The unplated Drain caisson is a vertical oil/water separator. Length in excess of 10 feet does not increase the oil removing efficiency of the device; however, increasing the length does increase the retention time. The only method of improving the removal efficiency of this type of separator is to increase the inside diameter of the caisson which reduces the exit velocity of the incoming flow, allowing smaller oil particles with low rising velocities to ascend.

This type of design is primarily used for disposal of pre-treated produced water and for containment of upstream upset or bypass conditions.