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.