This bill revises requirements for citizen enforcement suits brought under the Clean Water Act. Specifically, the bill limits the amount that may be awarded for the costs of litigation in citizen suits. An award of litigation costs may not exceed (1) an amount that is proportional to the number of successful claims included in the complaint, or (2) the amount of monetary penalties. The award of litigation costs must be based on the prevailing rates in the local market area where the violation occurred for the kind and quality of the services furnished. Citizen suits may not be commenced if the Environmental Protection Agency or a state has commenced and is diligently prosecuting a civil or criminal action through the issuance of a compliance order. The bill provides for certain affirmative defenses for a person who may be liable for the unlawful discharge of a pollutant.