The user charge system shall generate adequate annual revenues to pay: (i) costs of annual operation and maintenance including replacement; and (ii) costs associated with sewer bond retirement for bonds now outstanding including payments to all sinking, revenue, depreciation, extension and improvement funds established in the ordinances or resolutions authorizing such bonds; and (iii) when required, costs associated with sewer bond retirement of bonds to be issued in the future. That portion of the total sewer use charge which is designated for operation and maintenance including replacement of the treatment works shall be established by this chapter.