Issues Magazine

Progress in Ship Ballast Water Treatment

By Gustaaf Hallegraeff

A number of promising ballast water treatment systems that aim to eliminate the risk of translocating harmful marine phytoplankton, zooplankton and bacteria are in various stages of national or international certification.

Shipping moves over 80% of the world’s commodities, and in the process transfers approximately three to five billion tonnes of ballast water internationally each year. Ballast water is water carried by ships to ensure stability, trim and structural integrity, and is therefore essential to the safe and efficient operation of modern shipping, providing balance and stability to unladen ships. However, it also poses a serious ecological, economic and health threat that was only fully recognised in the late 1980s and 1990s.

It is estimated that at least 7000 different species are being carried in ships’ ballast tanks around the world. The vast majority of marine species carried in ballast water do not survive the journey, as the ballasting and deballasting cycle and the environment inside ballast tanks can be hostile to their survival. Even for those that do survive a voyage and are discharged, the chances of surviving in the new environmental conditions, including predation by and/or competition from native species, are reduced.

However, when all factors are favourable, an introduced species may survive to establish a reproductive population in the host environment, and ultimately it may become invasive, out-competing native species and multiplying into pest proportions. As a result, whole marine ecosystems can be irreversibly altered.

In the US, the European zebra mussel, Dreissena polymorpha, has infested over 40% of internal waterways and required US$1 billion expenditure on control measures. In the Black Sea, the filter-feeding North American jellyfish, Mnemiopsis leidyi, has on occasion depleted native plankton stocks to contribute to the collapse of Black Sea commercial fisheries. In Tasmania, introduced microscopic toxic dinoflagellates, Gymnodinium catenatum, have been ingested by filter-feeding shellfish, which when eaten by humans can cause paralysis and even death. It is even feared that diseases such as cholera might be able to be transported in ballast water.

It was not until the International Maritime Organisation (www.imo.org) introduced the International Convention for the Control and Management of Ships’ Ballast Water and Sediments in 2004 that significant financial investment was made in the research and development of ballast water management technologies. At present the Convention has been signed by 18 countries, representing about 15% of world merchant shipping tonnage, but the Convention will not enter into force until it is ratified by 30 states, representing 35% of merchant shipping tonnage – so we still have a long way to go.

While global agreement exists that “not doing anything” is no longer an option, the technological challenges involved in effectively treating five billion tonnes of seawater per annum are monumental, and the economic costs involved are estimated at US$8 billion. Once entered into force, the Convention requires all ships to conduct ballast water exchange with an efficacy of 95% volumetric exchange, and after a defined phasing-in period, depending on construction date and ballast capacity, ships shall meet a Ballast Water Performance Standard. This stipulates limits (depending on organism size) on the number of viable organisms in discharged ballast water, and (most difficult to achieve) concentration limits on indicator microbes such as E. coli as a human health standard.

Possible approaches being researched include:

  • treating ballast water to remove or destroy unwanted organisms;
  • redesigning new vessels to eliminate the need to discharge ballast water; and
  • retaining ballast water onboard.

The high flow rates and volumes of ballast water that must be treated pose significant technological challenges. Ballasting and deballasting operations on a proportion of ships take place at flow rates of 1000–20,000 m3/h, with the higher values applying to dry bulk carriers, ore carriers and tankers and the lower values to cruise liners, container ships and car carriers.

Major barriers still exist in scaling these various technologies up to deal effectively with the huge quantities of ballast water carried by large ships (e.g. about 60,000 tonnes of ballast water on a 200,000 dry weight tonnes bulk carrier). The presence of sediment in ballast tanks also reduces the efficacy of many treatment options, as this provides a habitat for resistant organisms such as resting stages of phytoplankton and zooplankton.

Treatment options must not interfere unduly with the safe and economical operation of the ship and must consider ship design limitations. Any control measure that is developed must meet a number of criteria: it must be safe, it must be environmentally acceptable, it must be cost-effective and it must work!

Ballast water exchange (Fig. 2) is currently the most widely practised procedure to minimise the risk of ballast water-mediated invasions, and is sometimes even legally required, but it will become mandatory for existing vessels following the entry into force of the above-mentioned Convention. After that date, ballast water exchange methods would continue to be acceptable but only if they can achieve an efficacy of 95% volumetric exchange.

Ballast water exchange involves replacing ballast water taken onboard in coastal and port areas with open oceanic water prior to discharge at subsequent ports of call. Open oceanic water is described by the Convention as water from at least 200 nautical miles from the nearest land and at least 200 metres in depth. If this is not possible, 200 nautical miles from land and 50 metres depth is acceptable, or designated ballast water exchange zones may be used.

The variable efficacy and operational limitations of this approach have led to significant financial investment in the past 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, ultraviolet irradiation, cavitation (ultrasonic treatment), de-oxygenation and the use of chemical biocides.

To date, no single treatment option has proved to be universally effective, and increasing attention has been given to multi-component treatment systems.

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.

Ultraviolet treatment systems are unlikely to eliminate all ballast water organisms because they are not able to deliver a stable lethal dose across a wide range of water quality conditions, and many organisms are resistant to ultraviolet 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 to 40–60ºC using waste heat from ships’ engines has been claimed to be a practical and cost-effective treatment option for eliminating ballast water zooplankton and phytoplankton (including resting stages), but concerns have been expressed that attainable temperatures may not eliminate all bacterial pathogens, and 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.

All treatment options require further research on their biological and operational efficacy and safety under full-scale shipboard conditions. Currently 16 promising systems using active substances have received basic approval, and eight systems have received final approval from the International Maritime Organisation, with four systems awarded type approval certification and two systems national approval certification. The still-limited production capacity for ballast water treatment systems may not be sufficient to equip all vessels by 2011, when the ballast water discharge standards are planned to apply to the first group of vessels.

Reducing the risk of ballast water-mediated invasions represents a significant technological challenge. Ideally, a treatment option that is 100% effective is required. At present, no treatment option or multi-component treatment system has proved completely effective as each is limited by one or more factors, including cost-effectiveness, space and energy requirements, environmental soundness, safety and biological efficacy. Many of these limitations relate to the high flow rates and volumes of water that must be treated.

Although many of the treatment options are suggested to be able to meet the Convention’s discharge standard, further research is required to increase their biological and operational efficacy and safety under full-scale shipboard conditions, in particular their ability to inactivate resistant organisms such as dinoflagellate cysts.

For further reading see http://www.aquaticinvasions.net/2009/AI_2009_4_3_Gregg_etal.pdf