70 ILCS 2805/32a.4
Any sanitary district may annex any territory which is not within the corporate limits of the sanitary district but which is contiguous to it and is served by the sanitary district or by a municipality with sanitary sewers that are connected and served by the sanitary district or by any other sewer system that is connected to and served by the sanitary district by the passage of an ordinance to that effect by the board of trustees, describing the territory to be annexed. A copy of the ordinance with an accurate map of the annexed territory, certified as correct by the clerk of the district shall be filed with the county clerk of the county in which the annexed territory is located. For purposes of this Act, a property is served by a sanitary district if a sewer that is part of the sanitary district's sewer system, part of the sewer system of a municipality that is connected to the sanitary district, or part of any other sewer system that connects to and is served by the sanitary district has been extended to, across, or along the property, whether or not the buildings on the property are physically connected to the sewer.
70 ILCS 2805/32a.4