Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1998-11-18
2001-04-24
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S764000, C252S178000, C252S181600
Reexamination Certificate
active
06221262
ABSTRACT:
This invention relates to a method and compositions for treating ship ballast water so as to reduce the numbers of viable organisms.
BACKGROUND OF THE INVENTION
The transfer of toxic and otherwise undesirable organisms between ports via ship ballast water has been a concern to maritime countries for a number of years. With the emergence of a number of well documented cases of foreign organisms establishing themselves and causing considerable damage to local ecology, the maritime industry is now showing an increased desire to find a solution to the problem. Solutions which have been proposed include the addition of biocides and herbicides to ballast water, heat sterilisation of ballast water, and mid-ocean flushing of ballast tanks. However, each of these proposed solutions have difficulties or requirements which make them impractical or undesirable.
For example, tests conducted on the use of biocides and herbicides show that the treatment of ballast water with these agents would both be expensive and cause unpredictable pollution and destruction to the local ecology upon discharge of the treated ballast. Further, it has been found that when sub-optimal concentrations of the biocides and herbicides are used, the numbers of organisms in the ballast water may actually increase.
Heat sterilisation of ballast water with steam or engine heat would also be expensive and also has the problem of causing accelerated corrosion due to the increased ballast water temperature.
Mid-ocean flushing is expected to be of limited value due to the complex interior surfaces of ballast tanks which tend to ensure considerable ballast water residues when the tanks are emptied, Further, the multiple pumping cycles required are expensive and may cause considerable stress in the ship's hull.
DESCRIPTION OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for treating ballast water so that ballast water transfer of organisms between ports may be prevented or reduced.
Thus, in a first aspect, the present invention provides a method for reducing the number of viable organisms present in ship ballast water comprising, treating the ballast water so as to reduce the concentration of dissolved oxygen and/or carbon dioxide to a level(s) at which a substantial portion of the organisms present cease to remain viable.
By the term “substantial portion” it is meant that at least 50% of the organisms present in the ballast water cease to remain viable following said treatment. It is, however, preferred that at least 70%, more preferably 95%, of the organisms present in the ballast water cease to remain viable following said treatment. “Organisms” is to be understood as including animals and plants and single and multicellular microorganisms. Thus, the kinds of organisms that might be destroyed by the method of the invention include fish, starfish, crabs, shrimps, bacteria (e.g. cholera, typhoid), dinoflagellates, pest flora species (e.g. Japanese kelp) and various algal blooms (e.g. red algal bloom, blue/green algal bloom) which are commonly found in ballast water.
Preferably, the concentration of dissolved oxygen is reduced to 5 mg l
−1
, more preferably 3 mg l
−1
, or lower. Preferably, the concentration of carbon dioxide is reduced to ≦2 mg l
−1
. Most preferably, the concentrations of both dissolved oxygen and carbon dioxide is reduced to ≦2 mg l
−1
, which ensures that the numbers of viable organisms after 24 hours is negligible.
The treatment of the ballast water may involve the addition of a suitable amount of an anti-oxidant(s). Many anti-oxidant agents may be suitable including sulphites, nitrites and chlorites of metals, organic phosphites and phosphates, and hydroquinones. Further, various oxygen consuming flora and fauna (e.g. yeasts) could be used as anti-oxidant agents. However, it has been found that metal powder(s) are particularly effective in reducing the concentration of dissolved oxygen and carbon dioxide to the required levels. However, to avoid any undesirable side effects upon discharge of the treated ballast, it is most preferred that the treatment involve the addition of iron powder which may actually be beneficial to the local ecology (New Scientist, Dec. 3 1994, pp 32-35).
Iron powder in the preferred particle size range of 0.1 &mgr;m to 200 &mgr;m has been found to reduce the dissolved oxygen and carbon dioxide to the required levels. Preferably, the iron powder may be added to the ballast water with a suspension agent(s) such as xanthan gum, polyacrylic acid or the like. Such suspension agents have been found to keep the iron in suspension longer, which assists in the reduction of dissolved oxygen and carbon dioxide concentrations. In addition, a flocculating agent(s) such as alum (aluminium sulfate) may be added so that the iron will flocculate onto the organisms such that they are surrounded by a high concentration of iron particles thereby causing their rapid demise. Iron oxides such as FeO and Fe
2
O
3
may also be beneficially added.
The iron powder may be added in amounts greater than 10, more preferably 30, grams m
−3
of ballast water. Greater amounts may be added to reduce treatment times which may be required for short trips. Xanthan gum may be added in similar amounts, although variation may be required depending on local conditions and required treatment times.
The rate at which the numbers of viable organisms are reduced may depend on the amount of anti-oxidant(s) added, the flocculants, the temperature and the degree of suspension and mixing. These variables may be tailored to suit cost and performance requirements.
As foreshadowed, the treatment of ballast water with iron powder may actually be beneficial to local ecology. The reasoning is that the discharged ballast water will provide a source of iron to the usually iron-deficient sea water. Iron is an important trace element essential for photosynthesis and can therefore be expected to encourage the productivity and healthy growth of the local ecology.
It is, however, possible that some quarantine and/or port authorities may object to the discharge of ballast water including iron. In such circumstances, it will be possible to substantially remove the iron from the treated ballast water by, for example, magnetic means. Two general strategies are presently envisaged.
The first strategy involves removing the iron from the treated ballast water in the ballast tank(s) themselves. This strategy may utilise various types of magnetic apparatus using either electromagnets or permanent magnets. One preferred embodiment comprises a plurality of magnetisable units which can be magnetised by supplying coils surrounding each magnet with a direct current pulse typically supplied by discharging capacitors into the coil. This will have the effect of permanently magnetising the magnet and thereby attracting the iron in the ballast water. By subsequently applying a suitable alternating current to the coils, the magnets can be demagnetised to release all of the trapped iron. Most conveniently, the magnet(s) may be adapted so as to be able to be lowered into the ballast tank(s) through inspection hatches and then removed to dispose or reuse the iron.
The second strategy involves the use of a magnetic “interception” tank situated between the overboard discharge and the ballast tank(s) either before or after the pump. This strategy has the advantage of easier accessibility and maintenance. The interception tank is preferably provided with magnets and coils much like the units described above for use in ballast tanks, only with larger capacity and the lowest pumping resistance possible. The interception tank can be arranged so that the accumulated iron is either discharged to the open sea or diverted to the ships slop tanks as desired.
In a second aspect, the present invention provides a composition for use in the method according to the first aspect. Preferably, the composition comprises:
20-80% (w/w) iron powder,
20-80% (w/w) suspension agent(s), and
5-80% (w/w) fl
Bull Robert
Lansell Peter
MacDonald Robert W. G.
Porter Douglas F
Archer Walter J.
Hoey Betsey Morrison
Nixon & Vanderhye
Simmons David A.
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