53 Pa. Stat. § 14491
Any city of the first class may, in constructing any sewer or sewer system for the accommodation of a certain portion only of the city, constitute the territory into a sewer district or divide it into several sewer districts. In every such case of division into several districts, the city shall make an estimate of the proportion of the cost of the sewer or sewer system which should be equitably charged on each such district, and declare and establish such apportionment by resolution. No district shall be charged with more than its due proportion of the cost of the main sewers used jointly by more than one district, and the aggregate amount charged on property in any such district shall not exceed the estimate. Any amount not legally chargeable upon properties benefited shall be paid by the city.
53 P.S. § 14491