Pretreatment of oily wastewater

American Petroleum Institute (API) separator

API separators are commonly used to remove floating oil and precipitating solids simultaneously from water. Its desing is very similar to the rectangular clarifiers used for non-oily wastewater, but the design and operation of skimmer and scrapper are optimized for removing oily wastewater.

API separators are designed to capture oil globules larger than 0.15 mm. In theory, if there is no turbulence and short-circuiting in the separator, oil globules with a rise velocity greater than overflow velocity will be captured by the skimmer, where overflow velocity is calculated by dividing influent flow rate by the water surface area of the separator. API recommends to have a minimum of two parallel channels so that one can be inline while the other is out of service.


Fig. 1. Top and side views of API separator (Nalco Water Handbook, 2009).

Corrugated Plate Interceptor (CPI)

CPI contains corrugated plates that are 19-51 mm apart at an angle of 45o from vertical. Free oil globules need to rise only such distance to be captured by the plate and rise along the plate. Heavy particles are also captured by the other side of the plates. As a result, the space efficiency of the separate can be greatly improved compared to API separator. In order to make sure the laminar condition must be maintained in the channels among plates.

Pretre2Fig. 2. Corrugated plate interceptor, CPI (Nalco Water Handbook, 2009).

Parallel Plate Interceptor (PPI)

PPI is similar to a plate clarifier and not extensively used in petrochemical industry because CPI are more efficient in coalescing free oils.

Circular Oil/Water Separator

Circular separators are similar to the circular clarifier used in other industry. The sizing principle of this separator is same as API’s.


© Seong Hoon Yoon