The WSSC shall annually determine the amount necessary to pay the principal and interest requirements of the bonds, and shall annually fix the water service charge of the sanitary district at such a sum as to produce, in addition to the costs of the service, and the requirements of any other bonds issued and outstanding the annual requirements of which are to be paid out of the water service, the amount determined as necessary to pay the annual requirements of the bonds hereby authorized. The sum so collected annually from water service charges shall be deducted from the amount which the WSSC has determined to be necessary to be raised by direct taxation upon certification to the County Councils of the counties.