All industrial users shall promptly notify the Chief, I.P.P., in advance of any substantial changes in the volume or character of pollutants in their discharge, including the listed or characteristic hazardous wastes for which the Industrial User has submitted initial notification under 40 CFR 403.12(p). In no case shall this notification be later than when the changed discharge occurs. Industrial dischargers need only notify the Chief, I.P.P. of "substantial changes" in volume and character. A substantial change would be based on the magnitude of change to the usual discharge and not the anticipated effect of the changed discharge on the POTW.