CPI (corrugated plate interceptor) separators are used predominantly to separate free oil from wastewater/effluent. CPI unit is deployed at the primary stage of wastewater treatment wherein plate packs are used as the main separation device. The plate packs are designed with specific spacing and alignment to enable solids to settle and fall to the bottom while allowing oil drops to rise and coalesce. A CPI is also equipped with a secondary coalescer screen. This oleophilic coalescer traps oil particles down to 20 microns. Oil is skimmed using a suitable skimmer. Sludge at the bottom is removed by either gravity force or by a sludge removal pump.