The owner of any sewerage system or sewage disposal plant, provided for in sections 18-501 to 18-507, or the city or village in which such system or plant is located, is authorized to extend such system or plant beyond the corporate limits of the city or village which it serves, under the same conditions as nearly as may be as within the corporate limits of such city or village and to charge to users of its services reasonable and fair rates consistent with those charged or which might be charged within such corporate limits and consistent with the expense of extending and maintaining such system or plant for the users thereof outside such corporate limits at a fair return to the owner thereof. The mayor and city council of any city or the board of trustees of any village shall have authority to enter into contracts with users of such sewerage system or sewage disposal plant, except that no contract shall provide for furnishing of such service for a period in excess of twenty years.