typical unit can treat 7.5 m3/min (450 m3/h) at a concentration of one mg/l of oxidant. The use of chlorine gas would be inappropriate for enclosed spaces such as on board ship, but calcium hypochlorite or sodium hypochlorite could be used for treating ballast water. It is well known that the addition of a powerful oxidant such as chlorine to seawater can generate toxic compounds (byproducts) due to the oxidation of halogens (Cl-, Br-, I-). This issue needs to be addressed in the context of various options for treating ballast water.
Reviews of water treatment conducted in the past have focused on the use of oxidizing biocides. However, as noted above, there is a large inventory of nonoxidizing biocides that are used by industry for the effective control of a wide variety of organisms. An example is the group of glutaraldehyde-based chemicals used in industrial water treatment. The limited reactions of nonoxidizing biocides with compounds in the water may be a possible advantage for ballast water treatment. The residuals generated by nonoxidizing biocides generally decay fairly rapidly into nontoxic byproducts. Thus, treated water might not pose a substantial environmental hazard if it were discharged in large quantities.
The use of waste heat from a ship's propulsion and service cooling is an attractive option for the inactivation of organisms in ballast water because (1) waste heat from ship propulsion and onboard equipment may be used to raise the temperature of water and (2) no chemical byproducts or residuals are discharged (Rigby and Taylor, 1993). Additional piping would be needed to pump ballast water through the existing heat exchangers. To destroy most organisms, ballast water would need to be heated to temperatures in the range 35°C to 45°C (95°F to 113°F) and held there for a set period of time. Investigations of the relationships between treatment temperature and time needed to kill or inactivate certain organisms are continuing (Taylor, 1995).
A number of critical factors will probably limit the practicality of thermal treatment to certain vessels on specific trade routes:
Length of voyage. Some voyages will be too short to permit heating the water to the required temperature or holding the temperature for the required period.
Volume of ballast water requiring treatment. There is a limited amount of energy available from waste heat sources. Thus, there are constraints on the volume of ballast water that can be treated.