Ill. Admin. Code tit. 35, § 364.303 – Population Factor

a) Under this factor, populations are computed for each segment and converted into an adjustment factor. The population of a segment is defined by the 1980 U.S. census population of the segment except where inter-segment transfers of sewage occur, in which case the estimated population whose sewage is tributary to the segment is added to the resident population of the segment. Population figures of the incorporated and unincorporated communities are recorded for each segment. Where a segment boundary splits a township, the unincorporated population within the segment is prorated by applying a percentage of the township population equal to the ratio of the segment area in the township to the total township area. The populations of the various political subdivisions within each segment are totaled.
b) The following population factors are applied as multipliers to the water quality index values:
Segment Population Population Factor
0 - 20,000 1.00
20,001 - 50,000 1.05
50,001 - 100,000 1.10
OVER 100,000 1.20