The user charge article shall be reviewed not less than every two years regarding the wastewater contribution of users and user classes, the total costs of the operation and maintenance of the treatment works, and its approved user charge system. The charges for users or user classes shall be revised to accomplish the following:
1. Maintain the proportionate distribution of operation and maintenance costs among users and user classes;
2. Generate sufficient revenue to pay the total operation and maintenance costs necessary to the proper operation (including replacement) of the treatment works; and
3. Apply excess revenues collected from a class of users to the costs of operation and maintenance attributable to that class for the next year, and the rates shall be adjusted accordingly.