A citizen suit may not be commenced if the EPA or the State is already diligently prosecuting a civil action in a court to enforce compliance.  A citizen suit for penalties may not be commenced if the EPA or the State (under a State law comparable to the Clean Water Act) has commenced the issuuance of a penalty order for such violation prior to the citizen notice of violation or citizen suit.

In a citizen suit, the court may grant an injunction, impose penalties, and award attorney fees and litigation costs to any prevailing or substantially prevailing party whenever the court determines such an award is appropriate.  Attorney fees will not be assessed against a citizen unless the court determines the suit is frivolous.
Clean Water Act
Clean Water Act Outline

Summary of the Clean Water Act

Clean Water Act:  A Summary of the Law
(CRS Report for Congress 2014)

Clean Water Act Course
(River Network)

A Citizens Guide to Clean Water Act Enforcement
(Ohio Environmental Council)
The Clean Water Act authorizes citizens to enforce compliance with effluent standards or limitations and orders issued by the EPA Administrator or a State.  33 U.S.C. ยง 1365.  Citizens must be adversely affected by the violation and normally must give 60-days notice of the alleged violation to the alleged violator, State and EPA prior to filing suit.   40 C.F.R. Part 135, Subpart A.  This notice and delay period are intended to allow the violator an opportunity to correct his violation and to give the EPA or State an opportunity to enforce compliance, thus making citizen enforcement unnecessary.