The Orange County Sanitation District (District) employs chemically enhanced primary treatment (CEPT) at its Water Reclamation Plant No. 1 located in Fountain Valley, California. The plant’s 10 rectangular primary sedimentation tanks (PSTs) were the focus of this study. CEPT involves the use of chemicals to facilitate the removal of colloidal and suspended solids (SS) by sedimentation. Ferric chloride (FeCl3) and polymer are injected into the influent stream before entering the PSTs. There is no formal flocculation zone in the PSTs. A test program was devised using in-situ sampling and flocculation tests to determine the strengths and weaknesses of the existing system. The results from the special testing show that the PSTs are achieving very close to the maximum TSS removal at the current chemical dosage. Inefficiencies are due to incomplete flocculation rather than the loss of floc into the effluent. Both the profiling and kinetics study showed that the addition of chemicals significantly improves the rate and extent of the flocculation reaction and reduces the nonsettleable TSS concentration (and the BOD associated with these solids) and slightly reduces the soluble BOD concentration. The profile tests show that some breakup of flocs was observed at the inlet of the PST. Some minor improvements, such as moving the polymer addition point to the inlet and automating chemical addition, will optimize flocculation and improve tank performance.