What is ballast water?
Ballast is any material used to weight and/or balance
an object. One example is the sandbags carried on conventional
hot-air balloons, which can be discarded to lighten the
balloon¡¯s load, allowing it to ascend. Ballast water is
therefore water carried by ships to ensure stability,
trim and structural integrity.
Ballast water is carried in unladen ships to provide
stability. It is taken on board at the port before the
voyage begins and tiny stowaways, in the form of marine
organisms, are taken on board with it.
During the voyage, temperature changes in the ballast
water and lack of food and light kill many, but not all,
of these organisms. At the ships' destination, the cargo
is loaded and the ballast water, with its surviving stowaway
organisms, is pumped out. Some of these organisms then
establish populations in the surrounding waters.
Over 100 species of marine organisms are known to have
been introduced by ballast water. While some appear benign,
others have become pests, threatening biodiversity, fisheries
and aquaculture. Some introduced species severely deplete
native populations or deprive them of food. Others form
colonies which can smother existing fauna. Introduced
toxic dinoflagellates cause red tides and algal blooms
that can affect or even kill shellfish, fish, sea birds
and humans.
About 60 million tonnes of ballast water are discharged
annually into Australian ports. Many iron ore and coal
carrying ships arrive empty of cargo and fully ballasted,
so enormous volumes of foreign water are pumped into our
ports.
Procedures have been implemented to control ballast water
introductions. For example, Australia has a voluntary
code of mid-ocean exchange. Under this system, ships exchange
their ballast water for ocean water, which contains organisms
that are less likely to survive in the estuarine environment
of ports. Unfortunately, this practice is not always feasible
because it is dangerous to empty a ship of all ballast.
Furthermore, it is voluntary and difficult to monitor.
Research is continuing in order to find better methods
of reducing ballast water introductions. If the biodiversity
of Australian waters is to be protected, it is vital that
such research continues.
Cross section of ships showing ballast tanks and
ballast water cycle
Treatment Type
There are several types of possible
treatment procedures that can be grouped into either chemical
or physical processes. Both methods are available to treat
ballast water and the sediment found at the bottom of
the ballast tanks. The physical treatment options include
ultraviolet light radiation, heat application, straining,
filtering, and sedimentation. Chemical processes involve
treating the water with a chemical like chlorine or ozone
to kill the organisms. For a detailed comparison of various
treatment types, please refer to Appendix D. The following
paragraphs describe a few of treatment types. (Australian
Quarantine and Inspection Service, 1993)
A physical treatment such as a gravity process may be
used to treat ballast water. Gravity processes include
sedimentation, floatation, and centrifugation. All gravity
processes require time for the particles to settle, which
is the major constraint. This type of treatment is only
possible for land-based treatment facilities due to the
large basins required to clean the water. Using ballast
tanks as treatment facilities for gravity processes has
been considered, but this would require the implementation
of new machinery aboard all ships.
Centrifugation involves the use of force to separate
or group together particles found in the liquid. This
is accomplished using the difference in specific gravity
of the particles. A problem arises when the particles
have the same or similar specific gravity as the sea water.
The treatment cannot separate the particles because of
the small deviations in the specific gravity of the water
and the unwanted particles (Australian Quarantine and
Inspection Service, 1993).
Treatment by filtration requires passing water through
a medium that sifts out the larger particles. Granular
filtration is a process similar to that which many swimming
pool filters operate. This filtration involves passing
water through a sand or gravel like medium. This means
of treatment is extremely effective with the use of a
coagulant. The coagulant absorbs any particles that are
dissolved in the water that would normally slip through
a simple filtration device. A major disadvantage to the
use of a coagulant is that the pH of the water may fluctuate,
requiring further treatment in order to restore the water
to its former pH level before it can be released into
the port. The average deballasting rate is 4000 cubic
meters per hour and can last for more than ten hours.
This treatment would not be effective as an on-board process
due to the large amount of water the equipment would have
to handle and the space constraints onboard a vessel.
The treatment itself is extremely efficient but time consuming
(Australian Quarantine and Inspection Service, 1993).
Thermal processes involve changing the temperature of
the ballast water in order to kill organisms. The target
temperature that ensures pasteurization is sixty degrees
Celsius. The process of heating the water could be accomplished
en-route. A major disadvantage is that on average, ships
generate only 20 MW of excess heat energy. The necessary
amount of energy needed to kill the organisms inside the
tanks could be as high as an additional 90 MW. This is
a vast amount of energy to supply, and the ship would
require a larger fuel load to generate this amount of
energy. The engines will need to consume more fuel to
be able to arrive at their destination and still treat
the water in the ballast tanks. The second option is to
take the same amount of fuel, but decrease the amount
of cargo carried to lighten the load on the engines (Australian
Quarantine and Inspection Service, 1993).
Electromagnetic radiation has been used in the past to
cleanse drinking water, wastewater, sludge, and food products.
The water must flow past UV lamps for a specific amount
of time in order to kill the hazardous biological materials.
A drawback is that there are particles that exist in ballast
water that are immune to UV treatment. The treatment also
depends on the clarity of the water. Water that contains
large quantities of solid matter may reduce the effectiveness
of UV treatment. A combination of filtration with UV treatment
may be required to eliminate extraneous solids (Australian
Quarantine and Inspection Service, 1993). A ship can be
fitted with UV treatment equipment with an average cost
of $10,000 per ship (Carlton pp.143, 1995). There are
also portable UV treatment units that can be placed inside
a ballast tank while the ship is traveling or docked (Carlton
pp.143, 1995).
The alternative to a physical treatment process is a
chemical treatment. Three types of chemical processes
that can be used to treat ballast water are the use of
chlorine, the addition of ozone, and the injection of
the exhaust of a ship's diesel engine. These processes
involve either removing the oxygen from the ballast water
or poisoning the organisms with toxins (Carlton, 1995).
The chlorine treatment process is performed by the injection
of chlorine gas into the ballast water either at a plant
or inside the ship. The gas removes oxygen from the water,
thus suffocating any organisms in the water. Chlorine
is also very toxic to most organisms, resulting in an
almost perfect treatment process. The hazard of chlorine
treatment is the risk of a leak. If chlorine escapes from
a treatment facility and enters the atmosphere it can
cause acid rain and possible mutations in any organisms
that are exposed to it. Other hazards of chlorine are
that it is very corrosive to materials and that treatment
of the water might result in the formation of hazardous
byproducts such as trihalomethane compounds (Carlton,
1995).
The use of ozone gas is another chemical treatment that
is similar to chlorine treatment but has some differences.
Ozone is an oxidant that removes oxygen from the ballast
water, but is harder to manufacture than chlorine gas.
Ozone is a natural forming gas but is not easily obtainable
on the surface of the earth. Ozone is also an irritant
to most organisms and again there is the risk of a leak
or exposure to the atmosphere (Carlton, 1995).
Using the exhaust of the ship's diesel engines is also
a chemical process. This process uses the exhaust of the
engines which is composed of nitrogen oxide, sulfur oxide,
carbon monoxide and hydrocarbons. The exhaust is very
toxic and kills off most organisms that are in the ballast
tanks. The hazard of this treatment is the chemical contamination
of the water. The water in the tanks will eventually reach
coastal regions and could kill any organisms that make
contact with the contaminated water.
| Bilge Treatment System |
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What is bilge water?
Bilge water is the collective term
for a cocktail of wastewater, oil and fluids from operational
sources such as technical rooms, propulsion systems
and on board machinery. An international regulation
requires ships to treat this waste to a level where
it can be pumped overboard without impacting upon the
ocean's ecological balance.
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