Special revenue obligation bonds, within the meaning of sections 247.230 to 247.670, shall be bonds payable, both as to principal and interest, wholly and only out of the net income and revenues arising from the operation of the water supply system of the district, after providing for costs of operation, maintenance, depreciation and necessary extensions and enlargements, and such bonds shall not be deemed to be indebtedness of the district within the meaning of any constitutional or statutory limitation upon the incurring of indebtedness. Before or at the time of issuing any special revenue obligation bonds, the board shall pledge such net income and revenues to the payment of such bonds, both principal and interest, and shall covenant to fix, maintain and collect rates for water and water service supplied by such district so as to assure that such net income and revenues will be sufficient for the purpose herein required. The board may make such other covenants as may be appropriate for the purpose of protecting and safeguarding the revenues of the district and the payment of such special revenue obligation bonds.