First of all the oily water seperator shall be considered one equipment in the entire oily draiange system.However, depends on the operation modes( settlment of the crude/product in tank(s) or resident time before agitation) of your tank farm, the water disposed volum( BSW ) in the oil shall be caculated first through lab analysis. Then, you caculate the recovered oil in barrels. If the amount of the crude is appreciable( more than 0.2% by volume), the separator will be economicall and effecient specially if process big/large crude oil tanks. Otherwise, you go for good settlemnt and direct evapration or to a ground sump where you can repump it bcak to a pipeline or into one of the tanks in the tank farm. If you decide to go for a seperator( CPI or API( costly & require big space, yet it is good for high impurities oily water), approcah one of the vendors who have packaged oily sepeartors(CPI, requir less space and less costly, yet good for clean flow such as finished products in bulk plants) with a sump built in it, so you can pump the back wherever you decide. You may start with API-Pb-421 standard.