The characteristics of wastewaters emanating from refineries can be quite variable not only at a specific plant, but from plant-to-plant within the industry. This variability is due in large part to the different types of production operation at specific refineries. In December 1973, the EPA issued proposed effluent guidelines for refineries according to six specific categories (A-F). The categories were formed in relation to the “process technology employed and the waste severity from the operations” at the refinery.