Any district owning or operating works may contract with one or more other cities, counties, sanitation districts, or sanitary districts for the use of the works, but only to the extent of their capacity and without impairing their usefulness, upon such terms and conditions as may be fixed and approved by ordinances of the respective contracting entities. Contracts shall not be made for a period of more than fifteen years nor in violation of the provisions of the ordinance authorizing the bonds.