40 CFR § 122.5 - Effect of a permit.
(a) Applicable to State programs, see § 123.25.
(1) Except for any toxic effluent standards and prohibitions imposed under section 307 of the CWA and “ standards for sewage sludge use or disposal” under 405(d) of the CWA, compliance with a permit during its term constitutes compliance, for purposes of enforcement, with sections 301, 302, 306, 307, 318, 403, and 405 (a)-(b) of CWA. However, a permit may be modified, revoked and reissued, or terminated during its term for cause as set forth in §§ 122.62 and 122.64.
(2) Compliance with a permit condition which implements a particular “standard for sewage sludge use or disposal” shall be an affirmative defense in any enforcement action brought for a violation of that “standard for sewage sludge use or disposal” pursuant to sections 405(e) and 309 of the CWA.