion, as defined in section 750-1.2 of this Part, for all existing SPDES permittees that are not POTWs must be reported by submission of a letter to the regional water engineer. The department may determine that additional information must be submitted or that the information submitted by letter to the regional permit administrator must be submitted on a department application form. The department may determine, on the basis of such information, and any related investigation, inspection or sampling, that a modification of the permit is necessary to assure maintenance of water quality standards or compliance with other provisions of ECL, article 17 or the Clean Water Act. Conversely, the department may determine in accordance with this Part that the proposed activity does not require a permit modification. Unless the department determines that a permit modification is unnecessary, operations that fit the following criteria, which may result in discharges that are not discharges authorized by the SPDES permit, are prohibited until the permit has been modified in accordance with Part 621 of this Title:

(c) The permittee shall submit written notice to the department if the permitted facility experiences a decrease in production, a decrease of process flow, or a facility modification, where such change results in a greater than 20 percent decrease in the discharges of a pollutant explicitly limited in a SPDES permit and the limit was based on production or flow, provided that such decrease in discharge is expected to continue or has been existing for more than one year.