General obligation bonds, within the meaning of sections 247.230 to 247.670, shall be bonds issued within the limitations of indebtedness prescribed under Section 26 of Article VI of the Constitution of Missouri for payment of which, both principal and interest, a direct tax may be levied upon all taxable tangible property within the district. Before or at the time of issuing general obligation bonds, the board shall provide for the collection of an annual tax to be levied upon all taxable tangible property within the district sufficient to pay the interest and principal of the indebtedness as they fall due and to retire the same within twenty years from the date contracted. The net income and revenue arising from the operation of the water supply system of such district, after providing for costs of operation, maintenance, depreciation and necessary extensions and enlargements, shall be transferred to and become a part of the interest and sinking fund applicable to such general obligation bonds, unless or until such net revenues are pledged to the payment of special obligation bonds as herein provided.