The city council shall review, not less often than every two (2) years, the contribution of users and user classes, the total cost of operation and maintenance of treatment works, and its user charge system. If necessary, the city council shall revise the charges for users to accomplish the following:
A. Generate Sufficient Revenue: Generate sufficient revenue to pay the total operation and maintenance costs necessary for the proper operation and maintenance, including necessary equipment replacement, of the treatment works.
B. Apply Excess Revenues: Apply excess revenues collected from users to the costs of operation and maintenance. (1973 Code § 13.40.440)