Purification of water through oil–water separation is essential for preserving the ecosystem and protecting human health. Although a conventional polypropylene depth filter can effectively purify water, modifying the wettability of a filter for oil–water separation is difficult owing to its low reactivity. In this study, we developed a superhydrophilic polypropylene filter with a hydrogel layer that could enable effective oil–water separation by using plasma treatment and dip coating, which enabled an even distribution of the coating solution across the filter. The fabricated filter was superhydrophilic with a water contact angle of 0o. It showed a high repulsive force with oil in water with an underwater oil contact angle of 142.9o. When such filter was applied to an oil–water separation device, it effectively purified water with low oil content (< 15 ppm) at a flow rate of 300 mL/min. These results demonstrate potential applications of such filters in areas such as wastewater treatment and oil spill cleanup.

A separator used by marine engine functions as a purifier by dividing engine oil into pure oil and impurities. By rotating the separator at a high speed between 1800 to 4500 rpm, fine particles of oil inside the separator are centrifuged due to the weight difference when the engine oil is refined. The impurities, so-called sludge, should be manually removed by taking the separator apart from the oil purifier system unless the accumulated sludge is measured and monitored in real time. A manual discharge of the sludge causes an increase of cycle time to clean the engine oil as well as operator exposure of safety risks. Therefore, the development of automatic system with a sludge monitor and an actuator is necessary for ship engine efficiency. In this paper, the pressure difference from the accumulated sludge is monitored via a pressure sensor. By measuring the pressure exerted at the wall surface, the amount of sludge was quantitatively estimated within the error of 0.032-0.091 kg when the oil purifier system has been automated.