In every system substances that show up as “oil” in the laboratory test procedure will be dissolved in the water. This is especially true where samples are acidized for “stabilization” prior to extraction with a solvent. This soluble oil cannot be removed by the systems discussed in this chapter. The soluble oil concentration should be subtracted from the discharge criteria to obtain a concentration of dispersed oil for design. Soluble oil concentrations as high as 1,000 mg/1 have been measured on rare occasions.