Any sanitary district organized and created under the laws of the State of Illinois having a population of less than 500,000 and lying wholly or partly within the boundaries of any county which accepts the provisions of Division 5-15, as heretofore or hereafter amended, may contract with such county for sewerage service to or for the benefit of the inhabitants of the sanitary district. Any such contract may provide for the periodic payment to the county of a share of the amounts necessary to pay or provide for the expenses of operation and maintenance of the sewerage system (including insurance) of the county, to pay the principal of and interest on any revenue bonds issued by the county hereunder, and to provide an adequate depreciation fund and to maintain other reserves and sinking funds for the payment of the bonds or the extension or improvement of the sewage facilities of the county.
     Any such contract may be entered into without making a previous appropriation for the expense thereby incurred. Any such contract may be for a term not in excess of 20 years, if the contract is a general obligation of the sanitary district, or for a term not in excess of 40 years, if the obligation under the contract is payable solely from the revenues derived by the sanitary district from its sewerage system.
     If the contract is payable solely from the revenues derived by the sanitary district from its sewerage system, the amounts due under the contract shall be deemed an expense of operating and maintaining the sewerage system of the sanitary district.