40 CFR § 35.2120 - Infiltration/Inflow.
(a) General. The applicant shall demonstrate to the Regional Administrator's satisfaction that each sewer system discharging into the proposed treatment works project is not or will not be subject to excessive infiltration/inflow. For combined sewers, inflow is not considered excessive in any event.
(b) Inflow. If the rainfall induced peak inflow rate results or will result in chronic operational problems during storm events, or the rainfall-induced total flow rate exceeds 275 gpcd during storm events, the applicant shall perform a study of the sewer system to determine the quantity of excessive inflow and to propose a rehabilitation program to eliminate the excessive inflow. All cases in which facilities are planned for the specific storage and/or treatment of inflow shall be subject to a cost-effectiveness analysis.
(1) If the flow rate at the existing treatment facility is 120 gallons per capita per day or less during periods of high groundwater, the applicant shall build the project including sufficient capacity to transport and treat any existing infiltration. However, if the applicant believes any specific portion of its sewer system is subject to excessive infiltration, the applicant may confirm its belief in a cost-effectiveness analysis and propose a sewer rehabilitation program to eliminate that specific excessive infiltration.
(ii) If the flow rate is not significantly more than 120 gallons per capita per day, request the Regional Administrator to determine that he may proceed without further study, in which case the allowable project cost will be limited to the cost of a project with a capacity of 120 gallons per capita per day under appendix A.G.2.a.