The cost of constructing all sewers, including the erection of all necessary disposal plants, in and for any district where such sewers were constructed by assessment against the property of the district shall be paid for wholly in special tax bills against the lands embraced within the district, exclusive of public highways, streets and alleys, in proportion to the area of each tract. The boundaries of no sewer district that has been paid for wholly by special tax bills shall be altered or changed after any sewer has been constructed in and for the same.