In-situ cleaning of the bilgewater processing plates of stacks within a treatment tank chamber of a 50-gpm oil/water separator is performed by an injected chemical cleaning agent dispersed by diverted flow of pressurized air introduced into the tank chamber below the plate stacks by emergence of air bubbles from plural branches of an air sparging array system positioned in underlying alignment with crest formations on the processing plates. The air sparging array system consists of an upper and lower tank arrays.