3 phase disc centrifuge and 3 phase decanter centrifuge is 2 equipments normally used for oily water separation, both of them have their advantages and disadvantages.
The comparison is as below:
Both of the 3 phase disc centrifuge and 3 phase centrifuge are for light liquid, heavy liquid, and solids separation, just one is vertical centrifuge, the other one is horizontal centrifuge.
The working principal is different:

3 phase Disc centrifuge: the oily water is feed into the disc centrifuge, under the centrifugal force, the material pass through the disc bundle separation intervals, the light liquid and heavy liquid are discharged by the lower centrifugal pump and the upper centrifugal pump respectively, the solids is discharged regularly.
3 phase decanter centrifuge: when the oil sludge is feed to the centrifuge inlet pipe and enter into high speed bowl, the material rotates synchronously with the bowl.The centrifugal force became less from outside to inside. The solids are pushed by the screw conveyor, the water is discharged from the nozzle in the upper, the oil is discharged from the bottom nozzle .

The disc centrifuge can be applied for materials with fine solids and with less solids content, so the material to be feed to disc centrifuge is very strict limited, the solids content should be less than 3% and the particle size is no more than 1mm, so normally we will use a decanter centrifuge before the disc centrifuge, to reduce the solids content and separate the big solids from the oily water/ oil sludge. So the cost of the proposal with a decanter centrifuge and a disc centrifuge will be definitely higher than 3 phase decanter centrifuge, but the treatment result will be always better, it can meet the requirement for oil recovery, solids disposal safely and water to be sent to waste water treatment.