No person shall discharge or cause to be discharged into the sanitary sewage 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 a 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 plumbing inspector. (Ord. 1150 § 17, 1954)