It shall be unlawful for any person to discharge or cause to be discharged into the public sewer system any flammable or explosive liquid, solid or gas, any garbage not properly shredded, any ashes, cinders, sand, mud, oil, grease, straw, shavings, metal, glass, rags, feathers, tar, plastics, wood or any other solid or viscous substance capable of causing obstruction to the flow in sewers or other interference with the proper operation of the sewage treatment plant; provided, that waste fluids containing minute portions of commercial petroleum oils may be discharged into the public sewer system after the installation of a grease trap inspected and approved by the Public Works Supervisor. (Ord. 12, 2-1971)