Swimming pool filter backwash should be discharged to the sanitary sewer. Though low in volume, it has a higher concentration of contaminants compared to pool water.
Denton City Code of Ordinances Ch. 26-189 prohibits discharge of pollutants to storm sewer or natural outlet except where suitable treatment has been provided or where a Federal National Pollution Discharge Elimination Systems (NPDES) permit is issued. Fines for violating section 26-189 of the City Code could equal up to $2,000 per violation, per day.