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 and to maintain the equitability of the user charges with respect to proportional distribution of the costs of operation and maintenance in proportion to each user's contribution to the total wastewater loading of the treatment works. Any revisions of the user charges shall be based on actual operation, maintenance, replacement, and financing expenses, and/or on significant changes in the total number of equivalent users, the total daily flow, the total daily suspended BOD or solids. The city may install flow measuring devices and/or collect wastewater samples at any time in any user's service line to determine actual usage as a basis for revision of the user's charge. 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 normally only be made upon the recommendation of a registered professional engineer. All changes in user charges applicable to this chapter shall be computed by the methods outlined herein. (Ord. 284, 1980)