What is Dilution Method of Ballast Water Exchange and factors taken into account?

In the dilution method, replacement ballast water is filled through the top of the ballast tank and simultaneously discharged from the bottom at the same flow rate while maintaining a constant level in the tank throughout the ballast exchange operation. As with the flow-though method, ballast equal to approximately three times the tank capacity must be pumped through the tank to achieve 95% effectiveness in eliminating aquatic organisms.

The dilution method has the advantages of the flow-through method with regard to maintaining the stability and strength and other similar benefits. By discharging water from the bottom of the ballast tanks, sediments are more easily removed. This method avoids the use of air vent pipes and the removal of manhole covers to discharge water over the deck.
In addition to the design requirements , the following safety precautions are to be taken and noted in the Ballast Water Management Plan if the dilution method is chosen:

  1. Arrangements are to be made to automatically maintain the ballast water level in the tanks at a constant level.
  2. High and low water level alarms are to be provided where maintaining a constant level in a tank or hold is essential to the operational efficiency of the vessel during the ballast water exchange.
  3. The arrangements are to include the provision of a manual emergency stop for any operating ballast pump in case of valve malfunction or incorrect control actions.

As with all methods, the Ballast Water Reporting Form provided in Appendix 1 is to be completed and transmitted to the next port where ballast water is discharged.

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