The city shall, at least annually, adopt an adequate schedule of sewer service charges to defray the cost of operating and maintaining the sanitary sewerage system. The costs to be used as a basis of determining charges shall include, but is not necessarily limited to direct operation and maintenance, administration, collection and billing of charges, bond redemption, studies and reports, professional fees, repairs, capital improvements and depreciation. The sewer service charges adopted shall be such that each user pays at least their proportionate share of all cost herein noted.
(`95 Code, § 4-2-61)