In event such contract, as provided in section 31-551, is entered into and in event all activities, liabilities, obligations and assets then possessed by said sanitary district are assigned and transferred to said natural resources district so that there are no further functions, activities or liabilities of said sanitary district existing then and in that event said sanitary district may be dissolved by a three-fourths vote of its board of trustees in which event a certified copy of said resolution of dissolution shall be filed in the office of the county clerk of the county in which said sanitary district is organized.