Amoco Oil Company has a large refinery on the southern shore of Lake Michigan in Whiting, Indiana. It is a Class D refinery, lube subcategory, as classified under the EPA effluent Guidelines in the Federal Register of May 9, 1974. Because of the complexity and size of the refinery, a significant amount of process water which has been in contact with oil and chemicals is generated. Recognizing the need for further improvement of effluent water quality, the company carried out extensive pilot plant work to select a process that could be used effectively with the secondary bioflotation process. Air flotation was finally selected. The refinery effluent treatment sequence consisting of API separators, aerated lagoon, chemical destabilization, and dissolved air flotation provides generally good effluent quality. Extensive work on a heretofore ignored approach to optimization of this unit operation involving negating the mutual repulsion of negatively charged flotation bubbles and negatively charged suspended solids is reported. Extensive background on appropriate water chemistry, properties, and characteristics of chemicals used in destabilization of colloid systems, chemical antagonists in waste waters, principles involved in colloid destabilization, and chemical models proposing a mechanism for destabilizing negatively charged solids with negatively charged polyelectrolytes is presented. Important design considerations for air flotation units are discussed, and guidelines for optimization of performance by proper chemical destabilization are presented.