An evaluation of the performance of three different combined sewer overflow treatment units is presented, with influent and effluent flow, BOD, and suspended solids data for a single-cell flow-through lagoon; a retention lagoon followed by a two-cell facultative lagoon system and effluent chlorination; and a sedimentation tank with influent chlorination. The extent of preconstruction pollution from combined sewer overflow is discussed as well as the success of partial sewer separation in reducing the strength of combined sewer overflow. The performance of the two-cell facultative lagoon as a tertiary treatment system for the existing sewage treatment plant effluent is also analyzed in detail, demonstrating a dual use for this type of combined sewer overflow treatment unit.