The sewer user charges shall, as a minimum, be reviewed annually and updated to reflect actual costs of operation, maintenance, replacement and financing of the sewage collection and treatment facilities. Any revisions of the users charges shall be based upon actual operation, maintenance, replacement and financing expenses. Revisions due only to changes in expenses and user class shall be made by the City. Revisions involving user’s flow, BOD and/or suspended solids shall be computed by the methods outlined herein.