Ill. Admin. Code tit. 35, § 364.402 – F4 Factor

a) The F4 factor considers the corrective measures necessary to provide the degree of treatment required by applicable effluent limitations. F4 is derived through a summation of values assigned for additional treatment process required for currently overloaded facilities and for rehabilitation.
b) The F4 factor establishes relative values for the following treatment processes:
1. 1) addition of any required secondary or supplemental treatment processes where, at the present time, the required processes are non-existent.
2. 2) addition of capacity for currently (hydraulically and/or organically) overloaded treatment processes.
3. 3) modification of existing processes which do not increase capacity or degree of treatment but are necessary for the upgrading of an existing treatment plant.
4. 4) the construction of new trunk and lateral sewers to provide an extension of service from an existing collection system or the rehabilitation of existing sewers, which will not eliminate excessive infiltration/inflow.
5. 5) the construction of a new collection system and treatment works to service a presently unsewered community where a discharge of raw or partially treated sewage exists, or where no discharge exists.

c) The F4 values are the following:

TABLE OF F4 VALUES
Treatment Process	Average Dry Weather Flow	Maximum Dry Weather Flow	Maximum Wet Weather Flow
Primary	0	0	8
Secondary	14	10	0
Tertiary (10/12)	8	6	0
Disinfection	6	6	6
Phosphorus Removal	5	3	0
Nitrification	8	6	0
Inadequate Sludge Processing Facilities	6
Modification of existing processes which do not increase capacity or degree of treatment	1
or
Wastewater reuse or recycling; Flood control projects; Trunk and Lateral Sewers	1
Range of Values: 0 to 99