A petroleum refining complex was required to upgrade its wastewater treatment system to meet newly adopted toxicity requirements and to handle increased flows. A four-phase investigation led to the design and construction of a full-scale PACTÒ system. The first phase, waste stream characterization, indicated that the effluent toxicity was organic in nature. The second phase, bench-scale screening, indicated that the toxicity was removable by activated carbon adsorption. The third phase, comparative pilot testing, indicated that although both extended aeration and PACTÒ processes were effective in reducing COD concentrations, only PACTÒ could remove the toxicity. In the fourth phase investigation, the PACTÒ pilot plant was tested for various conditions, including dry-and wet-weather follow conditions. The full-scale PACTÒ plant has been operating for more than two years and is meeting all expectations.