More and more ports and specific water areas prohibit ships from discharging sewage and grey water with the increasing requirements of the international community for marine environmental protection. The prohibition goes beyond the discharge requirements of the ship sewage and grey water under the International Convention for the Prevention of Pollution from Ships (MARPOL) by the International Maritime Organization (IMO). Therefore, ships have to seek other possible tanks for temporary storage of the grey water and sewage that are not allowed to be discharged. This paper focuses on the current practice of temporarily storing sewage and grey water in ballast tanks during the operation of ships. The feasibility of storing sewage and grey water in ballast tanks has been explored by analyzing the discharge requirements of the D-2 standard for ship's ballast water under IMO BWM Convention and the discharge requirements of the ship sewage and grey water under the IMO MARPOL Convention. At the same time, in conjunction with the IMO's discussions on this issue, the technical and operational factors that need to be further considered for the temporary storage of the grey water and the treated sewage in ballast tanks are proposed to avoid the potential risk of violations caused by this approach.

Seakeeping is one of the most important performance aspects for ship sailing and operating at sea. It is a research field related to the complex physical problems of wave and structure interactions. Seakeeping performance of a ship is influenced by the weather condition, the wave conditions, and the costal and deepwater region characteristics, which is also affected by the geometry, scale, encountered wave angle, speed, loading condition and the inherent natural frequency of the ship. The paper summarizes the recent hydrodynamic methods and development trend, along with some comments on the key developments of seakeeping researches.