WO2010149638A1

WO2010149638A1 - Ballast water treatment

Info

Publication number: WO2010149638A1
Application number: PCT/EP2010/058776
Authority: WO
Inventors: Ole GRØNBORG; Jens SKJØLSTRUP
Original assignee: Desmi Ocean Guard A/S
Priority date: 2009-06-23
Filing date: 2010-06-22
Publication date: 2010-12-29

Abstract

A ballast water treatment apparatus and method configured for treatment of seawater to be carried as ballast water by a maritime structure such as a ship. The system comprising a seawater inlet (10), a seawater outlet (90), means (20) for adding ozone (30) to a flow of seawater through said apparatus wherein that apparatus comprising, downstream the means (20) for adding ozone, means (40) for irradiating the seawater/ozone blend (35) with ultraviolet light.

Description

Ballast water treatment

The present invention relates to treatment or purification of ballast water

The invention finds its primary use within ballasting applications on board maritime structures such as ships or rigs and the like.

According to a first aspect, the present invention relates to a ballast water treatment apparatus configured for treatment of seawater to be carried as ballast water by a maritime structure, the apparatus inter alia comprising:

- a seawater inlet connectable to a supply of seawater,

- a seawater outlet connectable to means for holding or distributing ballast water for the purpose of ballasting the maritime structure,

- means for adding ozone to a flow of seawater through the apparatus such that a flow of seawater/ozone blend is established.

- means, arranged downstream the means for adding ozone, for irradiating the flowing seawater/ozone blend with ultraviolet light.

According to a second aspect, the present invention relates to a method of purifying ballast water.

It is understood that the expressions, "maritime structure", "ship", "boat", "vessel" or "rig", throughout this specification is meant to denote any kind of maritime structure suitable for performing operations, carrying persons or cargo or a combination thereof.

It is further understood that the expression "seawater" is, throughout this specification, meant to denote any kind of water found in any kind of maritime environment incl. saltwater, freshwater and brackish water etc. Further, it is understood that the expression "inviable", which in this specification is used to describe a state of organisms, throughout this specification is meant to denote a state wherein the organisms are rendered unable to survive.

Background

On ships, such as cargo or passenger ships, it is often, for various technical reasons, required to carry water ballast

Water ballast typically serves to oppose an unfavourable resulting centre of gravity resulting from a high or off-centre centre of gravity of the loaded cargo. Further, in cargo ships, in so called ballast conditions, water ballast is often used to obtain sufficient draft in order to submerge for the propeller and rudder. A ballast condition is a loading condition of a ship where the weight of the stowed cargo by it self does not result in a sufficient draft of the ship

Another typical application of water ballast is for stabilizing or anti heeling purposes where a part of the carried seawater is shifted from side to side in order to counteract rolling of a ship

Seawater is widely used as ballast because of its ability to be easily loaded and unloaded via different pumping arrangements

Seawater is typically pumped on board the ship directly from the environments wherein the ship is located whereby various species of organisms are, together with the ballast water, moved together with the ship from the place of loading to a non native place of unloading

A part of the living organisms carried in the ballast tanks die during the pumping process as well as and under the voyage, and more die when they are released at the place of unload, however, a substantial part of the organisms survive both the voyage as well as being unloaded in a foreign environment.

The introduction of foreign organisms into a new and for the foreign organ- isms' unnatural environment has been known to have catastrophic consequences to the environment wherein the organisms are released.

It is estimated that about 3-5 billion tonnes of ballast water each year are moved from its natural environment and pumped out in foreign environments

New directives for treatment of ballast water, aimed at rendering the organisms carried by the ballast water systems harmless for the environment wherein they are released, have been adopted by the International Maritime Organisation (IMO). The directives dictate that measures should be taken such that abovementioned relocation of living organisms is hindered, and further, the directives emphasize that any applied treatment of ballast water must be safe and environmentally acceptable.

IMO have developed specific requirements to performance of the ballast wa- ter treatment system. The requirements dictates certain performance limits for the actual treatment of the ballast water as well as limits for toxic content in the ballast water discharged.

In particular, the directives state that any applied technique must show con- vincing effects as well as be environmentally friendly. Further, the directives state that any applied technique must be economically and technically reasonable to apply.

Background art According to techniques of various prior art, it is known to expose ballast water to a relatively brief under pressure, typically under loading or unloading actions Such under pressure render some larger living organisms such as crawfish inviable, however an under pressure has little or no effect on, for example, bacteria or viruses.

Ballast water may also be treated with chlorine, however chlorine, together with bromide which occurs naturally in seawater, may have adverse effects on the environment at the site of ballast water discharge as well as on the ballasting systems incl tanks.

Treatment of ballast water with ultraviolet light has also been applied previously, however the treatment has been found insufficient as various organisms and bacteria etc has shown no or little susceptibility to the irradiation

It is also known to use ozone (O₃) for the treatment of ballast water

Ozone is a colorless gas soluble in alkalis and cold water. Ozone is a strong oxidizing agent which can be produced e.g. by electric discharge in oxygen or by irradiating oxygen with ultraviolet light

Ozone, or tri-oxygen, is a tri-atomic molecule consisting of three oxygen atoms It is an allotrope of oxygen that is much less stable than the diatomic O₂.

Ground-level ozone is considered a pollutant with harmful effects on the environment as well as on human beings and animals; however ozone is also a necessity in our earth's upper atmosphere, as ozone prevent potentially damaging ultraviolet light from reaching the earth's surface. Ozone may be produced from ultraviolet light reacting with oxygen under the process:

O₂ +UV radiation< 200 nm → 2 O O + O₂ → O₃

In particular, wavelengths below 200 nano meters are effective in converting the oxygen in a medium to ozone.

Ozone may be used in various applications, e.g. for bleaching substances and for killing microorganisms in air and water sources.

Many drinking water systems and installations now kill bacteria with ozone instead of the previously more commonly applied chlorine.

Ozone has a very high oxidation potential, and further, one particular desirable property of ozone is, in conjunction with water purification and treatment, that ozone may be removed from water after treatment by means of reaction with free radicals.

Provided presence of radicals, or free radicals, the ozone may, possibly at a specific step, be neutralized under a reaction with oxygen:

O₃ + O → 2 O₂

Radicals, or free radicals, are atoms, molecules, or ions with unpaired electrons on an otherwise open shell configuration. The unpaired electrons are usually highly reactive, and therefore, radicals play an important role the reaction processes wherein they are present. According to various prior art techniques applied for purposes of rendering living organisms carried by ships ballast water inviable, ozone may be pumped into ballast tanks and/or into a flow of ballast water being lead to ballast tanks through one or more distributing pipes. This method consumes a relatively large amount of ozone in order to obtain a satisfactory effect, and the ozone remains in the ballast water whereby numerous concomitant disadvantages must be faced. Further, larger organisms are less sensitive to ozone which is why prior art solutions of applying ozone to the ballast water have shown to be inadequate

WO 2008/039147 A discloses a system and method for treating ballast water. The system and method is capable of treating ballast water, inter alia by means of ozone, while the ballast water is flowing to or from a piping system leading the water to or from the ballast tanks of a ship. According to the document, ozone is generated, e.g. from oxygen present in the water flowing through the system, by means of ultraviolet light having wavelengths below 200 nano metres. The generated ozone on one hand eliminates a part of the microorganisms present in the seawater while on the other hand, the ozone is subjected to UV light of a certain wavelength and broke down to radicals being more aggressive than ozone. The ozone molecules formed are, according to the document, at the same time or stage of the process, decomposed by ultraviolet light at wavelengths of below 400 nm.

To summarize the teachings of WO 2008/039147 A, UV radiation lamps are provided for producing ozone and free radicals simultaneously whereby any through flowing ballast water is subject to an instantaneous treatment or purification.

A selection of other attempts to provide ballast water treatment systems for ships and the like are disclosed in the following: WO 2009/022913 A teaches a method of treating ballast water. The ballast water is conveyed into a ballast tank via a down pipe in which an under pressure is created at an upper portion whereby a formation of gas bubbles is established. Ozone is added to the ballast water before or as the ballast wa- ter enters the down pipe.

DE 10 2006 037845 A teaches a ballast water system for, and a method of, killing micro-organisms, bacteria and germs by means of an ultraviolet light radiator The ballast water system comprises ballast pump, filter and a sterili- zation device generating ozone by means of an apparatus constituting a combination of a UV irradiator and an ozone generator.

US 2003/0015481 A1 teaches a method and apparatus for ballast water treatment According to the document, biological content in the ballast water is eliminated or reduced by injecting ozone into the ballast water during loading of the ballast water In particular, according to a sixth embodiment, a combination of hydro cyclone, UV treatment and ozone treatment is disclosed Application of UV radiation to the ballast water subsequent to the addition of ozone, is, according to the document, expected to further increase the effect of the ozone treatment, as this process results in the generation of reactive radical intermediates, particularly the hydroxyl radical (OH sup.-), which are capable of destroying complex organic substances. The authors of the document claim that the increased efficiency obtained by following the sixth embodiment, result in a decrease in ozone demand leading to shorter treatment time and lower costs, however the skilled reader is left with no further guidance.

Brief description of the invention

The present invention seeks to provide a system and a method for, in an efficient, environmentally friendly and economically reasonable manner, elimi- nate, or substantially reduce, biological contents in ballast water to be loaded and unloaded by maritime structures.

Up to this day, prior art has failed to teach a simple and yet reliable and inex- pensive system which in a safe and reliable manner, without increasing environmental impacts or power consumption, allows a maritime structure, such as a ship or a rig or the like, to load ballast water comprising biological contents, and, upon unloading the biological contents, unloading said contents to an environment in such a manner that the unloaded water will not cause damages to the receiving environment.

According to the first aspect of the invention, there is provided an apparatus as per the introductory part of this specification, and in particular upon configuring the apparatus such that the means for adding ozone to the flow of seawater through the apparatus and the means for irradiating the sea- water/ozone blend with ultraviolet light is arranged such that a carefully selected retention time of the ozone, between the addition of ozone and the irradiation with ultraviolet light, is between 2 and 500 seconds.

By configuring the apparatus such that the retention time, and thereby also the reaction time, is more than 2 seconds and less than 500 seconds, alternatively more than 3 seconds and less than 160 seconds or, as another alternative, even more than 4 seconds and less than 50 seconds, it has been found that the ballast water treatment apparatus will, in combination with other parameters, demonstrate a clear and astounding effect. Configuring the apparatus such that the retention time fall outside the mentioned intervals will result in an apparatus that may or may not operate according to the intention of the installer. It has been found that in case the apparatus is configured such that the retention time is below the mentioned intervals, unsatisfying water treatment will occur, and further, if the retention time is selected such that the retention time is above the mentioned intervals, the space required to accommodate the apparatus on or inside a maritime structure renders installation the apparatus unrealistic.

Conclusively, by configuring the system as outlined above, a particular reac- tion/retention time for the seawater/ozone blend is obtained followed by a twofold step of eliminating of the introduced ozone as well as organisms being irresponsive to the exposure of ozone, but responsive to either ultraviolet light or free radicals.

According to one embodiment, the means for irradiating the seawater/ozone blend may comprise at least one ultraviolet light emitting lamp capable of emitting ultraviolet light with a wave length below 300 nano metres.

According to one embodiment, the at least one lamp may be capable of emit- ting ultraviolet with a wave length below 200 nano metres.

According to one embodiment, the at least one lamp may constitute at least one coated or un-coated lamp.

According to one embodiment, the ozone may be injected into the seawater upstream the means for establishing ultraviolet light radiation.

According to one embodiment, the apparatus may further comprise means for increasing an internal volume downstream the means for adding ozone and upstream the means for establishing ultraviolet light radiation. The means may ensure that the flow time of the seawater/ozone blend, from the means for adding ozone to the means for establishing ultraviolet light radiation, is at least 2 seconds, however between 1 and 2 minutes or even up to abt. 8 minutes may be preferred. The means for increasing the internal vol- ume may constitute a tank or equivalent. According to one embodiment, the ozone may be added to the stream of seawater as a gas or a liquid.

According to one embodiment, the apparatus may further comprises a me- chanical filter arranged upstream the means for irradiating the blend with ultraviolet light.

According to one embodiment, the apparatus may further comprises a mechanical filter arranged upstream the means for adding ozone.

According to one embodiment, the apparatus may further comprise a mechanical filter, where the filter constitute an open non-pressurized filter being able to retain debris, bodies or elements having a size in excess of 5-30 μm.

According to one embodiment, a ballast water treatment system comprising a ballast water treatment apparatus according to the present invention is provided.

According to one embodiment, a ship comprising a ballast water treatment apparatus according to the present invention is provided.

According to one embodiment, the apparatus according to the present invention treat seawater being pumped into a ship and/or seawater being pumped out of the ship.

According to one embodiment, the UV light means, which may be arranged downstream the means for adding ozone, may have a light intensity of 100- 300 mJ/cm².

According to one embodiment, the surface of the UV light source of the means for irradiating the flowing seawater/ozone blend with ultraviolet light may be arranged in a distance from the flowing seawater/ozone blend of 10- 30 mm

According to one embodiment, the ozone constituting a part of the sea- water/ozone blend may be generated by means of blowing or guiding air such that the air is exposed to the means for establishing ultraviolet light radiation. The air may, prior to being exposed to UV light, be dried to a dew point temperature between -80⁰C to +10⁰C.

According to a second aspect of the present invention, a method of, in a stream of ballast water, rendering living organisms inviable is provided. The method comprises the steps of:

- establishing a stream of seawater,

- adding ozone to the stream of seawater such that a seawater/ozone blend is obtained,

- downstream the introduction of ozone, irradiating the seawater/ozone blend with ultraviolet light such that any residual ozone is decomposed into free radicals,

- providing a retention time of the ozone in the seawater/ozone blend in- between the addition of ozone and the irradiation with ultraviolet light, of between 2 and 500 seconds.

According to one embodiment of the second aspect, the retention time may be between 3 and 160 seconds.

According to one embodiment of the second aspect, the retention time may be between 4 and 50 seconds According to one embodiment of the second aspect, the method may further comprises a step of suppressing or eliminating the ozone by means of the ultra violet light under a reaction forming free radicals.

According to one embodiment of the second aspect, the method may further comprises a step of suppressing or eliminating the ozone by means of the ultra violet light under a reaction forming free radicals

Brief description of the drawings

Figure 1 illustrates a ballast water treatment apparatus in a simplified embodiment

Figure 2 illustrates an embodiment of a ballast water treatment apparatus

Figure 3 illustrates an embodiment of a ballast water treatment apparatus

Figure 4 illustrates an embodiment of a ballast water treatment apparatus.

Detailed description with reference to the figures

As can be seen in figure 1 , the apparatus according to the present invention may, in a simplified form, be built up around a conduit suitable for accommodating a flow of seawater 12.

The seawater 12 is, possibly via not in figure 1 shown filtration means, lead to or through means 20 for adding ozone 30, whereby a seawater/ozone blend 35 is established.

The means 20 for adding ozone 30 to the flow of seawater may constitute any suitable means for adding gas or liquid to flowing liquid such as a perfo- rated sleeve or one or more injectors (not shown), one or more aspirators or equivalent.

An aspirator, also known as an eductor-jet pump, is a device that produces vacuum or a reduced pressure by means of a Venturi effect. In an aspirator, fluid (liquid or gaseous) flows through a conduit, and upon meeting a narrowing of the conduit, the fluid's speed increases, and due to the Venturi effect, the pressure decreases, and a fluid may thereby be drawn into the flow.

The ozone 20 may be supplied from an external and not shown source of ozone.

Means or measures may, although not depicted, be taken in order to secure uniform blend of ozone into the seawater. Such means could constitute guides or baffles and the like.

The concentration of ozone in the obtained seawater/ozone blend 35 may be controlled inter alia by regulating the supply pressure of ozone or, as an equal alternative, controlling the flow of seawater 12 through the means 30 for adding ozone.

Various organisms are after the inflow of ozone gradually eliminated as the ozone blends into the flowing seawater and is allowed to impose its fierce effect on the organisms present in the flow.

The seawater/ozone blend 35 is hereafter via conduits etc. lead to means 40 for irradiating the seawater/ozone blend 35 with ultraviolet light.

The means for irradiating the flow of seawater/ozone blend 35 may constitute any suitable arrangement ensuring that the through flowing seawater/ozone blend is efficiently exposed to the ultraviolet light emitted by the sources of ultraviolet light

In figure 1 , the means for irradiating the through flowing seawater/ozone blend constitute a sleeve configured to lead ultraviolet light established by one or more lamps 41 , to the flowing blend. The shown means for establishing radiation of the through flowing seawater/ozone blend is a simplified embodiment of a part of the present invention. A more sophisticated device for establishing the ultraviolet radiation will be discussed at a later stage of this specification.

The source of ultraviolet light may constitute any source capable of emitting ultraviolet light, such as LED s, light bulbs, tubes or equivalent.

The ultraviolet light may stem from one or more light sources and in embodiments wherein a plurality of light sources are applied, the light sources may or may not have similar or identical properties such as effect or light spectres.

One or more of the selected light sources may or may not constitute coated or doped light sources, possibly comprising quarts glass. The quarts glass may or may not constitute synthetic quarts glass.

According to a well-functioning embodiment of the present invention, non- coated low pressure ultraviolet lamps being able to emit ultraviolet light with at least two different wavelengths, one being below 200 nano metres and another being below 350 nano metres, and at least partially being made up from quarts glass, may be provided to the apparatus and method.

It may be preferred to adapt the ultraviolet lamps to emit ultraviolet light with the wavelengths of 185 nano metres and 254 nano meters. The ultraviolet light emitting lamps may, at least partially, be made up from synthetic quartz

Before the flow leaves the treatment with ultraviolet light, the ozone is elimi- nated and the seawater 85 hereafter contains only lifeless or non viable remains of organisms and bacteria, which will cause no harm when introduced into a foreign environment

The conduit, which in figure 1 leads the treated seawater 85 away from the treatment with ultraviolet light, is connectable to an inlet of a conventional ballasting system, or, if the apparatus and method according to the present invention is applied on a ballast water outlet, ballast water being pumped out of the ship.

The apparatus and method according to the present invention may at convenience be applied to seawater inlets and/or outlets or even to internal circulating or distribution systems Further, the apparatus and method according to the present invention may be coupled such that one apparatus, by means of a not shown valve system, may serve all aforesaid applications, i.e inlet or outlet of seawater as well as internal circulation or distribution

Figure 2 illustrates an enhanced embodiment of the ballast water treatment apparatus according to figure 1

The enhanced embodiment according to figure 2 deviates from the embodiment according to figure 1 in that ozone is not supplied from an external source of ozone.

As can be seen in figure 2, the conduit 33 leading ozone to the means 20 for adding ozone 30 to the flow of seawater, is connected to the means 40 for irradiating the seawater/ozone blend 35 with ultraviolet light. Air or oxygen 60 is irradiated by ultraviolet light whereby, as per the introductory part of this specification, ozone is generated from the oxygen lead into the means 40 for exposing the seawater/ozone blend to ultraviolet light.

The generated ozone is, through pumping or controlling means 31 , lead to the means 20 for adding ozone 30 to the flow of seawater 12, whereby the system and method according to the present invention may operate without requiring external supply of ozone.

As ultraviolet light sources may require a certain start-up time, it may be preferred to provide the system with one or more not shown inlets for externally stored ozone, such that treatment of ballast water may be initiated more or less immediately after a request for start up of ballast water treatment.

Upon start-up, a part of the generated ozone may be lead back to the ozone storage facility

The embodiment according to figure 2 may, for logistic reasons as well as for reasons concerning the safety of seafarers and the environment, be pre- ferred, as no or only little ozone has to be stowed externally

Figure 3 illustrates an enhanced embodiment of the ballast water treatment apparatus according to figure 2

This embodiment deviates from the embodiment according to figure 2 in that a large volume for holding the through flowing seawater/ozone blend is arranged downstream the means 20 for adding ozone and upstream the means 40 for irradiating the through flowing seawater/ozone blend 35

The larger volume provides increased reaction/retention time for the seawater/ozone blend 35 The increased reaction time boosts the efficiency of the apparatus as well as increase the allowed rate of flow through the system Further, the retention/reaction time should preferably be selected in accordance with teachings set for the earlier in this specification.

Figure 4 illustrates another embodiment of ballast water treatment apparatus

As can be seen in figure 4, an optional filtration unit, which may constitute an open and pressure less micro screen filter capable of retaining particles down to about 10 micrometers, may be applied prior to subsequent steps of water treatment The purpose of the filter is to remove large organisms and general debris not being retained in the seawater iniet filters such as the very rough grilles typically provided in conjunction with sea chests and the like

The filtrated seawater 112 is lead to means for adding ozone 30 As can be seen in the figure, a part of the filtrated seawater may be drawn parallel conduits 134 comprising pumping means 131 and an aspirator 135 or a similar device.

The vacuum or reduced pressure may be used to draw in to the parallel con- duits 134 ozone 130 via the conduit 133. As can be seen in figure 4, the seawater/ozone blend is lead back to the main flow of seawater and subsequently into a reaction tank 135 which could constitute a closed reactor.

Although not shown in figure 4, ozone may be applied directly to the main flow of seawater, i.e the system could be built up without the parallel conduits 134 and aspirator 135 etc

The purpose of the tank 135 is similar to the purpose of the larger volume as described under figure 3. Although not shown in the figures, the closed reactor may comprise stirring or blending means such as guides and/or baffles and the like.

Typically, the seawater to be treated will be under a continuous flow; how- ever the system may also be able to operate intermittently.

Upon the seawater/ozone blend completing a predetermined period of time in the reaction chamber, typically anywhere from 2 seconds to about 8 minutes, the seawater/ozone blend reaches a combined ultraviolet sterilization and ozone eliminating unit 136. In the unit 136, the ozone is eliminated by the UV light, according to a previously discussed reaction, and also any organisms not already eradicated by the ozone, will, due to the ultraviolet light irradiation, be eliminated.

As can be seen in figure 4, the source of ultraviolet light may constitute elongated tubes 144 or equivalent arranged within conduits or tubes 142

The tubes or conduits 142 may be made up from at least partially transparent glass or plastic tubes. Further, the tubes or conduits 142 may be made up from quarts glass or synthetic quartz glass.

Further as can be seen in figure 4, air or oxygen 60 is, possibly by not shown blowing or pumping means, being lead through the tubes 142, wherein the source of ultraviolet light 141 is also arranged, whereby ozone is generated according to a reaction described earlier in this specification. The generated ozone 130 is collected and is, via conduit 133, lead to the means for adding ozone to the seawater, either via above mentioned parallel conduits, as shown in figure 4, or directly to a main flow of seawater. The apparatus and method according to the present invention may be provided to a ship as a retrofit or as an initially installed apparatus and method arranged in the ships during the building process.

Claims

C l a i m s

1. A ballast water treatment apparatus configured for treatment of seawater to be carried as ballast water by a maritime structure, said apparatus inter alia comprising:

- a seawater inlet (10) connectable to a supply of seawater,

- a seawater outlet (90) connectable to means for holding or distributing ballast water for the purpose of ballasting said maritime structure,

- means (20) for adding ozone (30) to a flow of seawater through said apparatus such that a flowing seawater/ozone blend (35) is established,

- means (40), arranged downstream said means (20) for adding ozone (30), for irradiating said flowing seawater/ozone blend (35) with ultraviolet light characterized in that said means (20) for adding ozone (30) to a flow of seawater through said apparatus and said means (40) for irradiating said seawater/ozone blend (35) with ultraviolet light is arranged such that a retention time of said ozone (30) between said addition of ozone (30) and said irradiation with ultraviolet light is between 2 and 500 seconds.

2. A ballast water treatment apparatus according to claim 1 , wherein said retention time is between 3 and 160 seconds.

3. A ballast water treatment apparatus according to claim 1 , wherein said retention time is between 4 and 50 seconds.

4. A ballast water treatment apparatus according to any of the preceding claims, wherein said means for irradiating said flow of seawater/ozone blend (35) constitute at least one lamp (41 ) capable of emitting ultraviolet light with a wave length below 200 nano metres.

5. A ballast water treatment apparatus according to any of the preceding claims, wherein said apparatus further comprise a tank (36) provided for increasing an internal volume downstream said means (20) for adding ozone (30) and upstream said means (40) for establishing ultraviolet light radiation.

6. A ballast water treatment apparatus according to any of the preceding claims, wherein said apparatus further comprises a mechanical filter arranged upstream said means (20) for adding ozone, where said filter constitute an open non-pressurized filter being able to retain bodies or elements having a size in excess of 5-30 μm.

7. A ballast water treatment apparatus according to claim 6, wherein said means (40), arranged downstream said means (20) for adding ozone (30), have a light intensity of 100-300 m J/cm².

8. A ballast water treatment apparatus according to any of the preceding claims, wherein a surface of a UV light source of said means for irradiating said flowing seawater/ozone blend (35) with ultraviolet light is arranged in a distance from said flowing seawater/ozone blend (35) of 10-30 mm.

9. A ballast water treatment apparatus according to any of the preceding claims, wherein said ozone (30) is generated by means of blowing or guiding air such that said air is exposed to said means (40) for establishing ultraviolet light radiation, where said air, prior to being exposed to said UV light, is dried to a dew point temperature between -80⁰C to +10^uC.

10 A ballast water treatment system comprising a ballast water treatment apparatus according to any of the preceding claims.

11. A ship comprising a ballast water treatment apparatus according to any of the preceding claims.

12. A method of, in a stream of ballast water, rendering living organisms invi- able, said method comprises the steps of:

- establishing a stream of seawater,

- adding ozone to said stream of seawater such that a seawater/ozone blend is obtained,

- downstream said introduction of ozone, irradiating said sea- water/ozone blend with ultraviolet light such that any residual ozone is decomposed into free radicals. characterized in that said method further comprises a step of providing a retention time of said ozone in said seawater/ozone blend in-between said addition of ozone (30) and said irradiation with ultraviolet light, of between 2 and 500 seconds.

13. A method according to claim 12, wherein said retention time is between 3 and 160 seconds.

14. A method according to claim 12, wherein said retention time is between 4 and 50 seconds.

15. A method according to any of the claims 12 - 14, wherein said method further comprises a step of suppressing or eliminating said ozone by means of said ultra violet light under a reaction forming free radicals.