1. Exchange of ballast water at mid-ocean This technique is useful to reduce the total number of organisms, but the percentage of organisms eliminated is not constant and unknown. It is highly suspected that organisms of benthic life forms may not be discharged easily, comparing to those floating in the water. Although this technique is practicable to both existing and new ships without installing new expensive mechanisms, operation is sometimes difficult. Because exchange is not preformed during a short voyage which does not pass mid-ocean and under bad sea weather. Also strength and stability of ship may have trouble.

　

2. Mechanical termination method using ballast pipes. This is a method to terminate organisms by using a special pipe which has nobs inside surface. The device is planed to be integrated in ballast pipe system for sterilization of seawater through its passage during ballasting and/or deballasting operations. This method can be adopted without changing usual ballast operation, being not necessary extra energy supply or maintenance. This device showed high termination rate for phyto- and zooplankton, if the passage through the pipe can be repeated. But it could not terminate resting cysts of dinoflagellates and bacteria well. It is presumed that organisms smaller than 10 micrometer may not be terminated. This method is excellent since it is harmless to environment, easy to operate, and not expensive to install and maintain. It gives no particular problem to ship's safety.

　

3. Heating method. This method is considered to have a high sterilizationeffect. As large-sized heat source is, however, required to raise ballast water to sufficient temperature to be effective, running cost of heating equipment is considered to be expensive. There are no concerns regarding possible environmental impacts. It is the advantage of the method that it can terminate all organisms taken in tanks including pathogens and resistant forms of microalgae such as dinoflagellates cysts.

　

4. Chemical method using ozone, chlorine and hydrogen peroxide Sterilization effects of chemicals, ozone, chlorine and hydrogen peroxide against pathogens and microplankton are very excellent. However there are several problems common to the three chemicals. For example, generating or storage facilities are expensive and large. Chemicals may cause hull corrosion and give environment impacts, unless decomposition of excess chemicals is made in ballast tanks rather quickly.