Over the last decade, membrane filtration has appeared as a rentable and a powerful process for purifying large volumes of wastewater (mostly in the form of bilge water and ballast water) generated by naval and commercial vessels. Ceramic membranes with high resistance to extreme conditions can treat more and more effluents. The aim of this paper is to develop an industrial process that can separate hydrocarbon (HC) from oily wastewater using a membrane process. The purpose is to produce a treated water stream suitable to be discharged in the sea and to reduce the volume of waste stream which must be subsequently treated either onboard or onshore. The separation is accomplished by the combination of a decantation–flotation step and an ultrafiltration step. We have studied the effect of pre-treatment and operating conditions (transmembrane pressure, temperature, oily wastewater concentration, etc.). Having tested several ceramic membranes from the laboratory scale, the process was scaled up and ran in two ferries, with a 300-kDa membrane composed of 19 channels. The process is economically and environmentally attractive: (i) it reduces by a factor of 6 the volume of effluents to be treated onshore, (ii) it rejects a very low HC concentrated (less than 1 ppm) effluent that respects nowdays and future environmental standards, (iii) the regeneration of the membrane is effective after each treatment, (iv) the permeate flux can reach 100 L h−1 m−2 bar−1 and it is possible to produce more than 1500 L h−1 of purified water, (v) it is possible to treat in continuous. This process “Klearsep” has obtained the United States Coast Guard (162050/9051/0), the International Maritime Organisation (IMO MEPC 107 49) and the European (19353/AO EC) accreditations. 7 ferries are equipped with this new process.