All sums collected by the WSSC for benefits levied against property for water supply and sewerage construction, as provided in § 5-101 of this article, shall be set aside as a separate fund to be known and designated as the "Current Bond Fund", from which fund interest shall be paid on all outstanding bonds, and the balance of the Fund shall be prorated monthly, and applied to the payment of the principal of maturing serial bonds and the payment into the joint "sinking fund" account, as provided under § 4-105 of this subtitle, of the proportionate part of the principal of outstanding sinking fund bonds as the outstanding parvalue of both types of bonds bear to each other. The WSSC, in order to determine the amount necessary to be levied under § 4-105 of this subtitle, shall deduct the amount to its credit in the "Current Bond Fund" account from the whole amount necessary to be raised in any 1 year for interest on all of its outstanding bonds, the payment of the principal of its maturing serial bonds and the proportionate part of principal of all outstanding sinking fund bonds, and the balance remaining to be raised shall be the amount to be certified to the County Councils of Montgomery and Prince George's counties for collection by taxation as provided under § 4-105 of this subtitle.