The spill occurred near the southeast corner of the plant between 8:10 p.m. and 9:40 p.m. Sunday night, the release states.
Public drinking water has not been affected, according to the city. However, fish have reportedly died in the creek, and a cleanup effort is underway.
The Brushy Creek Regional Wastewater Treatment Plant is located south of U.S. 79 and west of Red Bud Lane.
For individuals who use private drinking wells located within a half-mile of the spill site, the city advises using water that has been distilled or boiled at a rolling boil for at least one minute. This includes water used for drinking, cooking, bathing and tooth brushing, according to the release.