Ballast water treatment system for ship

A ballast water treatment system for a ship, the system comprising a rotary filtration apparatus including a filter that is cylindrically formed so as to surround an axis, that is rotatable around the axis, and that has a pleated shape that is folded in a cylinder radial direction;

• a cleaning nozzle that ejects untreated water toward an outer peripheral portion of the filter;
• a case that includes an outer cylindrical portion that is disposed so as to surround the filter, a nozzle opening of the cleaning nozzle being disposed inside the case;
• a filtered water channel through which filtered water that has passed through the filter flows from a cylindrical space inside the filter to outside of the case;
• a discharge channel through which discharged water that has not passed through the filter is discharged to outside of the case;
• and an electric motor for rotating the filter,
• wherein a distance between the cleaning nozzle and a recess of the filter is 120 mm or less, and
• wherein the ballast water treatment system is configured to perform a steady-state operation that satisfies the following conditions a to d:
• a) a rotation speed of the filter during a filtering operation is in the range of 20 to 100 rpm,
• b) a flow speed of cleaning water ejected from the cleaning nozzle is 250 m/min or more,
• c) a flow rate of filtered water per unit area of the filter is 5.1 m/h or less, and
• d) a flow rate of discharged water discharged from the discharge channel is 5% of the flow rate of filtered water or more.