A large bulk carrier transiting the Great Barrier Reef

Concern about the potential impacts on the north Queensland coast of exotic species introductions led to a research project being set up to investigate ways to control the importation of exotic species by ballast water. With funding and support from the Ports Corporation of Queensland and the Cooperative Research Centre for Ecologically Sustainable Development of the Great Barrier Reef (CRC Reef Research Centre), the investigators have been able to make significant advances towards developing ballast water treatment methods. Filtration, ultraviolet irradiation and ozonation have been investigated as potential disinfectants, and the roles which these technologies could play in ballast water treatment have been more fully defined than previously. The results from ultraviolet irradiation, in particular, have been both surprising and promising. A chemical and physical characterisation of ballast water was conducted, which has implications for ballast water sampling methods and supports views that ballast exchange at sea is probably not a long term solution to ballast water introductions.

A brief history of ballast water

Ballast water is carried by ships when they are carrying little or no cargo, so as to maintain adequate propeller depth, to adjust their depth in the water and to compensate for currents and wind forces. Water was first carried as ballast in the mid-1800s, slowly replacing solid ballast such as rock and bags of sand, (which is also implicated in the translocation of coastal species such as insects). Today all ballast carried by shipping is water, pumped from the port at which cargo is unloaded or en route. Australia is a net importer of ballast water - 121 million tonnes in 1991 - as we export large amounts of bulk product, such as coal, sugar, ore and wheat. Ships travel with these cargoes to our overseas trading partners and return to Australia 'in ballast'. Ballast water is also transported between Australian ports by coastal trade, which can further spread introduced species.

The first rigorous examination of these organisms was by Medcof and Scribner on ballast water arriving in Australia from Japan in 1975 (Medcof 1975). They found adult and larval zooplankton in the ballast water. Since then, the evidence for ballast water being a significant carrier of exotic species has become overwhelming and pressure to manage ballast water properly has been increasing.

Ballast water has been responsible for the introduction of some very high profile invaders in Australia and overseas. The zebra mussel, Dreissena polymorpha, which was accidently introduced to the North American Great Lakes, is thought to have cost $5 billion by blocking water intakes at heavy industry, power and water treatment plants and by fouling fishing nets, boat hulls and buoys. In Australia, the northern Pacific seastar, Asterias amurensis, has invaded the Derwent River estuary, Tasmania threatening yields in nearby scallop fisheries, and has recently spread to Port Philip Bay. The paralytic shellfish poisoning species of dinoflagellate alga Gymnodinium catenatum, also introduced to Tasmania, has been responsible for the closure of shellfisheries in southern Tasmania for periods of up to six months to protect public health. Several authors have speculated on the potential role of ballast in the dissemination of cholera since Vibrio chloerae, the cause of cholera, has been found in the ballast water of vessels entering the United States of America from South America.

A number of countries have introduced measures to reduce the risk of exotic species introductions via ballast water. In 1989, the voluntary Great Lakes Ballast Water Control Guidelines were established for vessels entering the North American Great Lakes, requiring them to exchange all coastal and freshwater ballast with mid-ocean water before entering the St Lawrence seaway. These regulations were recently extended to the whole of the United States of America and made mandatory for vessels entering Vancouver and the Great Lakes. In 1990 Australia was the first country to introduce national guidelines for voluntary ballast exchange at sea which were recently ratified by the International Maritime Organization for voluntary adoption on an international basis. New Zealand currently requires that ballast water from Tasmania be exchanged at sea during the northern Pacific seastar spawning period, and other ports have introduced mandatory ballast water exchange at sea.

Ballast water treatment processes

Ballast exchange at sea is conducted either by emptying and refilling tanks whilst in the ocean or by continuously flushing oceanic water, equivalent to three to four ballast volumes, through the ballast tanks during transit. Ships currently in service are not designed for these processes, which are considered dangerous by some members of the shipping industry, as they put stress on the structure of vessels. The rates of compliance with the voluntary ballast exchange guidelines are thought to be reasonably high, but serious concerns about the effectiveness of the process remain. Ballast exchange at sea is inefficient as it fails to remove all the original ballast water, and fails to remove sediments present in ballast tanks. A recent refinement on flow through ballast exchange is to pump the seawater through the engine cooling circuit and to use the waste heat to warm the ballast tanks up to about 38°C, which kills many species of plankton. This process has the potential to improve the ballast exchange process significantly for some vessels and routes. However, it takes a number of days, does not work for parasites and may not be possible for trips through cold waters, due to the difficulty of increasing ballast water temperatures.

Figure 1. Some potential locations for ballast treatment processes

The alternative to ballast exchange at sea is to disinfect the ballast water with a biocide either as it is pumped aboard, during transit, or after it is discharged. These options are shown in figure 1. Very little data has been available on the performance of water treatment technologies against the species which are of concern in ballast water, and it is essential to obtain good disinfection data so that cost-benefit analysis of ballast water treatment options can be conducted. Important treatment research has been conducted in Australia and overseas, which has improved the data available, but much more is needed. A lot of the research has focused on the control of dinoflagellate hypnocysts as they are considered, with reason, to be extremely difficult to disinfect. Treatment of ballast water with screens can remove dinoflagellate cysts, so the failure of a treatment to inactivate these cysts does not mean that it should be rejected as a treatment option.

Results from the Ports Corporation of Queensland-CRC Reef Research Centre experimental program

The ballast water project of the CRC Reef Research Centre and the Ports Corporation of Queensland started in 1995. The first component of the project was the measurement of physical and chemical characteristics of ballast water to determine characteristics which may affect screening, filtration, ultraviolet irradiation or ozonation, and to examine the potential of chemical characterisation to determine if ballast had been exchanged at sea. The second component of the program is testing the efficacy of ultraviolet and ozonation for their use as ballast disinfectants. The third phase is to assess the results and recommend a design for a pilot treatment plant.

Screens have excellent potential as a primary ballast water treatment. There are two benefits of screening; firstly the removal of organisms which cannot pass the filter, and the clarification of the water for a secondary ballast water treatment. For ship-based treatment during ballasting, for example, screens of between approximately 10 and 50 micrometres are likely to be appropriate for primary treatment. The actual size of screen which could be used can only really be determined by pilot testing.

Experiments on ultraviolet irradiation have been very promising and have demonstrated considerable potential for ultraviolet irradiation as a ballast water treatment. Ultraviolet has been demonstrated to be effective for the control of the dinoflagellate alga Amphidinium sp. and vegetative cells of Gymnodinium catenatum under conditions approximating those in ships ballast tanks. Research is continuing into the use of ultraviolet on these, and other species. Pre-screening would probably be required for ultraviolet irradiation during ballasting, to remove large flocs which can protect organisms from the effects of the ultraviolet. For ultraviolet treatment at deballasting (option d, figure 1) a more complex treatment plant would be required as iron from the ballast would need to be removed.

Ozone is not appropriate for the removal of species such as dinoflagellate cysts and most zooplankton, but could be used to control bacteria, viruses, amoebae and some protozoa. The most likely application for ozonation would be in a land-based treatment plant following a filtration system capable of removing organisms larger than about 5 micrometres. Ozone is also likely to be affected by oxidant demand from sediments in ballast tanks, may cause corrosion or interfere with corrosion protection, and be reduced by iron if used for shipboard treatment.

Finally, the physical and chemical characteristics of ships' ballast water were compared with what would be expected from oceanic water. The results suggested that either the process of ballast exchange at sea is inefficient at replacing the original ballast water, or that the compliance rate with guidelines for voluntary exchange at sea is poorer than has been thought.

Further research needs

It is vital that research into the efficacy of disinfectants on the species which are likely to be transported in ballast water is continued, as the current set of data is too small to choose between many of the treatment alternatives. This data is needed to estimate both the size and cost of ballast water treatment plants, and to determine optimal combinations of screens and disinfectants.

Pilot testing is also essential for the development of good ballast water treatment processes. A pilot treatment plant design will be one of the major outcomes of this research. It is anticipated that such a pilot treatment plant will be containerised and transportable from port to port, for testing in coastal waters. In this way the effect on treatment of many different ports with different suspended solids and organisms present can be tested. Such a pilot plant could be moved relatively quickly if a bloom of algae or spawning of a starfish were occurring in a port. In this way the effects of many different conditions and organisms on treatment plant performance could be assessed within a few years.

Worldwide, the rate of introduced marine pests arriving in ballast water appears to have been increasing for some time, possibly due to environmental changes in ports, faster ships or changing patterns of trade. It will continue unless effective control measures are implemented, which depends on continuing research into treatment processes.

Reference

Medcof, JC 1975, Living marine animals in ship's ballast water, Proceedings of the National Shellfishers Association 65: 11-12.