If the volume or character of the waste to be treated does not cause overloading of the sewage collection, transmission, or treatment facilities, then the person making the discharge pays a user charge to be determined from the schedule of charges. The user charges assessed to the discharger pursuant to this section shall include but not be limited to the user's proportionate share of the following:
A.
Amortization of all applicable capital outlays and the proportionate part of the value of the existing wastewater system used in handling and treating the waste;
B.
Operation and maintenance costs, including salaries and wages, power costs, costs of chemicals and supplies, proper allowances for maintenance, depreciation, overhead and office expenses. If the volume or character of the waste to be treated required that wastewater collection, treatment or other disposal facilities be improved, expanded or enlarged in order to treat the waste, then prior to issuance of a wastewater discharge permit, the Mayor and Council and the person making the discharge shall enter into an agreement which provides that the discharger pay in full all added costs that may be incurred due to acceptance of the waste.