Any district organized under the provisions of this act may become a part of a county sanitation district after the board of supervisors of the county within which the district is located, has, after a hearing, pursuant to the County Sanitation District Act, found and determined by resolution duly adopted that such inclusion is for the best interest of the district and the governing body of the district consents thereto by resolution adopted by the affirmative vote of four-fifths of its members.