Review of two decades of progress in the development of management options for reducing or eradicating phytoplankton, zooplankton and bacteria in ship's ballast water

* Corresponding author Abstract The worldwide transfer and introduction of non-indigenous invasive aquatic organisms via ships' ballast water has been amply demonstrated to cause significant ecological, economic and human health impacts. Possible solutions to the problem include: 1) treating ballast water to remove or destroy unwanted organisms; 2) re-designing new vessels to eliminate the need to discharge ballast water; and 3) retaining ballast water onboard. Ballast water exchange is currently the only widely acceptable and suggested (sometimes even required) procedure to minimise the risk of ballast water mediated invasions but the variable efficacy and operational limitations of this approach have led to significant financial investment in the last two decades in the research and development of more effective shipboard and shore based ballast water treatment technologies. Specific technologies under consideration include mechanical separation, heat treatment, UV irradiation, cavitation, de-oxygenation and active substances. To date, no single treatment option has proved to be universally effective and increasing attention has focused on multi- component treatment systems. The high flow rates and volumes of ballast water that must be treated pose significant technological challenges, and the presence of sediment in ballast tanks reduces the efficacy of many treatment options as this provides a habitat for resistant organisms such as resting stages of phytoplankton and zooplankton. Mechanical separation devices would best be used as a primary stage of a treatment system comprising multiple technologies because free-living organisms and sediment below a certain size are likely to be largely unaffected. UV treatment systems are unlikely to eliminate all ballast water organisms, as they are not able to deliver a stable lethal dose across a wide range of water quality conditions and many organisms are resistant to UV exposure or can recuperate after treatment. At the current stage of development, cavitation would not be considered appropriate for the shipboard treatment of ballast water due to high capital and operating costs and high power requirements. The heating of ballast water using waste heat from ships' engines has been claimed to be a practical and cost effective treatment options for eliminating ballast water zooplankton and phytoplankton (including resting stages) but concerns have been expressed that attainable temperatures may not eliminate all bacterial pathogens, that this approach does not apply to ships traversing colder seas and may impact on the integrity of vessel structures. Promising research has been conducted on several systems that are able to achieve temperatures capable of eliminating bacteria but these technologies are still under development. De-oxygenation by the addition of glucose or reducing agents are not effective treatment options, however de- oxygenation technologies based on the injection of an inert gas are more promising (notably against larval and adult zooplankton) as they could be cost effective and do not impact on the aquatic environment as ballast water is re-oxygenated prior to discharge. Biocide dosing systems have low capital costs and power requirements but the costs of active substances are significant. Chemical treatment costs and space requirements can be significantly reduced by using onboard chemical generators but the capital cost of these systems is significant and all have biological efficacy, safety, operational and environmental (poor biodegradation) concerns. Treatment systems that produce free hydroxyl radicals would be favourable over other chemical treatments as they are claimed to produce less or no toxic by-products at ballast discharge but these technologies have high power requirements. Each treatment option requires further research on their biological and operational efficacy and safety under full-scale shipboard conditions. As of July 2009, 16 promising systems using active substances had received basic approval and 8 systems final approval from IMO, with 4 systems receiving type approval certification and 2 systems receiving national approval certification. Effectively eliminating the risk of ballast water mediated invasions still remains a monumental technological and economical challenge.