The ability of the vessel to separate the water from the oil is a key feature of the MV Arca, since typically the oil/water mixture that is recovered at sea after an incident consists mainly of water. In other words, if you can separate the oil from the water at sea, and just store the oil on board, then you can significantly increase the amount of oil recovered in any given day, and save a lot of time, effort and money in the process. Otherwise, the voluminous oil/water mixture has to be transferred to a shore-based facility where separation can occur. But for the system to really work effectively, the quality of the 'effluent' has to be good enough such that it can be discharged back into the sea. Even Kevin Costner's now famous centrifugal oil spill separation system has only been able to get the oil content in the treated oil/water mixture down to 80 ppm, which is well below the international Marpol standard of 15 ppm. Until such time as the lower figure is achieved, Costner's oil and water will both have to be stored on board, which kind of defeats the purpose of separating the water out of it at sea.