70 ILCS 2405/23.3

Whenever any unincorporated territory, containing 60 acres or less, is wholly bounded by any sanitary district organized under this Act, that territory may be annexed by that sanitary district by the passage of an ordinance to that effect by the board of trustees of the sanitary district, describing the territory to be annexed. After the passage of such ordinance of annexation a copy of such ordinance, with an accurate map of the territory annexed, certified as correct by the clerk of the board of trustees, shall be filed with the County Clerk of the County in which the annexed territory is situated.